Use of colon in C# constructor header

Question

Below is the constructor of a struct named Complex with two member variables, Real and Imaginary:

public Complex(double real, double imaginary) : this()
{
 Real = real;
 Imaginary = imaginary;
}

What is the use of the part after the colon in the function header?

Answer 1

You can always make a call to one constructor from within another. Say, for example:

public class mySampleClass
{
    public mySampleClass(): this(10)
    {
        // This is the no parameter constructor method.
        // First Constructor
    }

    public mySampleClass(int Age) 
    {
        // This is the constructor with one parameter.
        // Second Constructor
    }
}

this refers to same class, so when we say this(10), we actually mean execute the public mySampleClass(int Age) method. The above way of calling the method is called initializer. We can have at the most one initializer in this way in the method.

In your case it going to call default constructor without any parameter

Answer 2

It's called constructor chaining - where it's in fact calling another constructor (which takes no params in this case), and then comes back and does any additional work in this constructor (in this case setting the values of Real and Imaginary).