C#'是'运营商澄清

Question

Does the is operator indicate whether or not an object is an instance of a certain class, or only if it can be casted to that class?

Assume I have a DbCommand called command that has actually has been initialized as a SqlCommand . What is the result of command is OracleCommand ?

( SqlCommand and OracleCommand both inherit from DbCommand )

Answer 1

It checks if the object is a member of that type, or a type that inherits from or implements the base type or interface. In a way, it does check if the object can be cast to said type.

command is OracleCommand returns false as it's an SqlCommand , not an OracleCommand . However, both command is SqlCommand and command is DbCommand will return true as it is a member of both of those types and can therefore be downcast or upcast to either respectively.

If you have three levels of inheritance, eg BaseClass , SubClass and SubSubClass , an object initialized as new SubClass() only returns true for is BaseClass and is SubClass . Although SubSubClass derives from both of these, the object itself is not an instance of it, so is SubSubClass returns false.

Answer 2

An is expression evaluates to true if the provided expression is non-null, and the provided object can be cast to the provided type without causing an exception to be thrown.

Source

Answer 3

From MSDN :

An is expression evaluates to true if [...] expression can be cast to type

Answer 4

http://msdn.microsoft.com/en-us/library/scekt9xw%28v=vs.80%29.aspx

An is expression evaluates to true if the provided expression is non-null, and the provided object can be cast to the provided type without causing an exception to be thrown.

Answer 5

is indicate if the object can be casted to a class or interface.

If you have a BaseClass and a SubClass then:

var obj = new SubClass();

obj is SubClass returns true;

obj is BaseClass also returns true;

Answer 6

if(something is X) checks if the underlying type of something is X . This is significantly different from checking if a type supports casting to X since many types can support casts to X without being of type X .

Conversely the as operator attempts a conversion to a particular type and assigns null if source type is not within the inheritance chain of the target type.