Simplest way to do a fire and forget method in C#?

To add to Will's answer, if this is a console application, just throw in an AutoResetEvent and a WaitHandle to prevent it exiting before the worker thread completes:

Using System;
Using System.Threading;

class Foo
{
    static AutoResetEvent autoEvent = new AutoResetEvent(false);

    static void Main()
    {
        ThreadPoolQueueUserWorkItem(new WaitCallback(FireAway), autoEvent);
        autoEvent.WaitOne(); // Will wait for thread to complete
    }

    static void FireAway(object stateInfo)
    {
        System.Threading.Thread.Sleep(5000);
        Console.WriteLine("5 seconds later");
        ((AutoResetEvent)stateInfo).Set();
    }
}

ThreadPool.QueueUserWorkItem(o => FireAway());

(five years later...)

Task.Run(() => FireAway());

as pointed out by luisperezphd.

For C# 4.0 and newer, it strikes me that the best answer is now given here by Ade Miller: Simplest way to do a fire and forget method in c# 4.0

Task.Factory.StartNew(() => FireAway());

Or even...

Task.Factory.StartNew(FireAway);

Or...

new Task(FireAway).Start();

Where FireAway is

public static void FireAway()
{
    // Blah...
}

So by virtue of class and method name terseness this beats the threadpool version by between six and nineteen characters depending on the one you choose :)

ThreadPool.QueueUserWorkItem(o => FireAway());

For .NET 4.5:

Task.Run(() => FireAway());