A Double divided by zero is returning a Divide by Zero error

I am experiencing an unexpected behaviour and was hoping someone could help with some guidance as to what areas to focus an investigation on.

I have two methods, one essentially performs a divide by zero test on a double, the second calls an extern method for an unmanaged dll.

Note: In the .Net runtime, dividing a Double by Zero should return an Infinity value (amusingly of either positive or negative flavours).

Pseudocode for what I am doing looks something like this:

InfinityTest(); // Returns an Infinity value as expected
DllCall();
InfinityTest(); // Divide by zero error on second call.

The first call to InfinityTest() returns the value Infinity as expected. The second call to InfinityTest() throws a Divide by Zero exception that I didn't expect.

Update

The effective InfinityTest() code below. For brevity I've removed try/catch elements, etc. I do not have permission to go into details about the DllCall() pseudocode element, apologies.

private double InfinityTest()
{
    double a = 1.0;
    int b = 0;
    return a / b;
}

Since it sounds like your DLL is changing the FP status word on you, your only choice may be to change it back. I would suggest P/Invoke to _clearfp or _fpreset. Here are their P/Invoke signatures:

    [DllImport("msvcrt.dll")]
    static extern UInt32 _clearfp();
    [DllImport("msvcrt.dll")]
    static extern void _fpreset();

This may not reset things back to exactly the way they were, but hopefully it will be close enough.