Mapping Excel rows to model
The indentation needs to be mentioned. Hopefully it's just an accidental tabs vs spaces issue that caused it to render this way, ...but I'm not seeing the Tab characters, so... the indentation needs to be mentioned, becauase this is not normal:


public IList&lt;Employee&gt; ReadEmployees(byte[] file, string companyId)
        {



Expected:

public IList&lt;Employee&gt; ReadEmployees(byte[] file, string companyId)
{


Just like you have here:


            using (var stream = new MemoryStream(file))
            {



And here:


                    for (var rowNumber = 3; rowNumber &lt;= worksheet.Dimension.End.Row; rowNumber++)
                    {



Now, with that cleared, the GetEmployeeRowSummary method looks much cleaner already:

private static Employee GetEmployeeRowSummary(ExcelWorksheet worksheet, int rowNumber, string companyId)

{
    var employeeId = Guid.NewGuid().ToString();
    return new Employee
    {
        CompanyId = companyId,
        EmployeeId = employeeId,
        EmployeeNumber = worksheet.Cells[rowNumber, 1].GetValue&lt;string&gt;(),
        FirstName = worksheet.Cells[rowNumber, 2].GetValue&lt;string&gt;(),
        LastName = worksheet.Cells[rowNumber, 3].GetValue&lt;string&gt;(),
        BirthDate = worksheet.Cells[rowNumber, 4].GetValue&lt;DateTime&gt;(),
        AppointmentDate = worksheet.Cells[rowNumber, 5].GetValue&lt;DateTime&gt;(),
        CompanyRunFrequencyId = 9,
        JobTitle = worksheet.Cells[rowNumber, 8].GetValue&lt;string&gt;(),
        DateCreated = DateTime.Now,
        EmployeeAddress = new List&lt;EmployeeAddress&gt;
        {
            new EmployeeAddress
            {
                EmployeeId = employeeId,
                StreetNumber = worksheet.Cells[rowNumber, 9].GetValue&lt;int&gt;(),
                Suburb = worksheet.Cells[rowNumber, 11].GetValue&lt;string&gt;(),
                City = worksheet.Cells[rowNumber, 12].GetValue&lt;string&gt;(),
                PostalCode = worksheet.Cells[rowNumber, 13].GetValue&lt;int&gt;()
            }
       }
   };
}


Consistent indentation is fundamental for clean, readable code.

But readability isn't the biggest problem here: the repeated calls to worksheet.Cells and {Range?}.GetValue are.

I'm not sure what ExcelPackage is, but it's definitely not VSTO, so maybe this isn't as much of a problem, but invoking worksheet.Cells[...].GetValue&lt;...&gt;() is spawning a RCW (Runtime Callable Wrapper - a .NET wrapper object to access a COM object) whose reference isn't captured, and thus could be leaking the object reference. If you were using VSTO, there would very likely be a "ghost" EXCEL.EXE process lingering in Task Manager forever, well after your application happily terminated.

In any case, working directly with worksheet cells is THE slowest thing you can do, whether in VBA or C# through COM interop, whether it be through VSTO or any other library.

Again I don't know what ExcelPackage is nor what this API has to offer, but in the native Excel object model Range.Value, when the Range object represents multiple cells, gives you a 2D variant array (object[,] in .NET) - if all you're ever interested in is the values that are in the cells (as seems to be the case), then you don't need to deal with worksheets and cells and .GetValue&lt;T&gt;().

So instead of looping through rows:


for (var rowNumber = 3; rowNumber &lt;= worksheet.Dimension.End.Row; rowNumber++)



Grab the values array (not sure what the syntax would be with this API - if your library doesn't allow you to grab a 2D array of values, drop that library and use VSTO):

var values = worksheet.Range(worksheet.Cells[3, 1], worksheet.Cells[lastRow, 13]).Value;


The idea is to get a Range object that encompasses all the cells you're interested in. Maybe that's everything from row 3 to worksheet.Dimension.End.Row, but even without knowing the slightest thing about this particular API I would fear that worksheet.Dimension.End.Row yields something like 1,048,576 - which is very very very unlikely the actual number of rows you need to care about.

With the Excel object model, you'd do something like sheet.Range("A" &amp; sheet.Rows.Count).End(xlUp).Row to get the last interesting row (aka lastRow), and use that row number to make the "interesting range" from which to read the values.

Now that you have a 2D array that contains all the values, GetEmployeeRowSummary doesn't make much sense anymore: we don't care for a rowNumber, and we don't need a ExcelWorksheet reference anymore.

In fact all we need is a "row slice" of the 2D array, by copying the row to a new array (this code looks like it):

public T[] GetRow(T[,] matrix, int rowNumber)
{
    return Enumerable.Range(0, matrix.GetLength(1))
            .Select(x =&gt; matrix[rowNumber, x])
            .ToArray();
}

private static Employee ReadEmployee(object[] row, string companyId)
{
    var id = Guid.NewGuid().ToString();
    return new Employee
    {
        CompanyId = companyId,
        EmployeeId = id,
        EmployeeNumber = (string)row[1],
        FirstName = (string)row[2],
        LastName = (string)row[3],
        BirthDate = (DateTime)row[4],
        //...
    };
}


As for mapping the properties automatically, I'm not a fan. Something, somewhere needs to do the mapping, and I'd much rather it be explicit in the code. Automatic mapping would need more metadata than just column indices - you'd likely need column names and data types, so instead of copying the cells to an array you'd be copying the cells to some DataTable, or have the mapper understand what an Excel Range or ListObject is... all of which sound like quite a lot of effort for an error-prone way to just list the properties at the left of an = operator. If your explicit casts blow up, you'll know why. If the automagic mapping blows up, pray the library throws meaningful exceptions.