Does Visual Studio ignore DefineConstants in csproj?

Question

I have a CSPROJ in which I define my own constant / compiler directive.

<PropertyGroup>
    <DefineConstants>$(DefineConstants);MY_DIRECTIVE</DefineConstants>
    <MY_DIRECTIVE>true</MY_DIRECTIVE>
</PropertyGroup>

Then I proceed to include its use somewhere in my application, like this:

#if MY_DIRECTIVE
    System.Console.WriteLine("Yes");
#else
    System.Console.WriteLine("No");
#endif

Picture:

That works fine after building and running the application, but Visual Studio has both lines ( Yes and No ) highlighted, so if I were to do something like <MY_DIRECTIVE>false</MY_DIRECTIVE> , Visual Studio should "hide" and grey out and ignore the line where it says System.Console.WriteLine("Yes"); , shouldn't it? I would expect it to, but it doesn't. Am I doing something wrong?

Answer 1

This is not going to work because the MSBuild property ( <MY_DIRECTIVE>true</MY_DIRECTIVE> ) has nothing to do with the C# preprocessor directive ( <DefineConstants>$(DefineConstants);MY_DIRECTIVE</DefineConstants> ) - these are two independent concepts.

If you want to trigger the #else branch you have to bring these concepts together, eg:

<PropertyGroup>
    <MY_DIRECTIVE>true</MY_DIRECTIVE>
    <DefineConstants Condition="'$(MY_DIRECTIVE)' == 'true'">$(DefineConstants);MY_DIRECTIVE</DefineConstants>
</PropertyGroup>