Question

Can someone please explain the following constructor syntax to me. I haven’t come across it before and noticed it in a colleagues code.

public Service () : this (Service.DoStuff(), DoMoreStuff())
{ }

Answer 1

It chains to another constructor in the same class. Basically any constructor can either chain to another constructor in the same class using : this (...), or to a constructor in the base class using : base(...). If you don’t have either, it’s equivalent to : base().

The chained constructor is executed after instance variable initializers have been executed, but before the body of the constructor.

See my article on constructor chaining or the MSDN topic on C# constructors for more information.

As an example, consider this code:

using System;

public class BaseClass
{
    public BaseClass(string x, int y)
    {
        Console.WriteLine("Base class constructor");
        Console.WriteLine("x={0}, y={1}", x, y);
    }
}

public class DerivedClass : BaseClass
{
    // Chains to the 1-parameter constructor
    public DerivedClass() : this("Foo")
    {
        Console.WriteLine("Derived class parameterless");
    }

    public DerivedClass(string text) : base(text, text.Length)
    {
        Console.WriteLine("Derived class with parameter");
    }

}

static class Test
{
    static void Main()
    {
        new DerivedClass();
    } 
}

The Main method calls the parameterless constructor in DerivedClass. That chains to the one-parameter constructor in DerivedClass, which then chains to the two-parameter constructor in BaseClass. When that base constructor completes, the one-parameter constructor in DerivedClass continues, then when that finishes, the original parameterless constructor continues. So the output is:

Base class constructor
x=Foo, y=3
Derived class with parameter
Derived class parameterless