# Exception Class

## Definition

Represents errors that occur during application execution.

``public ref class Exception``
``public ref class Exception : System::Runtime::Serialization::ISerializable``
``public ref class Exception : System::Runtime::InteropServices::_Exception, System::Runtime::Serialization::ISerializable``
``public class Exception``
``public class Exception : System.Runtime.Serialization.ISerializable``
``````[System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.AutoDual)]
[System.Serializable]
public class Exception : System.Runtime.Serialization.ISerializable``````
``````[System.Serializable]
[System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.None)]
[System.Runtime.InteropServices.ComVisible(true)]
public class Exception : System.Runtime.InteropServices._Exception, System.Runtime.Serialization.ISerializable``````
``````[System.Serializable]
[System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.None)]
[System.Runtime.InteropServices.ComVisible(true)]
public class Exception : System.Runtime.Serialization.ISerializable``````
``type Exception = class``
``````type Exception = class
interface ISerializable``````
``````[<System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.AutoDual)>]
[<System.Serializable>]
type Exception = class
interface ISerializable``````
``````[<System.Serializable>]
[<System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.None)>]
[<System.Runtime.InteropServices.ComVisible(true)>]
type Exception = class
interface ISerializable
interface _Exception``````
``````[<System.Serializable>]
[<System.Runtime.InteropServices.ClassInterface(System.Runtime.InteropServices.ClassInterfaceType.None)>]
[<System.Runtime.InteropServices.ComVisible(true)>]
type Exception = class
interface ISerializable``````
``Public Class Exception``
``````Public Class Exception
Implements ISerializable``````
``````Public Class Exception
Implements _Exception, ISerializable``````
Inheritance
Exception
Derived
Attributes
Implements

## Examples

The following example demonstrates a `catch` (`with` in F#) block that is defined to handle ArithmeticException errors. This `catch` block also catches DivideByZeroException errors, because DivideByZeroException derives from ArithmeticException and there is no `catch` block explicitly defined for DivideByZeroException errors.

``````using namespace System;
int main()
{
int x = 0;
try
{
int y = 100 / x;
}
catch ( ArithmeticException^ e )
{
Console::WriteLine( "ArithmeticException Handler: {0}", e );
}
catch ( Exception^ e )
{
Console::WriteLine( "Generic Exception Handler: {0}", e );
}
}
/*
This code example produces the following results:

ArithmeticException Handler: System.DivideByZeroException: Attempted to divide by zero.
at main()

*/
``````
``````using System;

class ExceptionTestClass
{
public static void Main()
{
int x = 0;
try
{
int y = 100 / x;
}
catch (ArithmeticException e)
{
Console.WriteLine(\$"ArithmeticException Handler: {e}");
}
catch (Exception e)
{
Console.WriteLine(\$"Generic Exception Handler: {e}");
}
}
}
/*
This code example produces the following results:

ArithmeticException Handler: System.DivideByZeroException: Attempted to divide by zero.
at ExceptionTestClass.Main()

*/
``````
``````module ExceptionTestModule

open System

let x = 0
try
let y = 100 / x
()
with
| :? ArithmeticException as e ->
printfn \$"ArithmeticException Handler: {e}"
| e ->
printfn \$"Generic Exception Handler: {e}"

// This code example produces the following results:
//     ArithmeticException Handler: System.DivideByZeroException: Attempted to divide by zero.
//        at <StartupCode\$fs>.\$ExceptionTestModule.main@()
``````
``````Class ExceptionTestClass

Public Shared Sub Main()
Dim x As Integer = 0
Try
Dim y As Integer = 100 / x
Catch e As ArithmeticException
Console.WriteLine("ArithmeticException Handler: {0}", e.ToString())
Catch e As Exception
Console.WriteLine("Generic Exception Handler: {0}", e.ToString())
End Try
End Sub
End Class
'
'This code example produces the following results:
'
'ArithmeticException Handler: System.OverflowException: Arithmetic operation resulted in an overflow.
'   at ExceptionTestClass.Main()
'
``````

## Remarks

This class is the base class for all exceptions. When an error occurs, either the system or the currently executing application reports it by throwing an exception that contains information about the error. After an exception is thrown, it is handled by the application or by the default exception handler.

In this section:

### Errors and exceptions

Run-time errors can occur for a variety of reasons. However, not all errors should be handled as exceptions in your code. Here are some categories of errors that can occur at run time and the appropriate ways to respond to them.

• Usage errors. A usage error represents an error in program logic that can result in an exception. However, the error should be addressed not through exception handling but by modifying the faulty code. For example, the override of the Object.Equals(Object) method in the following example assumes that the `obj` argument must always be non-null.

