VB -> C#: Converter ignores the three-valued logic for nullable types and relational operators.

VB.Net input code

Public Sub Main()
    Dim x As Integer? = Nothing
    If x <> 10 Then
        Console.WriteLine("Fail!")
    Else
        Console.WriteLine("Ok!")
    End If
End Sub

This will print OK because: x <> 10 results in Nothing with Boolean? type. Nothing is False so code will execute Else statement.

Erroneous output

public static void Main()
{
    int? x = default;
    if (x != 10 == true) {
        Console.WriteLine("Fail!");
    }
    else {
        Console.WriteLine("Ok!");
    }
}

Here the code will incorrectly print Fail because: x != 10 results in true and true is equal to true.

Expected output

  public static void Main()
  {
      int? x = default;
      if ((x == null ? null : x != 10) == true) {
          Console.WriteLine("Fail!");
      }
      else {
          Console.WriteLine("Ok!");
      }
  }

VB uses three-valued logic for nullable types. Any relational comparison (=, <>, >, >=, <, <=) that involves nullable types results in Boolean?. If lhs or rhs is null then the entire expression will be null. The expected output produces ugly code but I’m not sure if it’s possible to maintain the same logic using something simpler as we need to keep bool? as the result of the comparison. I will take a look if there is something in Microsoft.VisualBasic.dll that could simplify the code.

Comparing Nullable Types

Details

• Product in use: VS extension
• Version in use: https://github.com/icsharpcode/CodeConverter/commit/b5e798bb4baf03d84aeb71e55f000545f5d69938
• Did you see it working in a previous version, which?
• Any other relevant information to the issue, or your interest in contributing a fix.

[GrahamTheCoder]

Yep it definitely needs a bunch more tests than it has! I think in the general case you’d need to expand to x.HasValue ? x != 10 : null for the operator (with some consideration for whether x itself is an expression that shouldn’t be reevaluated). Then within the if statement, add the == true like now. Obviously for a single operator condition you can reduce that down.

For the re-evaluatable expression part, there is some code that can help extract temp variables, but you might be better off trying to use some syntax that introduces a variable in scope along the lines of x is {} xVal ? xVal != 10 : null

So for this case you’d end up with x is {} xVal && xVal != 10 I think…which still isn’t exactly pretty

Note: Because the decompilation doesn’t show a call to the VB library I assumed (there I go again assuming) that there wasn’t one, but there may well be.

Is the problem here only with the <> operator, or other things too?

[Yozer]

Dim a As Boolean?
Dim b As Boolean?
Dim x = Operators.CompareObjectEqual(a, b, False) ' True 
Dim y = Operators.ConditionalCompareObjectEqual(a, b, False) ' True
Dim z = a = b 'Nothing

Nullables are value types so VB will box them before calling the methods: Sharplab.io