What is “this” keyword in C#? When to use it?

In C#, "this" keyword refers to the current instance of the class. It is used to distinguish the class members (variables, methods) from the parameters when they have the same names.

Here is how you can use it:

1. To refer to the current instance of the class: If you have a method in a class that needs to refer to the object that the method was called on, you can use "this" keyword.

public class MyClass
{
    private int myVariable;

    public void SetVariable(int myVariable)
    {
        this.myVariable = myVariable;
    }
}

In the above example, "this.myVariable" refers to the class member and "myVariable" refers to the method parameter.

2. To pass the current object as a parameter to other methods: Sometimes, you may need to pass the current object to another method. In such cases, you can use "this" keyword.

public class MyClass
{
    public void MyMethod()
    {
        OtherMethod(this);
    }

    public void OtherMethod(MyClass myClass)
    {
        // Do something with myClass
    }
}

In the above example, "this" in "OtherMethod(this)" refers to the current instance of MyClass.

3. To declare indexers: "this" keyword is also used to declare indexers.

public class MyClass
{
    private int[] array = new int[100];

    public int this[int i]
    {
        get { return array[i]; }
        set { array[i] = value; }
    }
}

In the above example, "this" keyword is used to declare an indexer that allows instances of MyClass to be indexed just like arrays.