What is a method group in C#?

Question

I have often encountered an error such as "cannot convert from 'method group' to 'string'" in cases like:

var list = new List<string>();
// ... snip
list.Add(someObject.ToString);

of course there was a typo in the last line because I forgot the invocation parentheses after ToString. The correct form would be:

var list = new List<string>();
// ... snip
list.Add(someObject.ToString()); // <- notice the parentheses

However I came to wonder what is a method group. Google isn't much of a help nor MSDN.

Answer 1

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.

Answer 2

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);
}

Answer 3

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);

Answer 4

The first result in your MSDN search said:

The method group identifies the one method to invoke or the set of overloaded methods from which to choose a specific method to invoke

my understanding is that basically because when you just write someInteger.ToString, it may refer to:

Int32.ToString(IFormatProvider) 

or it can refer to:

Int32.ToString()

so it is called a method group.

Answer 5

The ToString function has many overloads - the method group would be the group consisting of all the different overloads for that function.