``````using System;

public class Person
{
private string _name;

public string Name
{
get { return _name; }
set { _name = value; }
}

public override int GetHashCode()
{
return this.Name.GetHashCode();
}

public override bool Equals(object obj)
{
// This implementation contains an error in program logic:
// It assumes that the obj argument is not null.
Person p = (Person) obj;
return this.Name.Equals(p.Name);
}
}

public class Example
{
public static void Main()
{
Person p1 = new Person();
p1.Name = "John";
Person p2 = null;

// The following throws a NullReferenceException.
Console.WriteLine("p1 = p2: {0}", p1.Equals(p2));
}
}
``````
``````// In F#, null is not a valid state for declared types
// without 'AllowNullLiteralAttribute'
[<AllowNullLiteral>]
type Person() =
member val Name = "" with get, set

override this.GetHashCode() =
this.Name.GetHashCode()

override this.Equals(obj) =
// This implementation contains an error in program logic:
// It assumes that the obj argument is not null.
let p = obj :?> Person
this.Name.Equals p.Name

let p1 = Person()
p1.Name <- "John"
let p2: Person = null

// The following throws a NullReferenceException.
printfn \$"p1 = p2: {p1.Equals p2}"
``````
``````Public Class Person
Private _name As String

Public Property Name As String
Get
Return _name
End Get
Set
_name = value
End Set
End Property

Public Overrides Function Equals(obj As Object) As Boolean
' This implementation contains an error in program logic:
' It assumes that the obj argument is not null.
Dim p As Person = CType(obj, Person)
Return Me.Name.Equals(p.Name)
End Function
End Class

Module Example
Public Sub Main()
Dim p1 As New Person()
p1.Name = "John"
Dim p2 As Person = Nothing

' The following throws a NullReferenceException.
Console.WriteLine("p1 = p2: {0}", p1.Equals(p2))
End Sub
End Module
``````

The NullReferenceException exception that results when `obj` is `null` can be eliminated by modifying the source code to explicitly test for null before calling the Object.Equals override and then re-compiling. The following example contains the corrected source code that handles a `null` argument.

``````using System;

public class Person
{
private string _name;

public string Name
{
get { return _name; }
set { _name = value; }
}

public override int GetHashCode()
{
return this.Name.GetHashCode();
}

public override bool Equals(object obj)
{
// This implementation handles a null obj argument.
Person p = obj as Person;
if (p == null)
return false;
else
return this.Name.Equals(p.Name);
}
}

public class Example
{
public static void Main()
{
Person p1 = new Person();
p1.Name = "John";
Person p2 = null;

Console.WriteLine("p1 = p2: {0}", p1.Equals(p2));
}
}
// The example displays the following output:
//        p1 = p2: False
``````
``````// In F#, null is not a valid state for declared types
// without 'AllowNullLiteralAttribute'
[<AllowNullLiteral>]
type Person() =
member val Name = "" with get, set

override this.GetHashCode() =
this.Name.GetHashCode()

override this.Equals(obj) =
// This implementation handles a null obj argument.
match obj with
| :? Person as p ->
this.Name.Equals p.Name
| _ ->
false

let p1 = Person()
p1.Name <- "John"
let p2: Person = null

printfn \$"p1 = p2: {p1.Equals p2}"
// The example displays the following output:
//        p1 = p2: False
``````
``````Public Class Person
Private _name As String

Public Property Name As String
Get
Return _name
End Get
Set
_name = value
End Set
End Property

Public Overrides Function Equals(obj As Object) As Boolean
' This implementation handles a null obj argument.
Dim p As Person = TryCast(obj, Person)
If p Is Nothing Then
Return False
Else
Return Me.Name.Equals(p.Name)
End If
End Function
End Class

Module Example
Public Sub Main()
Dim p1 As New Person()
p1.Name = "John"
Dim p2 As Person = Nothing

