Set VB.NET variable to C#'s null

Question

According to the MSDN entry for Nothing (Visual Basic)

Nothing represents the default value of a data type.

It has also been noted by some that "...the Nothing keyword is actually equivalent to C#’s default(T) keyword".

This has been giving me some anomalous behaviour in a multi-language solution I have been working on recently. Specifically I have been getting with more than a few TargetInvocationExceptions being thrown at the C# end when the VB.NET async methods return Nothing.

Is it possible to set a variable in the VB.NET projects to C#'s null and be able to test for this null value in both C# and VB.NET.

---

Here is a snippet that is not behaving as expected. The C# project imports the VB.NET project as a reference.

VB.NET Side

Public Function DoSomething() As Task(Of Object)
    Dim tcs = New TaskCompletionSource(Of Object)
    Dim params = Tuple.Create("parameters", tcs)

    AnotherMethod(params)

    Return tcs.Task
End Function

Public Sub AnotherMethod(params As Tuple(Of String, TaskCompletionSource(Of Object))
    ' do some activities
    If result = "Success" Then
        params.Item2.SetResult("we were successful") ' result can also be of different type
    Else
        params.Item2.SetResult(Nothing)  ' could this be the source of the exception?
    End If
End Sub

C# Side

public async void AwaitSomething1()
{
    var result = "";
    result = (await DoSomething()).ToString(); // fails if Result is Nothing
}

public async void AwaitSomething2()
{
    var result = "";
    result = (string)(await DoSomething());    // fails if Result is Nothing
}

public async void AwaitSomething3()
{
    var task = DoSomething();
    await task;                                // also fails if Result is Nothing
}

There is no exception thrown when VB.NET's AnotherMethod is successful. However, when it is not successful and tcs's result is set to Nothing, everything falls on its head.

How can I effectively SetResult to Nothing without resulting in an exception or, otherwise, how can I SetResult to C#'s null?

Answer 1

This is not because of conversion from Nothing to null. Here i have some example for you where C# accepts Nothing as null:

Vb Class Library Code:

Public Class ClassVb
    Public Function Dosomething() As Task(Of Object)
        Return Nothing
    End Function
End Class

C# that Invoke This Class Library :

using vbclassLib;
  class Program
    {
     static void Main(string[] args)
        {
            ClassVb classLibObj = new ClassVb();
            var result = classLibObj.Dosomething();//result=null
        }
    } 

Which works fine and gives result=null, ie., Nothing is converted as null

Let me come to your scenario:

In your scenario, when the function returns Nothing its definitely converted to null but .ToString() method or await() is not capable of handling null that's the reason for the exception you are getting.

• null.ToString() or (null).ToString() says that The operator '.' cannot be applied to operand of type '<null>'.

• await(null) will not be allowed by c#, it says cannot await null.

This may help you:

ClassVb classLibObj = new ClassVb();
var temp = classLibObj.Dosomething();
var result = temp == null ? "" : temp.ToString();