Roslyn: 'is' and 'as' operators can guarantee each other's result

[SherifRefaat]

Can you confirm that if a statement that use is and returns true, substituting the 'is' operator with 'as' operator will result in not null result in all circumstances.

Example:

var result = instance is FooObject;
var result2 = instance as FooObject;

so if 'result' variable is 'true' it will guarantee that 'result2' will always be not null in all cases?

[fuszenecker]

Hi,

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.

See also: https://msdn.microsoft.com/en-us/library/scekt9xw.aspx

[dodyg]

if (instance is FooObject){
   var x = instance as FooObject
}

Will x guaranteed to be a valid casting and not return null in above case?

[0xd4d]

Yes, is and as use the same IL instruction, isinst.

EDIT: assuming instance isn't accessed by multiple threads. Otherwise it's theoretically possible for another thread to modify instance so it fails the 2nd time.

[orthoxerox]

Yes, and that's why we're getting the new value is Type variableofType construct in C#7.