Conflicting compile time behaviour using as keyword against generic types in C#

问题

When attempting to use the C# "as" keyword against a non-generic type that cannot be cast to, the compiler gives an error that the type cannot be converted.

However when using the "as" keyword against a generic type the compiler gives no error:

public class Foo { }

public class Bar<T> { }

public class Usage<T> {
   public void Test() {
      EventArgs args = new EventArgs();
      var foo = args as Foo;     // Compiler Error: cannot convert type
      var bar = args as Bar<T>;  // No compiler error
   }
}

I discovered this behaviour in a much larger code base where the lack of a compile time error led to an issue at runtime.

Is the conflicting behaviour by design? If so, does anyone have any insight as to why?

回答1:

In §7.10.11 The as operator C# 5.0 Specification says:

In an operation of the form E as T, E must be an expression and T must be a reference type, a type parameter known to be a reference type, or a nullable type. Furthermore, at least one of the following must be true, or otherwise a compile-time error occurs:

An identity (§6.1.1), implicit nullable (§6.1.4), implicit reference (§6.1.6), boxing (§6.1.7), explicit nullable (§6.2.3), explicit reference (§6.2.4), or unboxing (§6.2.5) conversion exists from E to T.

The type of E or T is an open type.

E is the null literal.

So args as Foo gives an error because none of this is true. But in the second case, Bar<T> is an open type, and the spec explains open type as, §4.4.2 Open and closed types :

An open type is a type that involves type parameters. More specifically:

A type parameter defines an open type. [...]

回答2:

"Note that the as operator performs only reference conversions, nullable conversions, and boxing conversions. The as operator can't perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions."

https://msdn.microsoft.com/en-us/library/cscsdfbt.aspx

来源：https://stackoverflow.com/questions/28149927/conflicting-compile-time-behaviour-using-as-keyword-against-generic-types-in-c-s

标签

generics