Question

Simple question, how do you force the C# compiler to throw a compilation error.

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Update: Perhaps it’s better to use an Assert.Fail() instead?

I have a custom-attribute that should only be applied to ONE member of a class. Inside of my other class’ static method it looks for that one member and I want it to fail (not throw an exception) if more than one member had the attribute applied to it.

public class Foo
{
    [MyCustomAttribute]
    public String FooString { get; set; }

    [MyCustomAttribute]
    public String OtherFooString { get; set; }
}


public class Bar<T>
    where T : class, new()
{
    static Bar()
    {
         //If more than one member of type Foo has MyCustomAttribute
         //applied to it compile error or Assert.Fail()?
    }
}

Answer 1

You can use a diagnostic directive:

#error Oops. This is an error.

or for just a warning:

#warning This is just a warning.

You’d normally want to put these in conditional blocks, I’d expect…

EDIT: Okay, now you’ve updated your question, you simply can’t do this at compile-time. Your suggestion of using Assert.Fail punts the problem to execution time.

I would suggest you write unit tests to detect this (iterate over all the types in the assembly, and check that the attribute has only been applied at most once per type).

EDIT: In 2016… while Code Analysis as suggested by the OP isn’t actually a compiler error, now that Visual Studio uses Roslyn, it’s feasible to hook into the compiler and genuinely get an error from the compiler, using a Roslyn code analyzer. However, I would still personally prefer unit tests for this, as then the code could be built and tested by anyone, regardless of whether they had the Roslyn analyzer installed. There’s still no way of validating this with a “purely vanilla” C# compiler.