c# - Using [NotNull] for a method's parameters

Question

Consider this code from ASP.NET MVC's source code:

public static IApplicationBuilder UseMvc(
            [NotNull] this IApplicationBuilder app,
            [NotNull] Action<IRouteBuilder> configureRoutes) {...}

According to this answer, the annotated parameters must not be null. Then why can I pass null to the method? That is, in the case below, why doesn't the compiler give me any error?

app.UseMvc(null);

question from:https://stackoverflow.com/questions/32806369/using-notnull-for-a-methods-parameters

Answer 1

The only attribute that can cause the compiler to generate an error is the ObsoleteAttribute. It is because this attribute's behavior is hard-coded into the compiler.

Attributes like the NotNull attribute are generally meant for tools (like ReSharper) to generate warnings or errors while writing code. Please read about this particular attribute here.

You can also use tools like PostSharp to issue additional build-time errors.