Conditional operator assignment with Nullable<value> types?

The problem occurs because the conditional operator doesn't look at how the value is used (assigned in this case) to determine the type of the expression -- just the true/false values. In this case, you have a null and an Int32, and the type can not be determined (there are real reasons it can't just assume Nullable<Int32>).

If you really want to use it in this way, you must cast one of the values to Nullable<Int32> yourself, so C# can resolve the type:

EmployeeNumber =
    string.IsNullOrEmpty(employeeNumberTextBox.Text)
    ? (int?)null
    : Convert.ToInt32(employeeNumberTextBox.Text),

or

EmployeeNumber =
    string.IsNullOrEmpty(employeeNumberTextBox.Text)
    ? null
    : (int?)Convert.ToInt32(employeeNumberTextBox.Text),

I think a utility method could help make this cleaner.

public static class Convert
{
    public static T? To<T>(string value, Converter<string, T> converter) where T: struct
    {
        return string.IsNullOrEmpty(value) ? null : (T?)converter(value);
    }
}

then

EmployeeNumber = Convert.To<int>(employeeNumberTextBox.Text, Int32.Parse);

While Alex provides the correct and proximal answer to your question, I prefer to use TryParse:

int value;
int? EmployeeNumber = int.TryParse(employeeNumberTextBox.Text, out value)
    ? (int?)value
    : null;

It's safer and takes care of cases of invalid input as well as your empty string scenario. Otherwise if the user inputs something like 1b they will be presented with an error page with the unhandled exception caused in Convert.ToInt32(string).

You can cast the output of Convert:

EmployeeNumber = string.IsNullOrEmpty(employeeNumberTextBox.Text)
   ? null
   : (int?)Convert.ToInt32(employeeNumberTextBox.Text)

//Some operation to populate Posid.I am not interested in zero or null
int? Posid = SvcClient.GetHolidayCount(xDateFrom.Value.Date,xDateTo.Value.Date).Response;
var x1 = (Posid.HasValue && Posid.Value > 0) ? (int?)Posid.Value : null;

EDIT: Brief explanation of above, I was trying to get the value of Posid (if its nonnull int and having value greater than 0) in varibale X1. I had to use (int?) on Posid.Value to get the conditional operator not throwing any compilation error. Just a FYI GetHolidayCount is a WCF method that could give null or any number. Hope that helps

As of C# 9.0, this will finally be possible:

Target typed ?? and ?: Sometimes conditional ?? and ?: expressions don’t have an obvious shared type between the branches. Such cases fail today, but C# 9.0 will allow them if there’s a target type that both branches convert to: Person person = student ?? customer; // Shared base type int? result = b ? 0 : null; // nullable value type

That means the code block in the question will also compile without errors.

EmployeeNumber =
string.IsNullOrEmpty(employeeNumberTextBox.Text)
    ? null
    : Convert.ToInt32(employeeNumberTextBox.Text),