In C#, Is "This" Keyword Required

In C#, is this keyword required?

No, this is purely optional in almost all cases. The only reason it is required in your example is to disambiguate between local variables and parameters and member variables that happened to have the same identifier name.

You could avoid the use of this by renaming either the parameter or the field to something unique.

When do you use the this keyword?

There are several usages of this keyword in C#.

To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class

You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.

Whether it is not necessary to use the `this.` keyword for variables within c# class?

In your case it's not required or necessary since that variable is not marked as static, compiler can identify that it's a instance member and not a static one.

In other case, where your instance variable and constructor variable names are same, you can use this to specify the instance member explicitly like

public ClassA(int x, int y)
{
    this.x = x;
    this.y = y;
}

Why is the 'this' keyword required to call an extension method from within the extended class

A couple points:

First off, the proposed feature (implicit "this." on an extension method call) is unnecessary. Extension methods were necessary for LINQ query comprehensions to work the way we wanted; the receiver is always stated in the query so it is not necessary to support implicit this to make LINQ work.

Second, the feature works against the more general design of extension methods: namely, that extension methods allow you to extend a type that you cannot extend yourself, either because it is an interface and you don't know the implementation, or because you do know the implementation but do not have the source code.

If you are in the scenario where you are using an extension method for a type within that type then you do have access to the source code. Why are you using an extension method in the first place then? You can write an instance method yourself if you have access to the source code of the extended type, and then you don't have to use an extension method at all! Your implementation can then take advantage of having access to the private state of the object, which extension methods cannot.

Making it easier to use extension methods from within a type that you have access to is encouraging the use of extension methods over instance methods. Extension methods are great, but it is usually better to use an instance method if you have one.

Given those two points, the burden no longer falls on the language designer to explain why the feature does not exist. It now falls on you to explain why it should. Features have enormous costs associated with them. This feature is not necessary and works against the stated design goals of extension methods; why should we take on the cost of implementing it? Explain what compelling, important scenario is enabled by this feature and we'll consider implementing it in the future. I don't see any compelling, important scenario that justifies it, but perhaps there is one that I've missed.

What is the purpose of 'this' keyword in C#

There are a couple of cases where it matters:

If you have a function parameter and a member variable with the same names, you need to be able to distinguish them:
```
class Foo {
  public Foo(int i) { this.i = i; }

  private int i;
}
```
If you actually need to reference the current object, rather than one of its members. Perhaps you need to pass it to another function:
```
class Foo {
  public static DoSomething(Bar b) {...}
}

class Bar {
  public Bar() { Foo.DoSomething(this); }
}
```
Of course the same applies if you want to return a reference to the current object

Is using this keyword with every class variable of class is good practice?

It wont effect your compile time, or your run time.
it's not a good practice neither.

the reason for using this keyword is to separate between class props then to another properties inside a particular section.

If your talking about code convention and styling, just adopt Microsofts one:

https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/inside-a-program/coding-conventions

you should read Microsoft documentation:

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/this

Is `this` keyword optional when accessing members in C#?

Yes, it's optional. The only times you'd have to use it are when you have a local variable that hides a member variable, or you want to refer to an indexed property (aka indexer).

In C#, Is "This" Keyword Required