Console.WriteLine("p1 = p2: {0}", p1.Equals(p2))
End Sub
End Module
' The example displays the following output:
'       p1 = p2: False
``````

Instead of using exception handling for usage errors, you can use the Debug.Assert method to identify usage errors in debug builds, and the Trace.Assert method to identify usage errors in both debug and release builds. For more information, see Assertions in Managed Code.

• Program errors. A program error is a run-time error that cannot necessarily be avoided by writing bug-free code.

In some cases, a program error may reflect an expected or routine error condition. In this case, you may want to avoid using exception handling to deal with the program error and instead retry the operation. For example, if the user is expected to input a date in a particular format, you can parse the date string by calling the DateTime.TryParseExact method, which returns a Boolean value that indicates whether the parse operation succeeded, instead of using the DateTime.ParseExact method, which throws a FormatException exception if the date string cannot be converted to a DateTime value. Similarly, if a user tries to open a file that does not exist, you can first call the File.Exists method to check whether the file exists and, if it does not, prompt the user whether they want to create it.

In other cases, a program error reflects an unexpected error condition that can be handled in your code. For example, even if you've checked to ensure that a file exists, it may be deleted before you can open it, or it may be corrupted. In that case, trying to open the file by instantiating a StreamReader object or calling the Open method may throw a FileNotFoundException exception. In these cases, you should use exception handling to recover from the error.

• System failures. A system failure is a run-time error that cannot be handled programmatically in a meaningful way. For example, any method can throw an OutOfMemoryException exception if the common language runtime is unable to allocate additional memory. Ordinarily, system failures are not handled by using exception handling. Instead, you may be able to use an event such as AppDomain.UnhandledException and call the Environment.FailFast method to log exception information and notify the user of the failure before the application terminates.

### Try/catch blocks

The common language runtime provides an exception handling model that is based on the representation of exceptions as objects, and the separation of program code and exception handling code into `try` blocks and `catch` blocks. There can be one or more `catch` blocks, each designed to handle a particular type of exception, or one block designed to catch a more specific exception than another block.

If an application handles exceptions that occur during the execution of a block of application code, the code must be placed within a `try` statement and is called a `try` block. Application code that handles exceptions thrown by a `try` block is placed within a `catch` statement and is called a `catch` block. Zero or more `catch` blocks are associated with a `try` block, and each `catch` block includes a type filter that determines the types of exceptions it handles.

When an exception occurs in a `try` block, the system searches the associated `catch` blocks in the order they appear in application code, until it locates a `catch` block that handles the exception. A `catch` block handles an exception of type `T` if the type filter of the catch block specifies `T` or any type that `T` derives from. The system stops searching after it finds the first `catch` block that handles the exception. For this reason, in application code, a `catch` block that handles a type must be specified before a `catch` block that handles its base types, as demonstrated in the example that follows this section. A catch block that handles `System.Exception` is specified last.

If none of the `catch` blocks associated with the current `try` block handle the exception, and the current `try` block is nested within other `try` blocks in the current call, the `catch` blocks associated with the next enclosing `try` block are searched. If no `catch` block for the exception is found, the system searches previous nesting levels in the current call. If no `catch` block for the exception is found in the current call, the exception is passed up the call stack, and the previous stack frame is searched for a `catch` block that handles the exception. The search of the call stack continues until the exception is handled or until no more frames exist on the call stack. If the top of the call stack is reached without finding a `catch` block that handles the exception, the default exception handler handles it and the application terminates.

#### F# try..with Expression

F# does not use `catch` blocks. Instead, a raised exception is pattern matched using a single `with` block. As this is an expression, rather than a statement, all paths must return the same type. To learn more, see The try...with Expression.

### Exception type features

Exception types support the following features:

• Human-readable text that describes the error. When an exception occurs, the runtime makes a text message available to inform the user of the nature of the error and to suggest action to resolve the problem. This text message is held in the Message property of the exception object. During the creation of the exception object, you can pass a text string to the constructor to describe the details of that particular exception. If no error message argument is supplied to the constructor, the default error message is used. For more information, see the Message property.

• The state of the call stack when the exception was thrown. The StackTrace property carries a stack trace that can be used to determine where the error occurs in the code. The stack trace lists all the called methods and the line numbers in the source file where the calls are made.

### Exception class properties

The Exception class includes a number of properties that help identify the code location, the type, the help file, and the reason for the exception: StackTrace, InnerException, Message, HelpLink, HResult, Source, TargetSite, and Data.

When a causal relationship exists between two or more exceptions, the InnerException property maintains this information. The outer exception is thrown in response to this inner exception. The code that handles the outer exception can use the information from the earlier inner exception to handle the error more appropriately. Supplementary information about the exception can be stored as a collection of key/value pairs in the Data property.

The error message string that is passed to the constructor during the creation of the exception object should be localized and can be supplied from a resource file by using the ResourceManager class. For more information about localized resources, see the Creating Satellite Assemblies and Packaging and Deploying Resources topics.

To provide the user with extensive information about why the exception occurred, the HelpLink property can hold a URL (or URN) to a help file.

The Exception class uses the HRESULT COR_E_EXCEPTION, which has the value 0x80131500.

For a list of initial property values for an instance of the Exception class, see the Exception constructors.

### Performance considerations

Throwing or handling an exception consumes a significant amount of system resources and execution time. Throw exceptions only to handle truly extraordinary conditions, not to handle predictable events or flow control. For example, in some cases, such as when you're developing a class library, it's reasonable to throw an exception if a method argument is invalid, because you expect your method to be called with valid parameters. An invalid method argument, if it is not the result of a usage error, means that something extraordinary has occurred. Conversely, do not throw an exception if user input is invalid, because you can expect users to occasionally enter invalid data. Instead, provide a retry mechanism so users can enter valid input. Nor should you use exceptions to handle usage errors. Instead, use assertions to identify and correct usage errors.

In addition, do not throw an exception when a return code is sufficient; do not convert a return code to an exception; and do not routinely catch an exception, ignore it, and then continue processing.

### Re-throwing an exception

In many cases, an exception handler simply wants to pass the exception on to the caller. This most often occurs in:

• A class library that in turn wraps calls to methods in the .NET Framework class library or other class libraries.

• An application or library that encounters a fatal exception. The exception handler can log the exception and then re-throw the exception.

The recommended way to re-throw an exception is to simply use the throw statement in C#, the reraise function in F#, and the Throw statement in Visual Basic without including an expression. This ensures that all call stack information is preserved when the exception is propagated to the caller. The following example illustrates this. A string extension method, `FindOccurrences`, wraps one or more calls to String.IndexOf(String, Int32) without validating its arguments beforehand.

