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

Question

Since variables declared inside a method are available only within that method, and variables declared private within a class are only available within a class. What is the purpose of the this key word? Why would I want to have the following:

private static class SomeClass : ISomeClass
{
    private string variablename;
    private void SomeMethod(string toConcat)
    {
        this.variablename = toConcat+toConcat;
        return this.variablename;
    }
}

When this will do the exact same thing:

private static class SomeClass : ISomeClass
{
    private string variablename;
    private void SomeMethod(string toConcat)
    {
        variablename = toConcat+toConcat;
        return variablename;
    }
}

to practice my typing skills?

Answer 1

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

Answer 2

In your example it is purely a matter of taste, but here is a case where it is not:

private string SomeMethod(string variablename)
{
    this.variablename = variablename;
    return this.variablename;
}

Without this, the code would work differently, assigning the parameter to itself.