Set thread processor affinity in Microsoft .Net

(Posting this with answer because I couldn't find a full explanation of how to do this anywhere, so I thought it might have some value for someone)

How can I set the processor affinity of a particular thread in Microsoft .Net? Setting the process's affinity is trivial via System.Diagnostics.Process.ProcessorAffinity, but the System.Threading.Thread class offers no such functionality and .Net doesn't guarantee a managed thread is linked to any particular operating system thread.


The separation between managed and operating system threads dates back to .Net 2.0, and plans by the SQL Server team to implement .Net threads using fibers. This never really went anywhere, so while there is no guarantee that a managed thread will always run on the same operating system thread, in practice this is always the case for all current .Net hosts. Given that this hasn't changed in all the years since .Net 2.0's introduction, it is unlikely this will ever change.

It is possible to strengthen our confidence even for future versions of .Net by using the System.Threading.Thread.BeginThreadAffinity method. This guarantees that the managed thread will stay on the same operating system thread (so it does nothing on the default CLR host, as that is already true by default). I suppose it is still possible that other managed threads can share the same operating system thread, but this seems unlikely and is definitely not the case in any current .Net hosts.

.Net provides the ability to access native operating system threads using the System.Diagnostics.ProcessThread class, and this class has the ability to change the thread's processor affinity using the ProcessorAffinity property. However, linking a particular managed thread to its ProcessThread was made deliberately difficult.

The only real way to do it is from inside the thread itself. Use the System.AppDomain.GetCurrentThreadId method (or PInvoke the GetCurrentThreadId function if you don't want to call a deprecated method, though that would not work with Mono on operating systems other than Windows). This can then be matched to the ProcessThread.Id property.

This makes it possible to set the thread's processor affinity with the following code (to be called from inside the thread):

/// <summary>
/// Sets the processor affinity of the current thread.
/// </summary>
/// <param name="cpus">A list of CPU numbers. The values should be
/// between 0 and <see cref="Environment.ProcessorCount"/>.</param>
public static void SetThreadProcessorAffinity(params int[] cpus)
    if( cpus == null )
        throw new ArgumentNullException("cpus");
    if( cpus.Length == 0 )
        throw new ArgumentException("You must specify at least one CPU.", "cpus");

    // Supports up to 64 processors
    long cpuMask = 0;
    foreach( int cpu in cpus )
        if( cpu < 0 || cpu >= Environment.ProcessorCount )
            throw new ArgumentException("Invalid CPU number.");

        cpuMask |= 1L << cpu;

    // Ensure managed thread is linked to OS thread; does nothing on default host in current .Net versions

#pragma warning disable 618
    // The call to BeginThreadAffinity guarantees stable results for GetCurrentThreadId,
    // so we ignore the obsolete warning
    int osThreadId = AppDomain.GetCurrentThreadId();
#pragma warning restore 618

    // Find the ProcessThread for this thread.
    ProcessThread thread = Process.GetCurrentProcess().Threads.Cast<ProcessThread>()
                               .Where(t => t.Id == osThreadId).Single();
    // Set the thread's processor affinity
    thread.ProcessorAffinity = new IntPtr(cpuMask);

Keep in mind that while this works on current versions of .Net, theoretically the lack of a guarantee that managed threads are bound to OS threads could break this code in the future. However, I consider this extremely unlikely.

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