``````using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

public static class Library
{
public static int[] FindOccurrences(this String s, String f)
{
var indexes = new List<int>();
int currentIndex = 0;
try {
while (currentIndex >= 0 && currentIndex < s.Length) {
currentIndex = s.IndexOf(f, currentIndex);
if (currentIndex >= 0) {
currentIndex++;
}
}
}
catch (ArgumentNullException e) {
// Perform some action here, such as logging this exception.

throw;
}
return indexes.ToArray();
}
}
``````
``````open System

module Library =
let findOccurrences (s: string) (f: string) =
let indexes = ResizeArray()
let mutable currentIndex = 0
try
while currentIndex >= 0 && currentIndex < s.Length do
currentIndex <- s.IndexOf(f, currentIndex)
if currentIndex >= 0 then
currentIndex <- currentIndex + 1
with :? ArgumentNullException ->
// Perform some action here, such as logging this exception.
reraise ()
indexes.ToArray()
``````
``````Imports System.Collections.Generic
Imports System.Runtime.CompilerServices

Public Module Library
<Extension()>
Public Function FindOccurrences(s As String, f As String) As Integer()
Dim indexes As New List(Of Integer)
Dim currentIndex As Integer = 0
Try
Do While currentIndex >= 0 And currentIndex < s.Length
currentIndex = s.IndexOf(f, currentIndex)
If currentIndex >= 0 Then
currentIndex += 1
End If
Loop
Catch e As ArgumentNullException
' Perform some action here, such as logging this exception.

Throw
End Try
Return indexes.ToArray()
End Function
End Module
``````

A caller then calls `FindOccurrences` twice. In the second call to `FindOccurrences`, the caller passes a `null` as the search string, which causes the String.IndexOf(String, Int32) method to throw an ArgumentNullException exception. This exception is handled by the `FindOccurrences` method and passed back to the caller. Because the throw statement is used with no expression, the output from the example shows that the call stack is preserved.

``````public class Example
{
public static void Main()
{
String s = "It was a cold day when...";
int[] indexes = s.FindOccurrences("a");
ShowOccurrences(s, "a", indexes);
Console.WriteLine();

String toFind = null;
try {
indexes = s.FindOccurrences(toFind);
ShowOccurrences(s, toFind, indexes);
}
catch (ArgumentNullException e) {
Console.WriteLine("An exception ({0}) occurred.",
e.GetType().Name);
Console.WriteLine("Message:\n   {0}\n", e.Message);
Console.WriteLine("Stack Trace:\n   {0}\n", e.StackTrace);
}
}

private static void ShowOccurrences(String s, String toFind, int[] indexes)
{
Console.Write("'{0}' occurs at the following character positions: ",
toFind);
for (int ctr = 0; ctr < indexes.Length; ctr++)
Console.Write("{0}{1}", indexes[ctr],
ctr == indexes.Length - 1 ? "" : ", ");

Console.WriteLine();
}
}
// The example displays the following output:
//    'a' occurs at the following character positions: 4, 7, 15
//
//    An exception (ArgumentNullException) occurred.
//    Message:
//       Value cannot be null.
//    Parameter name: value
//
//    Stack Trace:
//          at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
//    ngComparison comparisonType)
//       at Library.FindOccurrences(String s, String f)
//       at Example.Main()
``````
``````open Library

let showOccurrences toFind (indexes: int[]) =
printf \$"'{toFind}' occurs at the following character positions: "
for i = 0 to indexes.Length - 1 do
printf \$"""{indexes[i]}{if i = indexes.Length - 1 then "" else ", "}"""
printfn ""

let s = "It was a cold day when..."
let indexes = findOccurrences s "a"
showOccurrences "a" indexes
printfn ""

let toFind: string = null
try
let indexes = findOccurrences s toFind
showOccurrences toFind indexes

with :? ArgumentNullException as e ->
printfn \$"An exception ({e.GetType().Name}) occurred."
printfn \$"Message:\n   {e.Message}\n"
printfn \$"Stack Trace:\n   {e.StackTrace}\n"

// The example displays the following output:
//    'a' occurs at the following character positions: 4, 7, 15
//
//    An exception (ArgumentNullException) occurred.
//    Message:
//       Value cannot be null. (Parameter 'value')
//
//    Stack Trace:
//          at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
//    ngComparison comparisonType)
//       at Library.findOccurrences(String s, String f)
//       at <StartupCode\$fs>.main@()
``````
``````Module Example
Public Sub Main()
Dim s As String = "It was a cold day when..."
Dim indexes() As Integer = s.FindOccurrences("a")
ShowOccurrences(s, "a", indexes)
Console.WriteLine()

Dim toFind As String = Nothing
Try
indexes = s.FindOccurrences(toFind)
ShowOccurrences(s, toFind, indexes)
Catch e As ArgumentNullException
Console.WriteLine("An exception ({0}) occurred.",
e.GetType().Name)
Console.WriteLine("Message:{0}   {1}{0}", vbCrLf, e.Message)
Console.WriteLine("Stack Trace:{0}   {1}{0}", vbCrLf, e.StackTrace)
End Try
End Sub

Private Sub ShowOccurrences(s As String, toFind As String, indexes As Integer())
Console.Write("'{0}' occurs at the following character positions: ",
toFind)
For ctr As Integer = 0 To indexes.Length - 1
Console.Write("{0}{1}", indexes(ctr),
If(ctr = indexes.Length - 1, "", ", "))
Next
Console.WriteLine()
End Sub
End Module
' The example displays the following output:
'    'a' occurs at the following character positions: 4, 7, 15
'
'    An exception (ArgumentNullException) occurred.
'    Message:
'       Value cannot be null.
'    Parameter name: value
'
'    Stack Trace:
'          at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
'    ngComparison comparisonType)
'       at Library.FindOccurrences(String s, String f)
'       at Example.Main()
``````

In contrast, if the exception is re-thrown by using the

``````throw e;
``````
``````Throw e
``````
``````raise e
``````

statement, the full call stack is not preserved, and the example would generate the following output:

