Code with undefined behavior in C#

As others have mentioned, pretty much anything in the "unsafe" block can yield implementation-defined behaviour; abuse of unsafe blocks allows you to change the bytes of code that make up the runtime itself, and therefore all bets are off.

The division int.MinValue/-1 has an implementation-defined behaviour.

Throwing an exception and never catching it causes implementation-defined behaviour -- terminate the process, start a debugger, and so on.

There are a number of other situations in C# where we are forced to emit code which has implementation-determined behaviour. For example, this situation:

https://docs.microsoft.com/en-us/archive/blogs/ericlippert/odious-ambiguous-overloads-part-two

However, the situations in which a safe, well-behaved C# program has implementation-defined behaviour should be quite rare.

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