Introduction

ThreadLocal<T> was introduced in .NET 4 and didn’t get much attention because it didn’t do much over the ThreadStaticAttribute which we have had since version 1 of the framework, so let’s just review what it does. In short it gives every unique thread that uses it, it’s own global field. Let’s look at this code:

static ThreadLocal<int> balances = new ThreadLocal<int>(() =>
    {                
        return 10;
    });

static void Main(string[] args)
{
    for (int i = 0; i < 10; i++)
    {
        new Thread(AddMoney).Start();
    }

    Console.ReadLine();
}

static void AddMoney()
{
    Console.WriteLine("Before {0}", balances.Value);
    balances.Value += new Random().Next(0, 1000);
    Console.WriteLine("After {0}", balances.Value);
}

Which produces:

Note that ever Before is set to 10 and that is because the lambda method that we pass to the ThreadLocal<T> constructor is run for each unique thread.

What’s new in .NET 4.5?

.NET 4.5 improves the usefulness of this by including the .Values parameter which allows you to list the results from each thread! To make use of this you need to opt-in in the constructor by adding true:

static ThreadLocal<int> balances = new ThreadLocal<int>(() =>
{
    return 10;
}, true);

foreach (var item in balances.Values)
{
    Console.WriteLine("Balance at end: {0}", item);
}

This is VERY useful when working with threads and doing individual calculations and then collating the results at the end!

Warning

ThreadLocal<T> only works with unique threads! So using with the TPL or ThreadPool which reuse threads will not work as expected!