Why can I use a lambda expression in place of a callback delegate?

Question

I discovered some new C# syntax and do not understand what it means. Here is the syntax-related code:

1)

BeginInvoke(new Action(() =>
    {
        PopulateUI(ds);
    }));

2)

private void OnFormLoad() 
{ 
    ThreadPool.QueueUserWorkItem(() => GetSqlData()); 
}

What is the meaning of new Action() and what is the meaning of the => symbol?

The syntax of ThreadPool.QueueUserWorkItem was ThreadPool.QueueUserWorkItem(new WaitCallback(PrintOut), "Hello"); but here it shows ThreadPool.QueueUserWorkItem(() => GetSqlData());, so how does it work? Why is WaitCallback missing? Please explain in detail.

Thanks a lot.

Answer 1

As others have said, it is a lambda, which is basically an anonymous (unnamed) local function.

This might make a bit more sense if you look at some similar code that doesn't use lambdas:

// With a lambda
private void OnFormLoad()
{
    ThreadPool.QueueUserWorkItem(() => GetSqlData()); 
}

// Without a lambda
private void OnFormLoad() 
{ 
    ThreadPool.QueueUserWorkItem(ExecuteGetSqlData);
}

private void ExecuteGetSqlData()
{
    // If GetSqlData returns something, change this to "return GetSqlData();"
    GetSqlData();
}

As for the other code, normally you shouldn't have to do new Action. The problem is that the BeginInvoke method takes a Delegate, which is sort of old school, and breaks how most new code works.

With newer code (that takes something like Action, or a specific type of delegate, like WaitCallback), you either write a lambda, or simply give the name of a function inside your class. The example code I wrote above demonstrates both of these.

Also note that if you see something like: (Action) (() => Blah()), it is pretty much the same as new Action(() => Blah()).