C# Interlocked Exchange
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 meanInterlocked.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.
Although ugly, it is actually possible to perform an atomic Exchange or CompareExchange on an enum or other blittable value type of 64 bits or less using unsafe
C# code:
enum MyEnum { A, B, C };
MyEnum m_e = MyEnum.B;
unsafe void example()
{
MyEnum e = m_e;
fixed (MyEnum* ps = &m_e)
if (Interlocked.CompareExchange(ref *(int*)ps, (int)(e | MyEnum.C), (int)e) == (int)e)
{
/// change accepted, m_e == B | C
}
else
{
/// change rejected
}
}
The counterintuitive part is that the ref expression on the dereferenced pointer does actually penetrate through to the address of the enum. I think the compiler would have been within its rights to have generated an invisible temporary variable on the stack instead, in which case this wouldn't work. Use at your own risk.
[edit: for the specific type requested by the OP]
static unsafe uint CompareExchange(ref uint target, uint v, uint cmp)
{
fixed (uint* p = &target)
return (uint)Interlocked.CompareExchange(ref *(int*)p, (int)v, (int)cmp);
}
[edit: and 64-bit unsigned long]
static unsafe ulong CompareExchange(ref ulong target, ulong v, ulong cmp)
{
fixed (ulong* p = &target)
return (ulong)Interlocked.CompareExchange(ref *(long*)p, (long)v, (long)cmp);
}
(I also tried using the undocumented C# keyword __makeref
to achieve this, but this doesn't work because you can't use ref
on a dreferenced __refvalue
. It's too bad, because the CLR maps the [comment mooted by JIT interception, see below]) InterlockedExchange
functions to a private internal function that operates on TypedReference
[edit: July 2018] You can now do this more efficiently using the System.Runtime.CompilerServices.Unsafe library package. Your method can use Unsafe.As<TFrom,TTo>()
to directly reinterpret the type referenced by the target managed reference, avoiding the dual expenses of both pinning and transitioning to unsafe
mode:
static uint CompareExchange(ref uint target, uint value, uint expected) =>
(uint)Interlocked.CompareExchange(
ref Unsafe.As<uint, int>(ref target),
(int)value,
(int)expected);
static ulong CompareExchange(ref ulong target, ulong value, ulong expected) =>
(ulong)Interlocked.CompareExchange(
ref Unsafe.As<ulong, long>(ref target),
(long)value,
(long)expected);
Of course this works for Interlocked.Exchange
as well. Here are those helpers for the 4- and 8-byte unsigned types.
static uint Exchange(ref uint target, uint value) =>
(uint)Interlocked.Exchange(ref Unsafe.As<uint, int>(ref target), (int)value);
static ulong Exchange(ref ulong target, ulong value) =>
(ulong)Interlocked.Exchange(ref Unsafe.As<ulong, long>(ref target), (long)value);
This works for enumeration types also--but only so long as their underlying primitive integer is exactly four or eight bytes. In other words, int
(32-bit) or long
(64-bit) sized. The limitation is that these are the only two bit-widths found among the Interlocked.CompareExchange
overloads. By default, enum
uses int
when no underlying type is specified, so MyEnum
(from above) works fine.
static MyEnum CompareExchange(ref MyEnum target, MyEnum value, MyEnum expected) =>
(MyEnum)Interlocked.CompareExchange(
ref Unsafe.As<MyEnum, int>(ref target),
(int)value,
(int)expected);
static MyEnum Exchange(ref MyEnum target, MyEnum value) =>
(MyEnum)Interlocked.Exchange(ref Unsafe.As<MyEnum, int>(ref target), (int)value);
I'm not sure whether the 4-byte minimum is a fundamental to .NET, but as far as I can tell it leaves no means of atomically swapping (values of) the smaller 8- or 16-bit primitive types (byte
, sbyte
, char
, ushort
, short
) without risking collateral damage to adjacent byte(s). In the following example, BadEnum
explicitly specifies a size that is too small to be atomically swapped without possibly affecting up to three neighboring bytes.
enum BadEnum : byte { }; // can't swap less than 4 bytes on .NET?
If you're not constrained by interop-dictated (or otherwise fixed) layouts, a workaround would be to ensure that the memory layout of such enums is always padded to the 4-byte minimum to allow for atomic swapping (as int
). It seems likely, however, that doing so would defeat whatever purpose there might have been for specifying the smaller width in the first place.
[edit: April 2017] I recently learned that when .NET
is running in 32-bit mode (or, i.e. in the WOW subsystem), the 64-bit Interlocked
operations are not guaranteed to be atomic with respect to non-Interlocked
, "external" views of the same memory locations. In 32-bit mode, the atomic guarantee only applies globablly across QWORD accesses which use the Interlocked
(and perhaps Volatile.*
, or Thread.Volatile*
, TBD?) functions.
In other words, to obtain 64-bit atomic operations in 32-bit mode, all accesses to those QWORD locations must occur through Interlocked
in order to preserve the guarantees, and you can't get cute assuming that (e.g.) direct reads are protected just because you always use Interlocked
functions for writing.
