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ProcessThread.ProcessorAffinity Property

Updated: August 2008

Sets the processors on which the associated thread can run.

Namespace:  System.Diagnostics
Assembly:  System (in System.dll)

public IntPtr ProcessorAffinity { set; }

Property Value

Type: System.IntPtr
An IntPtr that points to a set of bits, each of which represents a processor that the thread can run on.


The processor affinity could not be set.


The platform is Windows 98 or Windows Millennium Edition.


The process is on a remote computer.

The processor affinity of a thread is the set of processors it has a relationship to. In other words, those it can be scheduled to run on.

ProcessorAffinity represents each processor as a bit. Bit 0 represents processor one, bit 1 represents processor two, and so on. The following table shows a subset of the possible ProcessorAffinity for a four-processor system.

Property value (in hexadecimal)

Valid processors






1 or 2




1 or 3


1, 2, or 3


1, 2, 3, or 4

You can also specify the single, preferred processor for a thread by setting the IdealProcessor property. A process thread can migrate from processor to processor, with each migration reloading the processor cache. Specifying a processor for a thread can improve performance under heavy system loads by reducing the number of times the processor cache is reloaded.

The following example shows how to set the ProcessorAffinity property for an instance of Notepad to the first processor.

using System;
using System.Diagnostics;

namespace ProcessThreadIdealProcessor
    class Program
        static void Main(string[] args)
            // Make sure there is an instance of notepad running.
            Process[] notepads = Process.GetProcessesByName("notepad");
            if (notepads.Length == 0)
            ProcessThreadCollection threads;
            //Process[] notepads; 
            // Retrieve the Notepad processes.
            notepads = Process.GetProcessesByName("Notepad");
            // Get the ProcessThread collection for the first instance
            threads = notepads[0].Threads;
            // Set the properties on the first ProcessThread in the collection
            threads[0].IdealProcessor = 0;
            threads[0].ProcessorAffinity = (IntPtr)1;

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0




August 2008

Added an example.

Customer feedback.