Why lambda expression has no return value?

Question

I read a code to send a email,here part of it:

MailUserState state = new MailUserState()
{
    AutoReleaseSmtp = m_autoDisposeSmtp,
    CurMailMessage = mMailMessage,
    CurSmtpClient = m_SmtpClient,
    IsSmpleMail = true,
    UserState = AsycUserState,
};
if (m_autoDisposeSmtp)
  m_SmtpClient = null;

ThreadPool.QueueUserWorkItem((userState) =>
{
    MailUserState curUserState = userState as MailUserState;
    curUserState.CurSmtpClient.SendAsync(mMailMessage, userState);
}, state);

Why the lambda expression has no return value?

I think it shoud return a callback instance object.But it has no return statement.Why?

Where would it return the value to? What would the thread pool do with the return value? — zerkms
– zerkms, Commented Dec 7, 2013 at 8:49
The lambda expression is a WaitCallback instance (or convertible to one, anyway). It doesn't need to return one. — Roger Lipscombe
– Roger Lipscombe, Commented Dec 7, 2013 at 8:50
@Dolphin, the QueueUserWorkItem function has two overloads, one of which takes two parameters. — Roger Lipscombe
– Roger Lipscombe, Commented Dec 7, 2013 at 8:51

Darin Dimitrov · Accepted Answer · 2013-12-07 08:53:36Z

7

Why the lambda expression has no return value?

The first argument of the ThreadPool.QueueUserWorkItem method is a WaitCallback delegate which looks like this:

public delegate void WaitCallback(object state)

So as you can see it is a function taking one object parameter and having no return value. And that's exactly what the lambda expression in your code is. The QueueUserWorkItem method will draw a thread from the thread pool if/when available and it will execute the code in the callback on this thread. There is no return value.

answered Dec 7, 2013 at 8:53

Darin Dimitrov

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