Call one constructor from another

Before the body of the constructor, use either:

: base (parameters)

: this (parameters)

Example:

public class People: User
{
   public People (int EmpID) : base (EmpID)
   {
      // Add more statements here.
   }
}

If what you want can't be achieved satisfactorily without having the initialization in its own method (e.g. because you want to do too much before the initialization code, or wrap it in a try-finally, or whatever) you can have any or all constructors pass the readonly variables by reference to an initialization routine, which will then be able to manipulate them at will.

public class Sample
{
    private readonly int _intField;
    public int IntProperty => _intField; 

    private void setupStuff(ref int intField, int newValue) => intField = newValue;

    public Sample(string theIntAsString)
    {
        int i = int.Parse(theIntAsString);
        setupStuff(ref _intField,i);
    }

    public Sample(int theInt) => setupStuff(ref _intField, theInt);
}

I am improving upon supercat's answer. I guess the following can also be done:

class Sample
{
    private readonly int _intField;
    public int IntProperty
    {
        get { return _intField; }
    }

    void setupStuff(ref int intField, int newValue)
    {
        //Do some stuff here based upon the necessary initialized variables.
        intField = newValue;
    }

    public Sample(string theIntAsString, bool? doStuff = true)
    {
        //Initialization of some necessary variables.
        //==========================================
        int i = int.Parse(theIntAsString);
        // ................
        // .......................
        //==========================================

        if (!doStuff.HasValue || doStuff.Value == true)
           setupStuff(ref _intField,i);
    }

    public Sample(int theInt): this(theInt, false) //"false" param to avoid setupStuff() being called two times
    {
        setupStuff(ref _intField, theInt);
    }
}

Like this:

public Sample(string str) : this(int.Parse(str)) { }