Finally, note that the Interlocked
functions in the CLR
are specially recognized by, and receive special treatment in, the .NET JIT compiler. See here and here This fact may help explain the counter-intuitiveness I mentioned earlier.
[edit:] Mea culpa and apologies to @AnorZaken since my answer is similar to his. I honestly didn't see it before posting mine. I'll keep this for now in case my text and explanations are useful or have additional insights, but credit for prior work properly goes to Anor.
Although I have another solution on this page, some people might be interested in a totally different approach. Below, I give a DynamicMethod
which implements Interlocked.CompareExchange
for any 32- or 64-bit blittable type, which includes any custom Enum
types, the primitive types that the built-in method forgot (uint
, ulong
), and even your own ValueType
instances--so long as any of these are dword (4-bytes, i.e., int
, System.Int32
) or qword (8-bytes, long
, System.Int64
) sized. For example, the following Enum
type won't work since it specifies a non-default size, byte
:
enum ByteSizedEnum : byte { Foo } // no: size is not 4 or 8 bytes
As with most DynamicMethod implementations of runtime-generated IL, the C# code isn't beautiful to behold, but for some people the elegant IL and sleek JITted native code make up for that. For example, in contrast to the other method I posted, this one doesn't use unsafe
C# code.
To allow automatic inference of the generic type at the call site, I wrap the helper in a static
class:
public static class IL<T> where T : struct
{
// generic 'U' enables alternate casting for 'Interlocked' methods below
public delegate U _cmp_xchg<U>(ref U loc, U _new, U _old);
// we're mostly interested in the 'T' cast of it
public static readonly _cmp_xchg<T> CmpXchg;
static IL()
{
// size to be atomically swapped; must be 4 or 8.
int c = Marshal.SizeOf(typeof(T).IsEnum ?
Enum.GetUnderlyingType(typeof(T)) :
typeof(T));
if (c != 4 && c != 8)
throw new InvalidOperationException("Must be 32 or 64 bits");
var dm = new DynamicMethod(
"__IL_CmpXchg<" + typeof(T).FullName + ">",
typeof(T),
new[] { typeof(T).MakeByRefType(), typeof(T), typeof(T) },
MethodInfo.GetCurrentMethod().Module,
false);
var il = dm.GetILGenerator();
il.Emit(OpCodes.Ldarg_0); // ref T loc
il.Emit(OpCodes.Ldarg_1); // T _new
il.Emit(OpCodes.Ldarg_2); // T _old
il.Emit(OpCodes.Call, c == 4 ?
((_cmp_xchg<int>)Interlocked.CompareExchange).Method :
((_cmp_xchg<long>)Interlocked.CompareExchange).Method);
il.Emit(OpCodes.Ret);
CmpXchg = (_cmp_xchg<T>)dm.CreateDelegate(typeof(_cmp_xchg<T>));
}
};
Technically, the above is all you need. You can now call CmpXchgIL<T>.CmpXchg(...)
on any appropriate value type (as discussed in the intro above), and it will behave exactly like the built-in Interlocked.CompareExchange(...)
in System.Threading
. For example, lets say you have a struct
containing two integers:
struct XY
{
public XY(int x, int y) => (this.x, this.y) = (x, y); // C#7 tuple syntax
int x, y;
static bool eq(XY a, XY b) => a.x == b.x && a.y == b.y;
public static bool operator ==(XY a, XY b) => eq(a, b);
public static bool operator !=(XY a, XY b) => !eq(a, b);
}
You can now atomically publish the 64-bit struct just as you would expect with any CmpXchg operation. This atomically publishes the two integers so that it is impossible for another thread to see a 'torn' or inconsistent pairing. Needless to say, easily doing so with a logical pairing is hugely useful in concurrent programming, even more so if you devise an elaborate struct that packs many fields into the available 64 (or 32) bits. Here's an example of the call-site for doing this:
var xy = new XY(3, 4); // initial value
//...
var _new = new XY(7, 8); // value to set
var _exp = new XY(3, 4); // expected value
if (IL<XY>.CmpXchg(ref xy, _new, _exp) != _exp) // atomically swap the 64-bit ValueType
throw new Exception("change not accepted");
Above, I mentioned that you can tidy up the call site by enabling type inference so that you don't have to specify the generic parameter. To do this, just define a static generic method in one of your non- generic global classes:
public static class my_globals
{
[DebuggerStepThrough, MethodImpl(MethodImplOptions.AggressiveInlining)]
public static T CmpXchg<T>(ref T loc, T _new, T _old) where T : struct =>
_IL<T>.CmpXchg(ref loc, _new, _old);
}
I'll show the simplified call site with a different example, this time using an Enum
:
using static my_globals;
public enum TestEnum { A, B, C };
static void CompareExchangeEnum()
{
var e = TestEnum.A;
if (CmpXchg(ref e, TestEnum.B, TestEnum.A) != TestEnum.A)
throw new Exception("change not accepted");
}
As for the original question, ulong
and uint
work trivially as well:
ulong ul = 888UL;
if (CmpXchg(ref ul, 999UL, 888UL) != 888UL)
throw new Exception("change not accepted");