VB -> C#: WithEvents replacement causes NullReferenceException

VB.Net input code

Imports System

Public Module Program
    Public Sub Main(args As String())
        Dim c As New SomeClass(New SomeDependency())
        Console.WriteLine("Done")
    End Sub
End Module

Public Class SomeDependency
    Public Event SomeEvent As EventHandler
End Class

Public Class SomeClass
    Private WithEvents _dep As SomeDependency

    Public Sub New(dep)
        _dep = dep
    End Sub

    Private Sub _dep_SomeEvent(sender As Object, e As EventArgs) Handles _dep.SomeEvent
        ' Do Something
    End Sub
End Class

Erroneous output

using System;
using System.Runtime.CompilerServices;

public static partial class Program
{
    public static void Main(string[] args)
    {
        var c = new SomeClass(new SomeDependency());
        Console.WriteLine("Done");
    }
}

public partial class SomeDependency
{
    public event EventHandler SomeEvent;
}

public partial class SomeClass
{
    private SomeDependency __dep;

    private SomeDependency _dep
    {
        [MethodImpl(MethodImplOptions.Synchronized)]
        get
        {
            return __dep;
        }

        [MethodImpl(MethodImplOptions.Synchronized)]
        set
        {
            if (__dep != null)
            {
                __dep.SomeEvent -= _dep_SomeEvent;
            }

            __dep = value;
            if (__dep != null)
            {
                __dep.SomeEvent += _dep_SomeEvent;
            }
        }
    }

    public SomeClass(object dep)
    {
        _dep.SomeEvent += _dep_SomeEvent;
        _dep = (SomeDependency)dep;
    }

    private void _dep_SomeEvent(object sender, EventArgs e)
    {
        // Do Something
    }
}

The VB code prints “Done”, whereas the C# code throws a NullReferenceException in the first line of the constructor of SomeClass, since _dep has not been initialized yet.

Expected output

The SomeClass constructor should not contain the line _dep.SomeEvent += _dep_SomeEvent;. This line only makes sense if _dep has an initializer.

Details

• Product in use: icsharpcode.github.io/CodeConverter
• Version in use: 9.1.3.0

[jrmoreno1]

FWIW I recently saw the same problem for an asp.net application.

[GrahamTheCoder]

Busy few months but finally got back to looking at this. I think I’ve moved it a step in the right direction in the version in master right now. I’ll wait a little before releasing in case you or anyone else spots an issue with it since there are quite a lot of situations to consider