``````'a' occurs at the following character positions: 4, 7, 15

An exception (ArgumentNullException) occurred.
Message:
Value cannot be null.
Parameter name: value

Stack Trace:
at Library.FindOccurrences(String s, String f)
at Example.Main()
``````

A slightly more cumbersome alternative is to throw a new exception, and to preserve the original exception's call stack information in an inner exception. The caller can then use the new exception's InnerException property to retrieve stack frame and other information about the original exception. In this case, the throw statement is:

``````throw new ArgumentNullException("You must supply a search string.",
e);
``````
``````raise (ArgumentNullException("You must supply a search string.", e) )
``````
``````Throw New ArgumentNullException("You must supply a search string.",
e)
``````

The user code that handles the exception has to know that the InnerException property contains information about the original exception, as the following exception handler illustrates.

``````try {
indexes = s.FindOccurrences(toFind);
ShowOccurrences(s, toFind, indexes);
}
catch (ArgumentNullException e) {
Console.WriteLine("An exception ({0}) occurred.",
e.GetType().Name);
Console.WriteLine("   Message:\n{0}", e.Message);
Console.WriteLine("   Stack Trace:\n   {0}", e.StackTrace);
Exception ie = e.InnerException;
if (ie != null) {
Console.WriteLine("   The Inner Exception:");
Console.WriteLine("      Exception Name: {0}", ie.GetType().Name);
Console.WriteLine("      Message: {0}\n", ie.Message);
Console.WriteLine("      Stack Trace:\n   {0}\n", ie.StackTrace);
}
}
// The example displays the following output:
//    'a' occurs at the following character positions: 4, 7, 15
//
//    An exception (ArgumentNullException) occurred.
//       Message: You must supply a search string.
//
//       Stack Trace:
//          at Library.FindOccurrences(String s, String f)
//       at Example.Main()
//
//       The Inner Exception:
//          Exception Name: ArgumentNullException
//          Message: Value cannot be null.
//    Parameter name: value
//
//          Stack Trace:
//          at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
//    ngComparison comparisonType)
//       at Library.FindOccurrences(String s, String f)
``````
``````try
let indexes = findOccurrences s toFind
showOccurrences toFind indexes
with :? ArgumentNullException as e ->
printfn \$"An exception ({e.GetType().Name}) occurred."
printfn \$"   Message:\n{e.Message}"
printfn \$"   Stack Trace:\n   {e.StackTrace}"
let ie = e.InnerException
if ie <> null then
printfn "   The Inner Exception:"
printfn \$"      Exception Name: {ie.GetType().Name}"
printfn \$"      Message: {ie.Message}\n"
printfn \$"      Stack Trace:\n   {ie.StackTrace}\n"
// The example displays the following output:
//    'a' occurs at the following character positions: 4, 7, 15
//
//    An exception (ArgumentNullException) occurred.
//       Message: You must supply a search string.
//
//       Stack Trace:
//          at Library.FindOccurrences(String s, String f)
//       at Example.Main()
//
//       The Inner Exception:
//          Exception Name: ArgumentNullException
//          Message: Value cannot be null.
//    Parameter name: value
//
//          Stack Trace:
//          at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
//    ngComparison comparisonType)
//       at Library.FindOccurrences(String s, String f)
``````
``````Try
indexes = s.FindOccurrences(toFind)
ShowOccurrences(s, toFind, indexes)
Catch e As ArgumentNullException
Console.WriteLine("An exception ({0}) occurred.",
e.GetType().Name)
Console.WriteLine("   Message: {1}{0}", vbCrLf, e.Message)
Console.WriteLine("   Stack Trace:{0}   {1}{0}", vbCrLf, e.StackTrace)
Dim ie As Exception = e.InnerException
If ie IsNot Nothing Then
Console.WriteLine("   The Inner Exception:")
Console.WriteLine("      Exception Name: {0}", ie.GetType().Name)
Console.WriteLine("      Message: {1}{0}", vbCrLf, ie.Message)
Console.WriteLine("      Stack Trace:{0}   {1}{0}", vbCrLf, ie.StackTrace)
End If
End Try
' The example displays the following output:
'       'a' occurs at the following character positions: 4, 7, 15
'
'       An exception (ArgumentNullException) occurred.
'          Message: You must supply a search string.
'
'          Stack Trace:
'             at Library.FindOccurrences(String s, String f)
'          at Example.Main()
'
'          The Inner Exception:
'             Exception Name: ArgumentNullException
'             Message: Value cannot be null.
'       Parameter name: value
'
'             Stack Trace:
'             at System.String.IndexOf(String value, Int32 startIndex, Int32 count, Stri
'       ngComparison comparisonType)
'          at Library.FindOccurrences(String s, String f)
``````

### Choosing standard exceptions

When you have to throw an exception, you can often use an existing exception type in the .NET Framework instead of implementing a custom exception. You should use a standard exception type under these two conditions:

• You are throwing an exception that is caused by a usage error (that is, by an error in program logic made by the developer who is calling your method). Typically, you would throw an exception such as ArgumentException, ArgumentNullException, InvalidOperationException, or NotSupportedException. The string you supply to the exception object's constructor when instantiating the exception object should describe the error so that the developer can fix it. For more information, see the Message property.

• You are handling an error that can be communicated to the caller with an existing .NET Framework exception. You should throw the most derived exception possible. For example, if a method requires an argument to be a valid member of an enumeration type, you should throw an InvalidEnumArgumentException (the most derived class) rather than an ArgumentException.

The following table lists common exception types and the conditions under which you would throw them.

Exception Condition
ArgumentException A non-null argument that is passed to a method is invalid.
ArgumentNullException An argument that is passed to a method is `null`.
ArgumentOutOfRangeException An argument is outside the range of valid values.
DirectoryNotFoundException Part of a directory path is not valid.
DivideByZeroException The denominator in an integer or Decimal division operation is zero.
DriveNotFoundException A drive is unavailable or does not exist.
FileNotFoundException A file does not exist.
FormatException A value is not in an appropriate format to be converted from a string by a conversion method such as `Parse`.
IndexOutOfRangeException An index is outside the bounds of an array or collection.
InvalidOperationException A method call is invalid in an object's current state.
KeyNotFoundException The specified key for accessing a member in a collection cannot be found.
NotImplementedException A method or operation is not implemented.
NotSupportedException A method or operation is not supported.
ObjectDisposedException An operation is performed on an object that has been disposed.
OverflowException An arithmetic, casting, or conversion operation results in an overflow.
PathTooLongException A path or file name exceeds the maximum system-defined length.
PlatformNotSupportedException The operation is not supported on the current platform.
RankException An array with the wrong number of dimensions is passed to a method.
TimeoutException The time interval allotted to an operation has expired.
UriFormatException An invalid Uniform Resource Identifier (URI) is used.

### Implementing custom exceptions

In the following cases, using an existing .NET Framework exception to handle an error condition is not adequate:

• When the exception reflects a unique program error that cannot be mapped to an existing .NET Framework exception.

• When the exception requires handling that is different from the handling that is appropriate for an existing .NET Framework exception, or the exception must be disambiguated from a similar exception. For example, if you throw an ArgumentOutOfRangeException exception when parsing the numeric representation of a string that is out of range of the target integral type, you would not want to use the same exception for an error that results from the caller not supplying the appropriate constrained values when calling the method.

The Exception class is the base class of all exceptions in the .NET Framework. Many derived classes rely on the inherited behavior of the members of the Exception class; they do not override the members of Exception, nor do they define any unique members.

To define your own exception class:

1. Define a class that inherits from Exception. If necessary, define any unique members needed by your class to provide additional information about the exception. For example, the ArgumentException class includes a ParamName property that specifies the name of the parameter whose argument caused the exception, and the RegexMatchTimeoutException property includes a MatchTimeout property that indicates the time-out interval.

2. If necessary, override any inherited members whose functionality you want to change or modify. Note that most existing derived classes of Exception do not override the behavior of inherited members.

3. Determine whether your custom exception object is serializable. Serialization enables you to save information about the exception and permits exception information to be shared by a server and a client proxy in a remoting context. To make the exception object serializable, mark it with the SerializableAttribute attribute.

4. Define the constructors of your exception class. Typically, exception classes have one or more of the following constructors:

• Exception(), which uses default values to initialize the properties of a new exception object.

• Exception(String), which initializes a new exception object with a specified error message.

• Exception(String, Exception), which initializes a new exception object with a specified error message and inner exception.

• Exception(SerializationInfo, StreamingContext), which is a `protected` constructor that initializes a new exception object from serialized data. You should implement this constructor if you've chosen to make your exception object serializable.

The following example illustrates the use of a custom exception class. It defines a `NotPrimeException` exception that is thrown when a client tries to retrieve a sequence of prime numbers by specifying a starting number that is not prime. The exception defines a new property, `NonPrime`, that returns the non-prime number that caused the exception. Besides implementing a protected parameterless constructor and a constructor with SerializationInfo and StreamingContext parameters for serialization, the `NotPrimeException` class defines three additional constructors to support the `NonPrime` property. Each constructor calls a base class constructor in addition to preserving the value of the non-prime number. The `NotPrimeException` class is also marked with the SerializableAttribute attribute.

