C# constructor execution order

Question

The order is:

• Member variables are initialized to default values for all classes in the hierarchy

Then starting with the most derived class:

• Variable initializers are executed for the most-derived type
• Constructor chaining works out which base class constructor is going to be called
• The base class is initialized (recurse all of this :)
• The constructor bodies in the chain in this class are executed (note that there can be more than one if they're chained with Foo() : this(...) etc

Note that in Java, the base class is initialized before variable initializers are run. If you ever port any code, this is an important difference to know about :)

I have a page with more details if you're interested.

It will call the base constructor first. Also keep in mind that if you don't put the :base(param1) after your constructor, the base's empty constructor will be called.

The constructor of the baseclass is called first.

Not sure if this should be a comment/answer but for those who learn by example this fiddle illustrates the order as well: https://dotnetfiddle.net/kETPKP

using System;

// order is approximately
/*
   1) most derived initializers first.
   2) most base constructors first (or top-level in constructor-stack first.)
*/
public class Program
{
    public static void Main()
    {
        var d = new D();
    }
}

public class A
{
    public readonly C ac = new C("A");

    public A()
    {
        Console.WriteLine("A");
    }
    public A(string x) : this()
    {
        Console.WriteLine("A got " + x);
    }
}

public class B : A
{
    public readonly C bc = new C("B");

    public B(): base()
    {
        Console.WriteLine("B");
    }
    public B(string x): base(x)
    {
        Console.WriteLine("B got " + x);
    }
}

public class D : B
{
    public readonly C dc = new C("D");

    public D(): this("ha")
    {
        Console.WriteLine("D");
    }
    public D(string x) : base(x)
    {
        Console.WriteLine("D got " + x);
    }
}

public class C
{
    public C(string caller)
    {
        Console.WriteLine(caller + "'s C.");
    }
}

Result:

D's C.
B's C.
A's C.
A
A got ha
B got ha
D got ha
D

[Edit: in the time it took me to answer, the question had totally changed].

The answer is that it calls the base first.

[Original answer to the old question below]

Are you asking when you would do the "base" bit of the constructor call?

If so, you would "chain" a call to the constructor base if the class is derived from another class which has this constructor:

  public class CollisionBase
    {
        public CollisionBase(Body body, GameObject entity)
        {

        }
    }

    public class TerrainCollision : CollisionBase
    {
        public TerrainCollision(Body body, GameObject entity)
            : base(body, entity)
        {

        }
    }

In this example, TerrainCollision derives from CollisionBase. By chaining the constructors in this way, it ensures the specified constructor is called on the base class with the supplied parameters, rather than the default constructor (if there is one on the base)