#warning (C# Reference)
#warning lets you generate a level one warning from a specific location in your code. For example:
#warning Deprecated code in this method.
Remarks
A common use of #warning is in a conditional directive. It is also possible to generate a user-defined error with #error (C# Reference).
Example
// preprocessor_warning.cs
// CS1030 expected
#define DEBUG
class MainClass
{
static void Main()
{
#if DEBUG
#warning DEBUG is defined
#endif
}
}