19.2 Conditional Compilation

The presence or absence of the DEBUG and TRACE symbols tell the compiler how to handle calls to methods in the Debug and Trace classes. If the DEBUG or TRACE symbols are not defined, many of the method calls into the Debug and Trace classes are optimized away completely. This can be a desirable feature for your own types, and there are a number of ways to accomplish this with the .NET Framework and the C# language.

One option is to bracket the calls with #if/#endif preprocessor directives, as follows:

void DoWork( ) {
#if METHODCALL
  Debug.WriteLine("MethodCall", "Entering MyClass::DoWork");
#endif
  Console.WriteLine("Working...");
}

Compiling this function with /d:METHODCALL,DEBUG results in a call to Debug.WriteLine and a log of the method entry. The downside to this approach is using preprocessor directives such as this is arduous and error-prone, and the code quickly becomes unreadable.

An alternative is to use the ConditionalAttribute custom attribute. When placed on a method, this attribute instructs the C# compiler to conditionally omit calls to the method.

Using this approach we can define a reflection-driven logger, as follows:

public class Logger {
  [Conditional("METHODCALL")]
  public static void LogMethodCall( ) {
    StackFrame sf = new StackFrame(1); // Get preceding stack frame
    MethodBase mb = sf.GetMethod( ); // Get refl. info for method
    Type t = mb.DeclaringType; // Get refl. info for type
    string s = String.Format("Entering {0}::{1}", t.Name, mb.Name);
    Debug.WriteLine("MethodCall", s); // Dump methodcall
  }
}

Client code can simply call this method on method entry, as follows:

void DoWork( ) {
  Logger.LogMethodCall( );
  Console.WriteLine("Working...");
}

When client code is compiled with METHODCALL defined, the C# compiler emits calls to the LogMethodCall function. If METHODCALL is not defined, the compiler omits the calls entirely.