When is it better practice to explicitly use the delegate keyword instead of a lambda?

Question

Is there any best practice with respect to coding style with respect to explicit use of the delegate keyword instead of using a lambda?

e.g.

new Thread(() =>
{
 // work item 1
 // work item 2
}).Start();

new Thread(delegate()
{
 // work item 1
 // work item 2
}).Start();

I think the lambda looks better. If the lambda is better style, what's the point of having a delegate keyword, other than for the fact that it existed before lambdas were implemented?

Answer 1

Lambda syntax is much more generalised, and the designers have said that they'd ideally remove the old overlapping syntaxes (dont have a citation, but it's probably Eric Lippert or Jon Skeet in a book or a podcast).

But delegate allows you to ignore parameters, e.g.:

object.Event += delegate { };

versus having to say:

object.Event += (sender,args) => { };

which can be very useful in large argument lists and/or to make the code more resilient to refactoring.

EDIT: As pointed out by Yann Schwartz in another answer (now unfortunately deleted), a very neat usage of this trick is in order to provide a default hander for an event using the Null Object pattern:-

class MyClassThatFiresWithoutTheTrick
{
    public event EventHandler MyEvent; // implicit = null

    // Need a method to keep this DRY as each fire requires a null check - see Framework Design Guidelines by Abrams and Cwalina
    protected virtual void OnMyEvent()
    {
        // need to take a copy to avoid race conditions with _removes
        var handler = MyEvent;
        // Need to do this check as it might not have been overridden
        if( handler == null)
            return;
        handler( this, EventArgs.Empty );
    }
}

class MyClassThatFiresWithTheTrick
{
    public event EventHandler MyEvent = delegate{};

    protected virtual void OnMyEvent()
    {
        MyEvent( this, EventArgs.Empty );
    }
}

(though what you might often end up doing is an Inline Method of OnMyEvent, making the code even shorter again.)