``````using System;
using System.Runtime.Serialization;

[Serializable()]
public class NotPrimeException : Exception
{
private int notAPrime;

protected NotPrimeException()
: base()
{ }

public NotPrimeException(int value) :
base(String.Format("{0} is not a prime number.", value))
{
notAPrime = value;
}

public NotPrimeException(int value, string message)
: base(message)
{
notAPrime = value;
}

public NotPrimeException(int value, string message, Exception innerException) :
base(message, innerException)
{
notAPrime = value;
}

protected NotPrimeException(SerializationInfo info,
StreamingContext context)
: base(info, context)
{ }

public int NonPrime
{ get { return notAPrime; } }
}
``````
``````namespace global

open System
open System.Runtime.Serialization

[<Serializable>]
type NotPrimeException =
inherit Exception
val notAPrime: int

member this.NonPrime =
this.notAPrime

new (value) =
{ inherit Exception(\$"%i{value} is not a prime number."); notAPrime = value }

new (value, message) =
{ inherit Exception(message); notAPrime = value }

new (value, message, innerException: Exception) =
{ inherit Exception(message, innerException); notAPrime = value }

// F# does not support protected members
new () =
{ inherit Exception(); notAPrime = 0 }

new (info: SerializationInfo, context: StreamingContext) =
{ inherit Exception(info, context); notAPrime = 0 }
``````
``````Imports System.Runtime.Serialization

