Regarding System.Object Constructor class in c#

Question

Object class constructor is called in the constructor of the class when an object is created . What happens in the Object Constructor?

Answer 1

In the Reference Source for object, this is the code for the constructor:

// Creates a new instance of an Object.
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
[System.Runtime.Versioning.NonVersionable]
public Object()
{            
}

Nothing happens there.

In the comments you ask how the class members are initialized to their default value. The Main() method in the following program...

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Program program = new Program();
        }
    }
}

is translated by the compiler to the following MSIL:

IL_0000:  nop
IL_0001:  newobj     instance void ConsoleApp1.Program::.ctor()
IL_0006:  stloc.0
IL_0007:  ret

The interesting instruction here is newobj. Which among other things:

allocates a new instance of the class associated with ctor and initializes all the fields in the new instance to 0 (of the proper type) or null references as appropriate.

So newobj initializes all class members to some type of 0 or to null.

In the comments you ask what happens if you initialize a field to a specific value. If we modify the program above:

namespace ConsoleApp1
{
    class Program
    {
        private int i = 1;

        public Program()
        {
            i = 2;
        }

        static void Main(string[] args)
        {
            Program program = new Program();
        }
    }
}

We have added a field i that is initialized to 1 and a constructor that sets i to 2.

The MSIL for the constructor of the Program class looks like this:

IL_0000:  ldarg.0
IL_0001:  ldc.i4.1
IL_0002:  stfld      int32 ConsoleApp1.Program::i
IL_0007:  ldarg.0
IL_0008:  call       instance void [mscorlib]System.Object::.ctor()
IL_000d:  nop
IL_000e:  nop
IL_000f:  ldarg.0
IL_0010:  ldc.i4.2
IL_0011:  stfld      int32 ConsoleApp1.Program::i
IL_0016:  ret

So now,

• newobj creates the object and initializes it's memory (setting i to 0).
• The constructor of Program runs and ldc.i4.1 followed by stfld ... i sets i to 1.
• Then the constructor of the base class System.Object is called.
• And then (still in the constructor of Program) i is set to 2 (ldc.i4.2 followed by stfld ... i).

So effectively i is set 3 times (to 0, 1, and 2) and when the constructor of the base class runs i has a different value then when the constructor of Program finishes.

For the order in which initializers and constructors run see these posts by Eric Lippert.