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

C# and the CLR have somewhat different conversion rules.

You can't directly cast between int[] and uint[] in C# because the language doesn't believe any conversion is available. However, if you go via object the result is up to the CLI. From the CLI spec section 8.7 (I hope - I'm quoting an email exchange I had on this topic with Eric Lippert a while ago):

Signed and unsigned integral primitive types can be assigned to each other; e.g., int8 := uint8 is valid. For this purpose, bool shall be considered compatible with uint8 and vice versa, which makes bool := uint8 valid, and vice versa. This is also true for arrays of signed and unsigned integral primitive types of the same size; e.g., int32[] := uint32[] is valid.

(I haven't checked, but I assume that this sort of reference type conversion being valid is what makes is return true as well.)

It's somewhat unfortunate that there are disconnects between the language and the underlying execution engine, but it's pretty much unavoidable in the long run, I suspect. There are a few other cases like this, but the good news is that they rarely seem to cause significant harm.

EDIT: As Marc deleted his answer, I've linked to the full mail from Eric, as posted to the C# newsgroup.

Now that's interesting. I found this in the ECMA-335 standard. 4.3 castclass. Note that:

• Arrays inherit from System.Array.

• If Foo can be cast to Bar, then Foo[] can be cast to Bar[].

• For the purposes of note 2 above, enums are treated as their underlying type: thus E1[] can be cast to E2[] if E1 and E2 share an underlying type.

You can cast int to uint, but that it behaves like this is very strange. Visual Studio does not recognize any of this, even the watch, when the debugger is attached just shows a question mark '?'.

You might wanna take a look at this, fast forward about 10 minutes in and listen to Anders explain the co-variant array implementation. I think that is the fundamentally underlying issue here.

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.