Question

What’s the best way to handle this situation:

I have this generic function

Function FieldValue(Of T)(row As DataRow,fieldName As String) As T
  Return If(row.IsNull(fieldName),Nothing,CType(row(fieldName),T))
End Function

in the special case where the field value is null and T is String, I want to return String.Empty instead of Nothing.

Answer 1

I think the easiest way is to provide a specific function for T = string:

Function FieldValue(row As DataRow ,fieldName As String) As String
  Return If(row.IsNull(fieldName), String.Empty, CType(row(fieldName), String))
End Function

(on top of your existing generic function)

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Here’s another option:

Function FieldValue(Of T)(row As DataRow,fieldName As String) As T
    If GetType(T) = GetType(String) And row.IsNull(fieldName) Then
        Return CType(CType(String.Empty, Object), T)
    Else
        Return If(row.IsNull(fieldName), Nothing, CType(row(fieldName), T))
    End If
End Function