C# Language Specification

The as operator

The as operator is used to explicitly convert a value to a given reference type or nullable type. Unlike a cast expression (§7.7.6), the as operator never throws an exception. Instead, if the indicated conversion is not possible, the resulting value is null.

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.
  

If the compile-time type of E is not dynamic, the operation E as T produces the same result as

E is T ? (T)(E) : (T)null

except that E is only evaluated once. The compiler can be expected to optimize E as T to perform at most one dynamic type check as opposed to the two dynamic type checks implied by the expansion above.

If the compile-time type of E is dynamic, unlike the cast operator the as operator is not dynamically bound (§7.2.2). Therefore the expansion in this case is:

E is T ? (T)(object)(E) : (T)null

Note that some conversions, such as user defined conversions, are not possible with the as operator and should instead be performed using cast expressions.

In the example

class X
{

public string F(object o) {
return o as string; // OK, string is a reference type
}

public T G(object o) where T: Attribute {

return o as T; // Ok, T has a class constraint
}

public U H(object o) {

return o as U; // Error, U is unconstrained
}
}

the type parameter T of G is known to be a reference type, because it has the class constraint. The type parameter U of H is not however; hence the use of the as operator in H is disallowed.

11.