VB -> C#: `Select Case [object]` can have incorrect converted code

[TymurGubayev]

VB.Net input code

Option Strict Off

Public Class TestSelectObjectCaseInt
    Enum E
        A
        B
        C
    End Enum
    Shared Sub Test()
        Dim o As Object
        Dim j As Integer

        o = E.C
        Select Case o
            Case 1
                j = 1
            Case 2
                j = 2
            Case Else
                j = -1
        End Select
        System.Diagnostics.Debug.Assert(j = 2)
    End Sub
End Class

Erroneous output

public class TestSelectObjectCaseInt
{
    public enum E
    {
        A,
        B,
        C
    }
    public static void Test()
    {
        object o;
        int j;

        o = E.C;
        switch (o)
        {
            case 1:
                {
                    j = 1;
                    break;
                }

            case 2:
                {
                    j = 2;
                    break;
                }

            default:
                {
                    j = -1;
                    break;
                }
        }
        System.Diagnostics.Debug.Assert(j == 2);
    }
}

Expected output

public class TestSelectObjectCaseInt
{
    public enum E
    {
        A,
        B,
        C
    }
    public static void Test()
    {
        object o;
        int j;

        o = E.C;
        switch (o is int ? (int)o : o)
        {
            case 1:
                {
                    j = 1;
                    break;
                }

            case 2:
                {
                    j = 2;
                    break;
                }

            default:
                {
                    j = -1;
                    break;
                }
        }
        System.Diagnostics.Debug.Assert(j == 2);
    }
}

Note with Option Strict On the VB.NET example doesn't compile. The issue is, in C# default branch is executed. Cast to int (=type of the cases) fixes it.

Changing the expression into (int)o can result in runtime errors in case the object can't be converted to an int, which would be better than executing wrong branch without errors. On the other hand, something like (o is int ? (int)o : o) would work correctly (~I think~most of the time*), but would require introducing temporary variable (in case o is a property, an invocation, or an index access - do I have them all?)

*) o = "2" is same as 2, which is a problem. The code decompiled to C# uses Operators.ConditionalCompareObjectEqual(o, 1, false)

Details

• Product in use: VS extension
• Version in use: 5143c08297ae65cb7490ae768645f61131bf96be
• Did you see it working in a previous version, which? no
• Any other relevant information to the issue, or your interest in contributing a fix.