Why does “int[] is uint[] == true” in C#

Question

Can somebody clarify the C# is keyword please. In particular these 2 questions:

Q1) line 5; Why does this return true?

Q2) line 7; Why no cast e

Answer 1

Suggestion:

Declaring intArray as "int [] intArray" rather then "object intArray" will allow the compiler to pick up the invalid C# cast. Unless you absolutely have to use object, I would take that approach.

Re Q2,Q3:

At runtime have you tried wrapping the cast in a checked block?

From this article at MSDN:

By default, an expression that contains only constant values causes a compiler error if the expression produces a value that is outside the range of the destination type. If the expression contains one or more non-constant values, the compiler does not detect the overflow.

...

By default, these non-constant expressions are not checked for overflow at run time either, and they do not raise overflow exceptions. The previous example displays -2,147,483,639 as the sum of two positive integers.

Overflow checking can be enabled by compiler options, environment configuration, or use of the checked keyword.

As it says, you can enforce overflow checking more globally via a compiler setting or environment config.

In your case this is probably desirable as it will cause a runtime error to be thrown that will ensure the likely invalid unsigned number to signed number overflow will not occur silently.

[Update] After testing this code, I found that using a declaration of type object instead of int [] appears to bypass the standard C# casting sytax, regardless of whether checked is enabled or not.

As JS has said, when you use object, you are bound by CLI rules and these apparently allow this to occur.

Re Q1:

This is related to the above. In short, because the cast involved it does not throw an exception (based on current overflow setting). Whether this is a good idea is another question.

From MSDN:

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.