IsDebuggingEnabled, #if DEBUG, Conditional("DEBUG") and Debugger.IsAttached

HttpContext.IsDebuggingEnabled checks the <compilation debug="..."/> value inside <system.web> node in web.config.

if (HttpContext.Current.IsDebuggingEnabled)
{
    // if debug mode is enabled in web.config
    . . .
}

#if DEBUG or #if RELEASE checks for particular build configuration. Whether a build configuration uses DEBUG or RELEASE, this is defined in the project build properties (right click the project -> Properties -> Build tab).
The drawback with this is if there is a property or method declared outside being renamed (through refactoring) then the property or method called inside one of its if ... else ... conditions might not be renamed as well causing error when the build configuration is switched.

#if DEBUG
. . .
#else
. . .
#endif

The Conditional attribute helps to overcome this issue. An example:

[System.Diagnostics.Conditional("DEBUG")]
public void MyMethod()
{ 
   . . . 
}

System.Diagnostics.Debugger.IsAttached checks whether an active debugger is attached to the system.

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