Add extension delegate

[IgorSivirinov]

Summary

Add the ability to create an extension delegate.

Example of creation:

public static class ObjectExtensions
{
    public static T Apply<T>(this T self, Action<this T> block)
    {
        block(self);
        return self;
    }
}

Example of use with Linq:

var users = new List<User> { new("Igor", 34), new("Lera", 23), new("Max", 54)};

var benefitUsers = users.Where(this => Age < 35).ToList();
// two variant
var benefitUsers = users.Where(() => Age < 35).ToList();

record User(string Name, int Age);

Motivation

Extension methods of this kind are often used (especially in Linq):

public static T Also<T>(this T self, Action<T> block)
    {
        block(self);
        return self;
   }   

Because of this, you often have to write the same name for the delegate parameter.

An example of using the ReactiveUI binding without adding this in C#:

this.WhenActivated(disposable =>
        {
            this.Bind(ViewModel, x => x.TheText, x => x.TheTextBox.Text)
                .DisposeWith(disposable);
            this.OneWayBind(ViewModel, x => x.TheText, x => x.TheTextBlock.Text)
                .DisposeWith(disposable);
            this.BindCommand(ViewModel, x => x.TheTextCommand, x => x.TheTextButton)
                .DisposeWith(disposable);
        });

With the addition of this:

this.WhenActivated(disposable =>
{
    this.Bind(ViewModel, vm => vm.TheText, this => TheTextBox.Text)
        .DisposeWith(disposable);
    this.OneWayBind(ViewModel, vm => vm.TheText, this => TheTextBlock.Text)
        .DisposeWith(disposable);
    this.BindCommand(ViewModel, vm => vm.TheTextCommand, this => TheTextButton)
        .DisposeWith(disposable);
});

[svick]

There have been proposals for achieving the same goal using different syntax in the past: #91, #1151, #602.

Specifically for this syntax, what confuses me is that your own example shows that this can make the code longer (this => TheText is one more character than x => x.TheText). So why would this be a worthwhile addition to the language?