Static fields, by default, are shared between threads within an application domain. You need to allow each thread to have its own nonshared copy of a static field, so that this static field can be updated on a per-thread basis.

Use ThreadStaticAttribute to mark any static fields as not being shareable between threads:

using System;
using System.Threading;

public class Foo
{
    [ThreadStaticAttribute( )]
    public static string bar = "Initialized string";
}

By default, static fields are shared between the threads that access these fields in an application domain as a whole. To see this, we’ll create a class with a static field called bar and a static method used to access and display the value contained in this static field:

using System;
using System.Threading;

public class ThreadStaticField
{
    public static string bar = "Initialized string";

    public static void DisplayStaticFieldValue( )
    {
        Console.WriteLine(Thread.CurrentThread.GetHashCode( ) + 
                  " contains static field value of: " + 
                  ThreadStaticField.bar);
    }
}

Next, create a test method that accesses this static field both on the current thread and on a newly spawned thread:

public void TestStaticField( )
{
    ThreadStaticField.DisplayStaticFieldValue( );

    Thread newStaticFieldThread = 
         new Thread(new ThreadStart(
           ThreadStaticField.DisplayStaticFieldValue));

    newStaticFieldThread.Start( );

    ThreadStaticField.DisplayStaticFieldValue( );
}

This code displays output that resembles the following:

21 contains static field value of: Initialized string
21 contains static field value of: Initialized string
23 contains static field value of: Initialized string

The current thread’s hash value is 21 and the new thread’s hash value is 23. Notice that both threads are accessing the same static bar field. Next, add the ThreadStaticAttribute to the static field:

public class ThreadStaticField
{
    [ThreadStaticAttribute( )]
    public static string bar = "Initialized string";

    public static void DisplayStaticFieldValue( )
    {
        //bar = Thread.CurrentThread.ThreadState.ToString( );
        Console.WriteLine(Thread.CurrentThread.GetHashCode( ) + 
                   " contains static field value of: " + ThreadStaticField.bar);
    }
}

Now, output resembling the following is displayed:

21 contains static field value of: Initialized string
21 contains static field value of: Initialized string
23 contains static field value of:

Notice that the new thread returns a null for the value of the static bar field. This is the expected behavior. The bar field is only initialized in the first thread that accesses it. On all other threads, this field is initialized to null. Therefore, it is imperative that you initialize the bar field on all threads before it is used.

The bar field is initialized to the "Initialized string" string literal before it is used on the first thread that accesses this field. In the previous test code, the bar field was accessed first, and, therefore, it was initialized, on the current thread. Suppose we were to remove the first line of the TestStaticField method, as shown here:

public void TestStaticField( )
{
//  ThreadStaticField.DisplayStaticFieldValue( );
    Thread newStaticFieldThread = 
         new Thread(new ThreadStart(ThreadStaticField.DisplayStaticFieldValue));

    newStaticFieldThread.Start( );

    ThreadStaticField.DisplayStaticFieldValue( );
}

This code now displays similar output to the following:

21 contains static field value of: 
23 contains static field value of: Initialized string

The current thread does not access the bar field first and therefore does not initialize it. However, when the new thread accesses it first, it does initialize it.

Note that adding a static constructor to initialize the static field marked with this attribute will still follow the same behavior. Static constructors are executed only one time per application domain.

See the “ThreadStaticAttribute Attribute” and “static Modifier (C#)” topics in the MSDN documentation.