C# if/then directives for debug vs release

DEBUG/_DEBUG should be defined in VS already.

Remove the #define DEBUG in your code. Set preprocessors in the build configuration for that specific build.

The reason it prints "Mode=Debug" is because of your #define and then skips the elif.

The right way to check is:

#if DEBUG
    Console.WriteLine("Mode=Debug"); 
#else
    Console.WriteLine("Mode=Release"); 
#endif

Don't check for RELEASE.

By default, Visual Studio defines DEBUG if project is compiled in Debug mode and doesn't define it if it's in Release mode. RELEASE is not defined in Release mode by default. Use something like this:

#if DEBUG
  // debug stuff goes here
#else
  // release stuff goes here
#endif

If you want to do something only in release mode:

#if !DEBUG
  // release...
#endif

Also, it's worth pointing out that you can use [Conditional("DEBUG")] attribute on methods that return void to have them only executed if a certain symbol is defined. The compiler would remove all calls to those methods if the symbol is not defined:

[Conditional("DEBUG")]
void PrintLog() {
    Console.WriteLine("Debug info");
}

void Test() {
    PrintLog();
}

I prefer checking it like this over looking for #define directives:

if (System.Diagnostics.Debugger.IsAttached)
{
   //...
}
else
{
   //...
}

With the caveat that of course you could compile and deploy something in debug mode but still not have the debugger attached.

If you are trying to use the variable defined for the build type you should remove the two lines ...

#define DEBUG  
#define RELEASE 

... these will cause the #if (DEBUG) to always be true.

Also there isn't a default Conditional compilation symbol for RELEASE. If you want to define one go to the project properties, click on the Build tab and then add RELEASE to the Conditional compilation symbols text box under the General heading.

The other option would be to do this...

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

Be sure to define the DEBUG constant in the Project Build Properties. This will enable the #if DEBUG. I don't see a pre-defined RELEASE constant, so that could imply that anything Not in a DEBUG block is RELEASE mode.

Remove your defines at the top

#if DEBUG
        Console.WriteLine("Mode=Debug"); 
#else
        Console.WriteLine("Mode=Release"); 
#endif

It is worth noting here that one of the most significant differences between conditionally executing code based on #if DEBUG versus if(System.Diagnostics.Debugger.IsAttached) is that the compiler directive changes the code that is compiled. That is, if you have two different statements in an #if DEBUG/#else/#endif conditional block, only one of them will appear in the compiled code. This is an important distinction because it allows you do do things such as conditionally compile method definitions to be public void mymethod() versus internal void mymethod() depending on build type so that you can, for example, run unit tests on debug builds that will not break access control on production builds, or conditionally compile helper functions in debug builds that will not appear in the final code if they would violate security in some way should they escape into the wild. The IsAttached property, on the other hand, does not affect the compiled code. Both sets of code are in all of the builds - the IsAttached condition will only affect what is executed. This by itself can present a security issue.

I got to thinking about a better way. It dawned on me that #if blocks are effectively comments in other configurations (assuming DEBUG or RELEASE; but true with any symbol)

public class Mytest
    {
        public DateTime DateAndTimeOfTransaction;
    }

    public void ProcessCommand(Mytest Command)
        {
            CheckMyCommandPreconditions(Command);
            // do more stuff with Command...
        }

        [Conditional("DEBUG")]
        private static void CheckMyCommandPreconditions(Mytest Command)
        {
            if (Command.DateAndTimeOfTransaction > DateTime.Now)
                throw new InvalidOperationException("DateTime expected to be in the past");
        }

Remove the definitions and check if the conditional is on debug mode. You do not need to check if the directive is on release mode.

Something like this:

#if DEBUG
     Console.WriteLine("Mode=Debug"); 
#else
    Console.WriteLine("Mode=Release"); 
#endif