C# this Keyword

In C#, this keyword refers to the current instance of a class. For example,

using System;
 
namespace ThisKeyword {
  class Test {

    int num;
    Test(int num) {
      // this.num refers to the instance field
      this.num = num;
      Console.WriteLine("object of this: " + this);
    }

    static void Main(string[] args) {

      Test t1 = new Test(4);
      Console.WriteLine("object of t1: " + t1);
      Console.ReadLine();
    }
  }
}

Output

object of this: ThisKeyword.Test
object of t1: ThisKeyword.Test

In the above example, we have created an object named t1 of the class Test. We have printed the name of the object t1 and this keyword of the class.

Here, we can see the name of both t1 and this is the same. This is because this keyword refers to the current instance of the class which is t1.

Here are some of the major uses of this keyword in C#.

C# this with Same Name Variables

We cannot declare two or more variables with the same name inside a scope (class or method). However, instance variables and parameters may have the same name. For example,

using System;
 
namespace ThisKeyword {
  class Test {

    int num;
    Test(int num) {

      num = num;
    }

    static void Main(string[] args) {

      Test t1 = new Test(4);
      Console.WriteLine("value of num: " + t1.num);
      Console.ReadLine();
    }
  }
}

Output

0

In the above program, the instance variable and the parameter have the same name: num. We have passed 4 as a value to the constructor.

However, we are getting 0 as an output. This is because the C# gets confused because the names of the instance variable and the parameter are the same.

We can solve this issue by using this.

Example: this with Same Name Variables

using System;
 
namespace ThisKeyword {
  class Test {

    int num;
    Test(int num) {
      
      // this.num refers to the instance field
      this.num = num;
    }

    static void Main(string[] args) {

      Test t1 = new Test(4);
      Console.WriteLine("value of num: " +t1.num);
      Console.ReadLine();
    }
  }
}

Output

value of num: 4

Now, we are getting the expected output that is 4. It is because this.num refers to the instance variable of the class.

So, there is no confusion between the names of the instance variable and the parameter.

Invoke Constructor of the Same Class Using this

While working with constructor overloading, we might have to invoke one constructor from another constructor. In this case, we can use this keyword. For example,

using System;
 
namespace ThisKeyword {
  class Test {
    
    Test(int num1, int num2) {

      Console.WriteLine("Constructor with two parameter");
    }
    
    // invokes the constructor with 2 parameters
    Test(int num) : this(33, 22) {

      Console.WriteLine("Constructor with one parameter");
    }

    public static void Main(String[] args) {

      Test t1 = new Test(11); 
      Console.ReadLine();   
    }
  }
}

Output

Constructor with two parameter
Constructor with one parameter

In the above example, we have used : followed by this keyword to call constructor Test(int num1, num2) from the constructor Test(int num).

When we call the Test(int num) constructor the Test(int num1, int num2) constructor executes first.

Note: Calling one constructor from another constructor is known as constructor chaining.

C# this as an object argument

We can use this keyword to pass the current object as an argument to a method. For example,

using System;
 
namespace ThisKeyword {
  class Test {
    int num1;
    int num2;
      
    Test() {
      num1 = 22;
      num2 = 33;
    }

    // method that accepts this as argument   
    void passParameter(Test t1)
 {
      Console.WriteLine("num1: " + num1);
      Console.WriteLine("num2: " + num2);
    }

    void display() {
      // passing this as a parameter
      passParameter(this);
    }
  
    public static void Main(String[] args) {
      Test t1 = new Test();
      t1.display();
      Console.ReadLine();
    }
  }
}

Output

num1: 22
num2: 33

In the above program, we have a method passParameter(). It accepts the object of the class as an argument.

passParameter(this);

Here, we have passed this to the passParameter() method. As this refers to the instance of the class, we are able to access the value of num1 and num2.

this to declare a C# indexer

Indexers allow objects of a class to be indexed just like arrays. We use this keyword to declare an indexer in C#. For example,

using System;
namespace ThisKeyword {
      
  class Student {
      
    private string[] name = new string[3];
  
    // declaring an indexer
    public string this[int index] {

      // returns value of array element
      get {
        return name[index];
      }
      
      // sets value of array element
      set { 
        name[index] = value;
      }
    }
  }
  
  class Program {
  
    public static void Main() {
      Student s1 = new Student();
      s1[0] = "Ram";
      s1[1] = "Shyam";
      s1[2] = "Gopal";

      for (int i = 0; i < 3; i++) {

        Console.WriteLine(s1[i] + " ");
      }
    }
  }
}

Output

Ram
Shyam
Gopal

In the above program, we have created an indexer using this keyword.

The array name[] is private. So, we cannot access it from the Program class.

Now, to access and set the value of the array, we use an indexer.

Student s1 = new Student();
s1[0] = "Ram";
s1[1] = "Shyam";
s1[2] = "Gopal";

As we have used this to create an indexer, we must use the object of the Student class to access the indexer. To know more about the indexer, visit C# indexer.