How does C# compiler remove Debug.Assert's in release builds?

Question

I was recently going through some code and considering whether I need to be careful with the expressions placed inside Debug.Assert statements, such as expensive op

Answer 1

The methods on the debugger use the pseudo-custom attribute, ConditionalAttribute, which the compiler detects and removes any calls to any methods with that attribute unless the specified compilation symbol (in this case, DEBUG) is defined. Anyone may use the attribute on void methods without any out parameters.

Answer 2

Debug.Assert is declared with ConditionalAttribute; as the documentation states, this "[i]ndicates to compilers that a method call or attribute should be ignored unless a specified conditional compilation symbol is defined."

The C# compiler has specific support for that attribute and removes the Debug.Assert during release builds, so it is never part of the built expression tree.

If you right-click on one of your Debug.Assert statements, you should be able to go to the definition. Visual Studio will show you "code" generated from the metadata, and there you can see the [Conditional("DEBUG")] attribute applied. So this code is only respected when DEBUG is #define'd as part of your build.

Answer 3

I don't believe that Debug.Assert is special in any way; it's just using the Conditional attribute so that the compiler removes it when it detects that the 'preprocessor' define does not exist (C# does not have a preprocessor!).

You can use it like so to do the same thing (so long as you've defined DEBUG (or whatever symbol you want to switch on, TRACE is another popular one):

[Conditional("DEBUG"), Conditional("TRACE")]
public void DebugOnlyMethod() {
    Console.WriteLine("Won't see me unless DEBUG or TRACE is defined");
}