Code with undefined behavior in C#

Question

In C++ there are a lot of ways that you can write code that compiles, but yields undefined behavior (Wikipedia). Is there something similar in C#? Can we write code in C# th

Answer 1

Yes! There is, even in a safe context! (Well, it's implementation defined to be undefined, at least)

Here's one from Marek Safar and VSadov in the Roslyn issues.There is a mismatch between C# and the CLI in regards to bool.

C# believes that there is only one kind of true, and one kind of false.

CLI believes that false is a byte containing 0, and all other values are true.

This discrepancy means we can coerce C# to do some a (marginally) interesting things thing:

//non-standard bool
//We're setting a bool's value to a byte value of 5.
var a = new bool[1];
Buffer.SetByte(a, 0, 5);

//non-standard bool
//We're setting a bool's value to a byte value of 10.
var b = new bool[1];
Buffer.SetByte(b, 0, 10);

//Both are true.
Console.WriteLine(a[0]);
Console.WriteLine(b[0]);

//But they are not the same true.
Console.WriteLine(a[0] == b[0]);

The above outputs:

true

true

false

Interestingly, the debugger disagrees (must evaluate truth differently?)

Anyways, the conclusion the C# team appears to have come to is (emphasis added):

I.E. the language will stay entirely unconcerned about nonstandard bools. The particular implementation (as in MS C# on CIL) will acknowledge the existence of nonstandard bools and specify their behavior as undefined