<Serializable()> _
Public Class NotPrimeException : Inherits Exception
Private notAPrime As Integer

Protected Sub New()
MyBase.New()
End Sub

Public Sub New(value As Integer)
MyBase.New(String.Format("{0} is not a prime number.", value))
notAPrime = value
End Sub

Public Sub New(value As Integer, message As String)
MyBase.New(message)
notAPrime = value
End Sub

Public Sub New(value As Integer, message As String, innerException As Exception)
MyBase.New(message, innerException)
notAPrime = value
End Sub

Protected Sub New(info As SerializationInfo,
context As StreamingContext)
MyBase.New(info, context)
End Sub

Public ReadOnly Property NonPrime As Integer
Get
Return notAPrime
End Get
End Property
End Class
``````

The `PrimeNumberGenerator` class shown in the following example uses the Sieve of Eratosthenes to calculate the sequence of prime numbers from 2 to a limit specified by the client in the call to its class constructor. The `GetPrimesFrom` method returns all prime numbers that are greater than or equal to a specified lower limit, but throws a `NotPrimeException` if that lower limit is not a prime number.

``````using System;
using System.Collections.Generic;

[Serializable]
{
private const int START = 2;
private int maxUpperBound = 10000000;
private int upperBound;
private bool[] primeTable;
private List<int> primes = new List<int>();

{
if (upperBound > maxUpperBound)
{
string message = String.Format(
"{0} exceeds the maximum upper bound of {1}.",
upperBound, maxUpperBound);
throw new ArgumentOutOfRangeException(message);
}
this.upperBound = upperBound;
// Create array and mark 0, 1 as not prime (True).
primeTable = new bool[upperBound + 1];
primeTable = true;
primeTable = true;

// Use Sieve of Eratosthenes to determine prime numbers.
for (int ctr = START; ctr <= (int)Math.Ceiling(Math.Sqrt(upperBound));
ctr++)
{
if (primeTable[ctr]) continue;

for (int multiplier = ctr; multiplier <= upperBound / ctr; multiplier++)
if (ctr * multiplier <= upperBound) primeTable[ctr * multiplier] = true;
}
// Populate array with prime number information.
int index = START;
while (index != -1)
{
index = Array.FindIndex(primeTable, index, (flag) => !flag);
if (index >= 1)
{
index++;
}
}
}

public int[] GetAllPrimes()
{
return primes.ToArray();
}

public int[] GetPrimesFrom(int prime)
{
int start = primes.FindIndex((value) => value == prime);
if (start < 0)
throw new NotPrimeException(prime, String.Format("{0} is not a prime number.", prime));
else
return primes.FindAll((value) => value >= prime).ToArray();
}
}
``````
``````namespace global

open System

[<Serializable>]
let start = 2
let maxUpperBound = 10000000
let primes = ResizeArray()
let primeTable =
upperBound + 1
|> Array.zeroCreate<bool>

do
if upperBound > maxUpperBound then
let message = \$"{upperBound} exceeds the maximum upper bound of {maxUpperBound}."
raise (ArgumentOutOfRangeException message)

// Create array and mark 0, 1 as not prime (True).
primeTable <- true
primeTable <- true

// Use Sieve of Eratosthenes to determine prime numbers.
for i = start to float upperBound |> sqrt |> ceil |> int do
if not primeTable[i] then
for multiplier = i to upperBound / i do
if i * multiplier <= upperBound then
primeTable[i * multiplier] <- true

// Populate array with prime number information.
let mutable index = start
while index <> -1 do
index <- Array.FindIndex(primeTable, index, fun flag -> not flag)
if index >= 1 then
index <- index + 1

member _.GetAllPrimes() =
primes.ToArray()

member _.GetPrimesFrom(prime) =
let start =
Seq.findIndex ((=) prime) primes

if start < 0 then
raise (NotPrimeException(prime, \$"{prime} is not a prime number.") )
else
Seq.filter ((>=) prime) primes
|> Seq.toArray
``````
``````Imports System.Collections.Generic

Private Const START As Integer = 2
Private maxUpperBound As Integer = 10000000
Private upperBound As Integer
Private primeTable() As Boolean
Private primes As New List(Of Integer)

Public Sub New(upperBound As Integer)
If upperBound > maxUpperBound Then
Dim message As String = String.Format(
"{0} exceeds the maximum upper bound of {1}.",
upperBound, maxUpperBound)
Throw New ArgumentOutOfRangeException(message)
End If
Me.upperBound = upperBound
' Create array and mark 0, 1 as not prime (True).
ReDim primeTable(upperBound)
primeTable(0) = True
primeTable(1) = True

' Use Sieve of Eratosthenes to determine prime numbers.
For ctr As Integer = START To CInt(Math.Ceiling(Math.Sqrt(upperBound)))
If primeTable(ctr) Then Continue For

For multiplier As Integer = ctr To CInt(upperBound \ ctr)
If ctr * multiplier <= upperBound Then primeTable(ctr * multiplier) = True
Next
Next
' Populate array with prime number information.
Dim index As Integer = START
Do While index <> -1
index = Array.FindIndex(primeTable, index, Function(flag)
Return Not flag
End Function)
If index >= 1 Then
index += 1
End If
Loop
End Sub

Public Function GetAllPrimes() As Integer()
Return primes.ToArray()
End Function

Public Function GetPrimesFrom(prime As Integer) As Integer()
Dim start As Integer = primes.FindIndex(Function(value)
Return value = prime
End Function)
If start < 0 Then
Throw New NotPrimeException(prime, String.Format("{0} is not a prime number.", prime))
Else
Return primes.FindAll(Function(value)
Return value >= prime
End Function).ToArray()
End If
End Function
End Class
``````

The following example makes two calls to the `GetPrimesFrom` method with non-prime numbers, one of which crosses application domain boundaries. In both cases, the exception is thrown and successfully handled in client code.

