Thread vs Threadstart

The Thread(ThreadStart) constructor can only be used when the signature of your SomeMethod method matches the ThreadStart delegate. Conversely, Thread(ParameterizedThreadStart) requires SomeMethod to match the ParameterizedThreadStart delegate. The signatures are below:

public delegate void ThreadStart()
public delegate void ParameterizedThreadStart(Object obj)

Concretely, this means that you should use ThreadStart when your method does not take any parameters, and ParameterizedThreadStart when it takes a single Object parameter. Threads created with the former should be started by calling Start(), whilst threads created with the latter should have their argument specified through Start(Object).

public static void Main(string[] args)
{
    var threadA = new Thread(new ThreadStart(ExecuteA));
    threadA.Start();

    var threadB = new Thread(new ParameterizedThreadStart(ExecuteB));
    threadB.Start("abc");

    threadA.Join();
    threadB.Join();
}

private static void ExecuteA()
{
    Console.WriteLine("Executing parameterless thread!");
}

private static void ExecuteB(Object obj)
{
    Console.WriteLine($"Executing thread with parameter \"{obj}\"!");
}

Finally, you can call the Thread constructors without specifying the ThreadStart or ParameterizedThreadStart delegate. In this case, the compiler will match your method to the constructor overload based on its signature, performing the cast implicitly.

var threadA = new Thread(ExecuteA);   // implicit cast to ThreadStart
threadA.Start();

var threadB = new Thread(ExecuteB);   // implicit cast to ParameterizedThreadStart
threadB.Start("abc");