Delegates, Why? [duplicate]

Answer 1

.NET Delegates: A C# Bedtime Story

Answer 2

You can use delegates to implement subscriptions and eventHandlers. You can also (in a terrible way) use them to get around circular dependencies.

Or if you have a calculation engine and there are many possible calculations, then you can use a parameter delegate instead of many different function calls for your engine.

Answer 3

Let me offer an example. If your class exposes an event, it can be assigned some number of delegates at runtime, which will be called to signal that something happened. When you wrote the class, you had no idea what delegates it would wind up running. Instead, this is determined by whoever uses your class.

Answer 4

To give a concrete example, a particularly recent use of a delegate for me was SendAsync() on System.Net.Mail.SmtpClient. I have an application that sends tons and tons of email and there was a noticeable performance hit waiting for the Exchange server to accept the message. However, it was necessary to log the result of the interaction with that server.

So I wrote a delegate method to handle that logging and passed it to SendAsync() (we were previously just using Send()) when sending each email. That way it can call back to the delegate to log the result and the application threads aren't waiting for the interaction to finish before continuing.

The same can be true of any external IO where you want the application to continue without waiting for the interaction to complete. Proxy classes for web services, etc. take advantage of this.

Answer 5

One example where a delegate is needed is when you have to modify a control in the UI thread and you are operating in a different thread. For example,

public delegate void UpdateTextBox(string data);

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    ...
    Invoke(new UpdateTextBox(textBoxData), data);
    ...

}

private void textBoxData(string data)
{
            textBox1.Text += data;
}

Answer 6

Did you read http://msdn.microsoft.com/en-us/library/ms173171(VS.80).aspx ?