C# Interlocked Exchange

Question

I have a bit of my game which looks like this:

public static float Time;

float someValue = 123;
Interlocked.Exchange(ref Time, someValue);

I want to change Time to be a Uint32, however when I try to use UInt32 instead of float for the values it protests that the type must be a reference type. Float is not a reference type, so I know it's technically possible to do this with non reference types, is there any practical way to make this work with UInt32?

Answer 1

Perhaps use int instead of uint; there are overloads for int. Do you need the extra bit of range? If so, cast / convert as late as possible.

Answer 2

There's an overload for Interlocked.Exchange specifically for float (and others for double, int, long, IntPtr and object). There isn't one for uint, so the compiler reckons the closest match is the generic Interlocked.Exchange<T> - but in that case T has to be a reference type. uint isn't a reference type, so that doesn't work either - hence the error message.

In other words:

• Your current code works because it calls Interlocked.Exchange(ref float, float).
• Changing it to uint fails because there's no applicable overload. The exact error message is caused by the compiler guessing that you mean Interlocked.Exchange<T>(ref T, T).

As for what to do, the options are any of:

• Potentially use int instead, as Marc suggests.
• If you need the extra range, think about using long.
• Use uint but don't try to write lock-free code

Although obviously Exchange works fine with some specific value types, Microsoft hasn't implemented it for all the primitive types. I can't imagine it would have been hard to do so (they're just bits, after all) but presumably they wanted to keep the overload count down.

Answer 3

You cannot pass a casted expression by reference, you should use a temporary variable:

public static float Time;
float value2 = (float)SomeValue;
Interlocked.Exchange(ref Time, ref value2);
SomeValue = value2;