What is a method group in C#?

A method group is the name for a set of methods (that might be just one) - i.e. in theory the ToString method may have multiple overloads (plus any extension methods): ToString(), ToString(string format), etc - hence ToString by itself is a "method group".

It can usually convert a method group to a (typed) delegate by using overload resolution - but not to a string etc; it doesn't make sense.

Once you add parentheses, again; overload resolution kicks in and you have unambiguously identified a method call.

Also, if you are using LINQ, you can apparently do something like myList.Select(methodGroup).

So, for example, I have:

private string DoSomethingToMyString(string input)
{
    // blah
}

Instead of explicitly stating the variable to be used like this:

public List<string> GetStringStuff()
{
    return something.getStringsFromSomewhere.Select(str => DoSomethingToMyString(str));
}

I can just omit the name of the var:

public List<string> GetStringStuff()
{
    return something.getStringsFromSomewhere.Select(DoSomethingToMyString);
}

You can cast a method group into a delegate.

The delegate signature selects 1 method out of the group.

This example picks the ToString() overload which takes a string parameter:

Func<string,string> fn = 123.ToString;
Console.WriteLine(fn("00000000"));

This example picks the ToString() overload which takes no parameters:

Func<string> fn = 123.ToString;
Console.WriteLine(fn);