The ‘if’ preprocessor directive for the compiler in C# .NET

You can decorate your C# source code with “messages” to the compiler. There are a couple of predefined preprocessors that the compiler understands.

A common scenario is when you’d like to run some part of your code in Debug mode but not in Release mode or any other Build type. The following method shows the ‘if’ and ‘elif’ preprocessors:

private static void TryPreprocessors()
{
# if DEBUG
	Console.WriteLine("You are running the Debug build");
# elif RELEASE
	Console.WriteLine("You are running the Release build");
#else
	Console.WriteLine("This is some other build.");
# endif
}

If we’re running this code with the Debug build type then the Debug section will be executed. Note the gray colour code in Visual Studio for the currently unavailable paths:

Now switch the Build type to Release. We’re expecting the RELEASE block to be highlighted. However, it’s the else-if block that is now the valid execution path.

Open the Properties window of the project and click the Build tab:

You’ll notice that DEBUG and TRACE can be selected in their respective boxes but RELEASE is missing. Enter RELEASE in the symbols textbox:

If you then go back to our code then the RELEASE section should be highlighted:

You can now switch between Debug and Release builds and the various execution paths will be automatically updated.

View all various C# language feature related posts here.