VB.NET Preprocessor Directives

Question

Why doesn't #IF Not DEBUG work the way I'd expect in VB.NET?

#If DEBUG Then
   Console.WriteLine("Debug")
#End If

#If Not DEBUG Then
   Console.WriteLine("Not Debug")
#End If

#If DEBUG = False Then
   Console.WriteLine("Not Debug")
#End If
' Outputs: Debug, Not Debug

But, a manually set const does:

#Const D = True
#If D Then
   Console.WriteLine("D")
#End If

#If Not D Then
   Console.WriteLine("Not D")
#End If
' Outputs: D

And, of course, C# has the expected behavior as well:

#if DEBUG
    Console.WriteLine("Debug");
#endif

#if !DEBUG
    Console.WriteLine("Not Debug");
#endif
// Outputs: Debug

Solution

Turns out, it's not all of VB.NET that's broken - just the CodeDomProvider (which both ASP.NET and Snippet Compiler use).

Given a simple source file:

Imports System
Public Module Module1
    Sub Main()
       #If DEBUG Then
          Console.WriteLine("Debug!")
       #End If

       #If Not DEBUG Then
          Console.WriteLine("Not Debug!")
       #End If
    End Sub
End Module

Compiling with vbc.exe version 9.0.30729.1 (.NET FX 3.5):

> vbc.exe default.vb /out:out.exe
> out.exe
  Not Debug!

That makes sense...I didn't define DEBUG, so it shows "Not Debug!".

> vbc.exe default.vb /out:out.exe /debug:full
> out.exe
  Not Debug!

And, using CodeDomProvider:

Using p = CodeDomProvider.CreateProvider("VisualBasic")
   Dim params As New CompilerParameters() With { _
      .GenerateExecutable = True, _
      .OutputAssembly = "out.exe" _
   }
   p.CompileAssemblyFromFile(params, "Default.vb")
End Using

> out.exe
Not Debug!

Okay, again - that makes sense. I didn't define DEBUG, so it shows "Not Debug". But, what if I include debug symbols?

Using p = CodeDomProvider.CreateProvider("VisualBasic")
   Dim params As New CompilerParameters() With { _
      .IncludeDebugInformation = True, _
      .GenerateExecutable = True, _
      .OutputAssembly = "C:\Users\brackett\Desktop\out.exe" _
   }
   p.CompileAssemblyFromFile(params, "Default.vb")
End Using

> out.exe
Debug!
Not Debug!

Hmm...I didn't define DEBUG, but maybe it defined it for me? But if it did, it must have defined it as "1" - because I can't get that behavior with any other value. ASP.NET, using the CodeDomProvider, must define it the same way.

Looks like the CodeDomProvider is tripping over VB.NET's stupid psuedo-logical operators.

Moral of the story? #If Not is not a good idea for VB.NET.

---

And now that source is available, I can verify that it does actually set it equal to 1 as I expected:

if (options.IncludeDebugInformation) {
      sb.Append("/D:DEBUG=1 ");
      sb.Append("/debug+ ");
}