this statement after contructor's arguments [duplicate]

Question

I saw this code block when i was trying build something with APN. Could someone explain me what does "this" statements do there ?

public ApplePushService(IPushChannelFactory pushChannelFactory, ApplePushChannelSettings channelSettings)
        : this(pushChannelFactory, channelSettings, default(IPushServiceSettings))

Is it like default values of those arguments ?

Answer 1

this calls the overloaded constructor for the ApplePushService class with the specified parameters.

For example

// Set a default value for arg2 without having to call that constructor
public class A(int arg1) : this(arg1, 1) 
{
}

public class A(int arg1, int arg2)
{
}

This lets you call one constructor that can invoke another.

Answer 2

Sure - that chains one constructor onto another. There are two forms - this to chain onto another constructor in the same class, and base to chain to another constructor in the base class. The body of the constructor you're chaining to executes, and then your constructor body executes. (Of course, the other constructor may chain onto another one first.)

If you don't specify anything, it automatically chains to a parameterless constructor in the base class. So:

public Foo(int x)
{
    // Presumably use x here
}

is equivalent to

public Foo(int x) : base()
{
    // Presumably use x here
}

Note that instance variable initializers are executed before the other constructor is called.

Surprisingly, the C# compiler doesn't detect if you end up with mutual recursion - so this code is valid, but will end up with a stack overflow:

public class Broken
{
    public Broken() : this("Whoops")
    {
    }

    public Broken(string error) : this()
    {
    }
}

(It does prevent you chaining onto the exact same constructor signature, however.)

For more details, see my article on constructor chaining.