Export (0) Print
Expand All

7.9.10 The as operator

Visual Studio .NET 2003

The as operator is used to explicitly convert a value to a given reference type using a reference conversion or a boxing conversion. Unlike a cast expression (Section 7.6.6), 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. The type of the result is T, and the result is always classified as a value. The operation is evaluated as follows:

  • If the compile-time type of e is the same as T, the result is simply the value of e.
  • Otherwise, if an implicit reference conversion (Section 6.1.4) or boxing conversion (Section 6.1.5) exists from the compile-time type of e to T, this conversion is performed and becomes the result of the operation.
  • Otherwise, if an explicit reference conversion (Section 6.2.3) exists from the compile-time type of e to T, a dynamic type check is performed:
    • If the value of e is null, the result is the value null with the compile-time type T.
    • Otherwise, let R be the run-time type of the instance referenced by e. If R and T are the same type, if R is a reference type and an implicit reference conversion from R to T exists, or if R is a value type and T is an interface type that is implemented by R, the result is the reference given by e with the compile-time type T.
    • Otherwise, the result is the value null with the compile-time type T.
  • Otherwise, the indicated conversion is never possible, and a compile-time error occurs.

Note that the as operator only performs reference conversions and boxing conversions. Other conversions, such as user defined conversions, are not possible with the as operator and should instead be performed using cast expressions.

Show:
© 2014 Microsoft