``````using System;
using System.Reflection;

class Example
{
public static void Main()
{
int limit = 10000000;
int start = 1000001;
try
{
int[] values = primes.GetPrimesFrom(start);
Console.WriteLine("There are {0} prime numbers from {1} to {2}",
start, limit);
}
catch (NotPrimeException e)
{
Console.WriteLine("{0} is not prime", e.NonPrime);
Console.WriteLine(e);
Console.WriteLine("--------");
}

AppDomain domain = AppDomain.CreateDomain("Domain2");
typeof(Example).Assembly.FullName,
BindingFlags.Default, null,
new object[] { 1000000 }, null, null);
try
{
start = 100;
Console.WriteLine(gen.GetPrimesFrom(start));
}
catch (NotPrimeException e)
{
Console.WriteLine("{0} is not prime", e.NonPrime);
Console.WriteLine(e);
Console.WriteLine("--------");
}
}
}
``````
``````open System
open System.Reflection

let limit = 10000000
let start = 1000001
try
let values = primes.GetPrimesFrom start
printfn \$"There are {values.Length} prime numbers from {start} to {limit}"
with :? NotPrimeException as e ->
printfn \$"{e.NonPrime} is not prime"
printfn \$"{e}"
printfn "--------"

let domain = AppDomain.CreateDomain "Domain2"
let gen =
domain.CreateInstanceAndUnwrap(
BindingFlags.Default, null,
[| box 1000000 |], null, null)
try
let start = 100
printfn \$"{gen.GetPrimesFrom start}"
with :? NotPrimeException as e ->
printfn \$"{e.NonPrime} is not prime"
printfn \$"{e}"
printfn "--------"
``````
``````Imports System.Reflection

Module Example
Sub Main()
Dim limit As Integer = 10000000
Dim start As Integer = 1000001
Try
Dim values() As Integer = primes.GetPrimesFrom(start)
Console.WriteLine("There are {0} prime numbers from {1} to {2}",
start, limit)
Catch e As NotPrimeException
Console.WriteLine("{0} is not prime", e.NonPrime)
Console.WriteLine(e)
Console.WriteLine("--------")
End Try

Dim domain As AppDomain = AppDomain.CreateDomain("Domain2")
Dim gen As PrimeNumberGenerator = domain.CreateInstanceAndUnwrap(
GetType(Example).Assembly.FullName,
BindingFlags.Default, Nothing,
{1000000}, Nothing, Nothing)
Try
start = 100
Console.WriteLine(gen.GetPrimesFrom(start))
Catch e As NotPrimeException
Console.WriteLine("{0} is not prime", e.NonPrime)
Console.WriteLine(e)
Console.WriteLine("--------")
End Try
End Sub
End Module
' The example displays the following output:
'      1000001 is not prime
'      NotPrimeException: 1000001 is not a prime number.
'         at Example.Main()
'      --------
'      100 is not prime
'      NotPrimeException: 100 is not a prime number.
'         at Example.Main()
'      --------
``````

### Windows Runtime and .NET Framework 4.5.1

In .NET for Windows 8.x Store apps for Windows 8, some exception information is typically lost when an exception is propagated through non-.NET Framework stack frames. Starting with the .NET Framework 4.5.1 and Windows 8.1, the common language runtime continues to use the original Exception object that was thrown unless that exception was modified in a non-.NET Framework stack frame.

## Constructors

 Initializes a new instance of the Exception class. Initializes a new instance of the Exception class with serialized data. Initializes a new instance of the Exception class with a specified error message. Initializes a new instance of the Exception class with a specified error message and a reference to the inner exception that is the cause of this exception.

## Properties

 Gets a collection of key/value pairs that provide additional user-defined information about the exception. Gets or sets a link to the help file associated with this exception. Gets or sets HRESULT, a coded numerical value that is assigned to a specific exception. Gets the Exception instance that caused the current exception. Gets a message that describes the current exception. Gets or sets the name of the application or the object that causes the error. Gets a string representation of the immediate frames on the call stack. Gets the method that throws the current exception.

## Methods

 Determines whether the specified object is equal to the current object. (Inherited from Object) When overridden in a derived class, returns the Exception that is the root cause of one or more subsequent exceptions. Serves as the default hash function. (Inherited from Object) When overridden in a derived class, sets the SerializationInfo with information about the exception. Gets the runtime type of the current instance. Gets the Type of the current instance. (Inherited from Object) Creates a shallow copy of the current Object. (Inherited from Object) Creates and returns a string representation of the current exception.

## Events

 Obsolete. Occurs when an exception is serialized to create an exception state object that contains serialized data about the exception.
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