Question

Why this doesn’t accept a parameter of a class-type? What can I read about it? Here is my useless code

using System;

public class Class1
{
  public int a = 5;
}

public class Class2
{
  private readonly int num;
  Class1 obj2 = new Class1();
  public Class2(Class1 obj)
  {
    num = obj.a;
  }
  public Class2(string l) : this (Class1 obj2)
  {

  }
}

Answer 1

Your constructor takes an instance of Class1, so you should use:

public Class2(string l) : this (new Class1())
{
}

You will probably also want to assign the constructor argument to your obj2 member instead of creating a new one:

public class Class2
{
    private readonly int num;
    Class1 obj2;
    public Class2(Class1 obj)
    {
        this.obj2 = obj;
        num = obj.a;
    }
    public Class2(string l) : this (new Class1()) { }
}