Function that builds dictionary based on lambda params
Ronnie,
You have a good starting point for the problem you are trying to solve.
To answer your concerns:

Is there a better way to work with the expressions that are passed in.
I think what you have done is fine. You are using Expression to let user specify methods/properties in type safe way. The only thing I would change is to use Expression&lt;Func&lt;T, object&gt;&gt;[] instead of Expression&lt;Func&lt;T, dynamic&gt;&gt;[] as dynamic will invoke the compiler at run time.

Can the expressions parameter be better constrained by a more appropriate type?
I don't think you can specify additional constraints at compile time but you can definitely add code to validate the expression at runtime.

Is there a better way to derive the cache key?
I think your cache key is fine. If you want to be more conservative, you can include Type.AssemblyQualifiedName. I would worry about it only if I was releasing to general public and not if I am using this class as part of a team.


With that said, here's the full re-factored code along with my notes. I hope it helps.
void Main()
{
    var ronnie = new Person();
    ronnie.Name = &quot;Ronnie&quot;;
    ronnie.Address = &quot;Greensboro, NC&quot;;
    
    var dictionary = ObjectToDictionaryConverter.GetDictionary(ronnie, x =&gt; x.Name, x =&gt; x.Address);
    
    // In following example, we are trying to pass in an expression which is not valid for our object.
    // Running this code will throw an appropriate exception like
    // Expression value(UserQuery+&lt;&gt;c__DisplayClass0).anotherDictionary.Keys is invalid. 
    // Expression Property/Member Type System.Collections.Generic.Dictionary`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]
    //, expecting Type: UserQuery+Person
    
    //var anotherDictionary = new Dictionary&lt;string, string&gt;();
    //var dictionary = ObjectToDictionaryConverter.GetDictionary(ronnie, x =&gt; anotherDictionary.Keys);    

    //I don't think you can enforce this constraint at compile time (I am not sure about it, I would love to be proved wrong.)
    
    Console.WriteLine(dictionary[&quot;Name&quot;]); //Prints Ronnie
    Console.WriteLine(dictionary[&quot;Address&quot;]); //Prints Greensboro, NC
    
}

public class Person
{
    public string Name { get; set; }
    
    public string Address { get; set; }
}

// Why would you name a class like 'DictionaryThing'? It doesn't explain anything about what the class does. 
// Let's give it a more descriptive 'ObjectToDictionaryConverter' name. You may prefer some other name but 
// make sure that you put same thought in to naming your class as the code which goes inside the class.

// Next, the original code was doing two things
// 1. Inspecting the expression to derive a key
// 2. Create the dictionary from the input object (while also catching the expression details)

// Following SRP, let's separate it out in two classes so that each class is responsible for doing one thing.
// This also means that you can use the Expression inspection code somewhere else if you choose to.
public static class ObjectToDictionaryConverter 
{
    //Always spell out the variable name in its full. (expCache -&gt; expressionCache)
    static ConcurrentDictionary&lt;string, Delegate&gt; expressionCache = new ConcurrentDictionary&lt;string, Delegate&gt;();

    public static Dictionary&lt;string, object&gt; GetDictionary&lt;T&gt;(T obj, params Expression&lt;Func&lt;T, object&gt;&gt;[] expressions)
    {
        //Always validate your arguments. 
        if(ReferenceEquals(obj, null))
            throw new ArgumentNullException(&quot;obj&quot;);
            
        if(expressions.Length == 0)
            throw new ArgumentException(&quot;You must specify at least one expression.&quot;);
            
        foreach (var expression in expressions)
        {
            if(expression == null)
                throw new ArgumentException(&quot;You can not specify NULL expression.&quot;);
        }
            
        var result = new Dictionary&lt;string, object&gt;();
        
        foreach (var expression in expressions)
        {
            //The purpose of ExpressionDetail is to inspect our expression 
            var expressionDetail = ExpressionDetail.Create(expression);

            //A lambda expression can be a valid expression referring to a property or function.
            //But for our need, we will need to compile this expression to a delegate and run on object of Type T, let's make
            //sure that expression refers to the correct type.
            
            //IMPORTANT: We should not put this check in ExpressionDetail class. It is not
            //the responsibility of ExpressionDetail to enforce this type constraint. 
            //This type constraint is only needed for ObjectToDictionaryConverter class. 
            if(expressionDetail.DeclaringType != typeof(T))
                throw new InvalidExpressionException(&quot;Expression &quot; + expression.Body + &quot; is invalid. Expression Property/Member Type &quot; + expressionDetail.DeclaringType.FullName + &quot;, expecting Type: &quot; + typeof(T).FullName);        
            
            //expressionDetail has properties like 'Name', 'FullName', 'Delegate' 
            var func = (Func&lt;T, object&gt;) expressionCache.GetOrAdd(expressionDetail.FullName, expressionDetail.Delegate);        
        
            result[expressionDetail.Name] = func(obj);
        }

        return result;
    }
}

public class ExpressionDetail
{
    private ExpressionDetail()
    {
    
    }
    
    public MemberInfo MemberInfo { get; private set; }
    
    public LambdaExpression Expression { get; private set; }
    
    //By figuring out MemberInfo from the Expression, 
    //we can now have all these read-only properties to get expression detail.
    public string Name { get { return MemberInfo.Name; } }
    
    public Type DeclaringType { get { return MemberInfo.DeclaringType; } }
    
    public string FullName { get { return DeclaringType.FullName + &quot;.&quot; + Name; } }
    
    //Depending on performance requirement, you may want to use Lazy&lt;T&gt; to calculate this value
    //only once. 
    public Delegate Delegate { get { return Expression.Compile(); } }

    //We are expecting a lambda expression which should either point to a method or a property access.
    //To get body, we have to handle the case of expression being UnaryExpression
    //To learn more: http://stackoverflow.com/questions/3567857/why-are-some-object-properties-unaryexpression-and-others-memberexpression
    private static Expression GetBody(LambdaExpression expression)
    {
        //We don't validate arguments here only because it's a private method.
    
        var unaryExpression = expression.Body as UnaryExpression;
        return unaryExpression != null ? unaryExpression.Operand : expression.Body;
    }
    
    //In your original method, you returned the name.
    //However, it could be even more useful to get the MemberInfo and store it.
    //Now we will have access to Name as well as the Type in which the property/method is declared.

    // There are lots of edge cases here. 
    // Refer to: http://stackoverflow.com/questions/671968/retrieving-property-name-from-lambda-expression and update this method to handle the edge cases that you care about.
    private static MemberInfo GetMemberInfo(Expression expression)
    {
        //We don't validate arguments here only because it's a private method.
        
        var memberExpression = expression as MemberExpression;
        
        if(memberExpression != null)
            return memberExpression.Member;
            
        var methodCallExpression = expression as MethodCallExpression;
        
        if(methodCallExpression != null)
            return methodCallExpression.Method;
            
        return null;
    }
    
    public static ExpressionDetail Create(LambdaExpression expression)
    {
        if(expression == null)
            throw new ArgumentNullException(&quot;expression&quot;);
            
        var body = GetBody(expression);
        
        var memberInfo = GetMemberInfo(body);
        
        if (memberInfo == null)
                throw new InvalidExpressionException(
                    string.Format(&quot;The expression '{0}' is invalid. You must supply an expression that references a property or a function.&quot;,
                        expression.Body));
                        
        return new ExpressionDetail { MemberInfo = memberInfo, Expression = expression};
    }

}