#warning
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#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.

A common use of #warning is in a conditional directive. It is also possible to generate a user-defined error with #error (C# Reference).

// preprocessor_warning.cs
// CS1030 expected
#define DEBUG
class MainClass 
{
    static void Main() 
    {
#if DEBUG
#warning DEBUG is defined
#endif
    }
}
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