Call one constructor from another

Like this:

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

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);
}

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.
   }
}

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);
    }
}

Here is an example that calls another constructor, then checks on the property it has set.

    public SomeClass(int i)
    {
        I = i;
    }

    public SomeClass(SomeOtherClass soc)
        : this(soc.J)
    {
        if (I==0)
        {
            I = DoSomethingHere();
        }
    }

Yeah, you can call other method before of the call base or this!

public class MyException : Exception
{
    public MyException(int number) : base(ConvertToString(number)) 
    {
    }

    private static string ConvertToString(int number) 
    { 
      return number.toString()
    }

}

Constructor chaining i.e you can use "Base" for Is a relationship and "This" you can use for same class, when you want call multiple Constructor in single call.

  class BaseClass
{
    public BaseClass():this(10)
    {
    }
    public BaseClass(int val)
    {
    }
}
    class Program
    {
        static void Main(string[] args)
        {
            new BaseClass();
            ReadLine();
        }
    }

When you inherit a class from a base class, you can invoke the base class constructor by instantiating the derived class

class sample
{
    public int x;

    public sample(int value)
    {
        x = value;
    }
}

class der : sample
{
    public int a;
    public int b;

    public der(int value1,int value2) : base(50)
    {
        a = value1;
        b = value2;
    }
}

class run 
{
    public static void Main(string[] args)
    {
        der obj = new der(10,20);

        System.Console.WriteLine(obj.x);
        System.Console.WriteLine(obj.a);
        System.Console.WriteLine(obj.b);
    }
}

Output of the sample program is

50 10 20

You can also use this keyword to invoke a constructor from another constructor

class sample
{
    public int x;

    public sample(int value) 
    {
        x = value;
    }

    public sample(sample obj) : this(obj.x) 
    {
    }
}

class run
{
    public static void Main(string[] args) 
    {
        sample s = new sample(20);
        sample ss = new sample(s);

        System.Console.WriteLine(ss.x);
    }
}

The output of this sample program is

20

Error handling and making your code reusable is key. I added string to int validation and it is possible to add other types if needed. Solving this problem with a more reusable solution could be this:

public class Sample
{
    public Sample(object inputToInt)
    {
        _intField = objectToInt(inputToInt);
    }

    public int IntProperty => _intField;

    private readonly int _intField;
}

public static int objectToInt(object inputToInt)
{
    switch (inputToInt)
        {
            case int inputInt:
                return inputInt;
            break;
            case string inputString:
            if (!int.TryParse(inputString, out int parsedInt))
            {
                throw new InvalidParameterException($"The input {inputString} could not be parsed to int");
            }
            return parsedInt;

            default:
                throw new InvalidParameterException($"Constructor do not support {inputToInt.GetType().Name}");
            break;
        }
}

In case you need to run something before calling another constructor not after.

public class Sample
{
    static int preprocess(string theIntAsString)
    {
        return preprocess(int.Parse(theIntAsString));
    }

    static int preprocess(int theIntNeedRounding)
    {
        return theIntNeedRounding/100;
    }

    public Sample(string theIntAsString)
    {
        _intField = preprocess(theIntAsString)
    }

    public Sample(int theIntNeedRounding)
    {
        _intField = preprocess(theIntNeedRounding)
    }

    public int IntProperty  => _intField;

    private readonly int _intField;
}

And ValueTuple can be very helpful if you need to set more than one field.

Please, please, and pretty please do not try this at home, or work, or anywhere really.

This is a way solve to a very very specific problem, and I hope you will not have that.

I'm posting this since it is technically an answer, and another perspective to look at it.

I repeat, do not use it under any condition. Code is to run with LINQPad.

void Main()
{
    (new A(1)).Dump();
    (new B(2, -1)).Dump();
    
    var b2 = new B(2, -1);
    b2.Increment();
    
    b2.Dump();
}

class A 
{
    public readonly int I = 0;
    
    public A(int i)
    {
        I = i;
    }
}

class B: A
{
    public int J;
    public B(int i, int j): base(i)
    {
        J = j;
    }
    
    public B(int i, bool wtf): base(i)
    {
    }
    
    public void Increment()
    {
        int i = I + 1;

        var t = typeof(B).BaseType;
        var ctor = t.GetConstructors().First();
        
        ctor.Invoke(this, new object[] { i });
    }
}

Since constructor is a method, you can call it with reflection. Now you either think with portals, or visualize a picture of a can of worms. sorry about this.

In my case, I had a main constructor that used an OracleDataReader as an argument, but I wanted to use different query to create the instance:

I had this code:

public Subscriber(OracleDataReader contractReader)
    {
        this.contract = Convert.ToString(contractReader["contract"]);
        this.customerGroup = Convert.ToString(contractReader["customerGroup"]);
        this.subGroup = Convert.ToString(contractReader["customerSubGroup"]);
        this.pricingPlan= Convert.ToString(contractReader["pricingPlan"]);
        this.items = new Dictionary<string, Member>();
        this.status = 0;
        
        
    }

So I created the following constructor:

public Subscriber(string contract, string customerGroup) : this(getSubReader(contract, customerGroup))
    { }

and this method:

 private static  OracleDataReader getSubReader(string contract, string customerGroup)
    { 
        cmdSubscriber.Parameters[":contract"].Value = contract + "%";
        cmdSubscriber.Parameters[":customerGroup"].Value = customerGroup+ "%";
        return  cmdSubscriber.ExecuteReader();
        
    }

notes: a statically defined cmdSubscriber is defined elsewhere in the code; My main constructor has been simplified for this illustration.