Deprecation behavior using the [Obsolete] attribute

Question

While removing some obsolete code I came across an unexpected scenario, recreated below:

class Program {     static void Main(string[] args)     {         ViableMethod();         Console.WriteLine("");             SoftDeprecatedMethod();//Compiler warning          //HardDeprecatedMethod();//Can't call that from here, compiler error          Console.ReadKey(true);     }      public static void ViableMethod ()     {         Console.WriteLine("ViableMethod, calls SoftDeprecatedMethod");         SoftDeprecatedMethod();//Compiler warning         //HardDeprecatedMethod();//Can't call that from here, compiler error     }      [Obsolete("soft", false)]     public static void SoftDeprecatedMethod()     {         Console.WriteLine("SoftDeprecatedMethod, calls HardDeprecatedMethod");         HardDeprecatedMethod();     }      [Obsolete("hard", true)]     public static void HardDeprecatedMethod()     {         Console.WriteLine("HardDeprecatedMethod");     } } 

Based in the output it seems that functions deprecated with a warning are permitted to call functions deprecated with an error and the code will execute.

My expectation was that I would see a compiler error complaining that the call to HardDeprecatedMethod() from SoftDeprecatedMethod() is not permitted. The observed behavior seems odd to me.

Does anyone know if this is the desired behavior (and if so, why), or could this be a flaw in the implementation of the [Obsolete] attribute?

Answer 1

In facts it's the other way around: It shows that C# compiler is smart and very clear on the use of methods marked with Obsolete.

Assume that you are providing this code as a public API in a class library to Bob.

1. You expect if Bob calls HardDeprecatedMethod in his code, he should get a compile time error; and he will.

2. You expect if Bob has called SoftDeprecatedMethod anywhere, from this time forward, he should be warned about that, BUT his code should still work; and it will.

So you get what exactly you want!