How call constructor inside other constructor?

Question

My class has quite some properties and one of my constructors sets them all, I want the default constructor to call this other one and use the set properties. But I need to prepare arguments first, so calling it from the header won't help.

Here's what I'd like to do:

public class Test
{
    private int result, other;

        // The other constructor can be called from header,
        // but then the argument cannot be prepared.
        public Test() : this(0)
        {
        // Prepare arguments.
        int calculations = 1;

        // Call constructor with argument.
        this(calculations);

        // Do some other stuff.
        other = 2;
    }

    public Test(int number)
    {
        // Assign inner variables.
        result = number;
    }
}

So this is not possible, does anyone know how to call my constructor to set parameters inside code? Currently I'm storing a copy of the object from the other constructor and copying all the properties, it's really annoying.

Charles Bretana · Accepted Answer · 2015-01-28T14:56:41.910

Yes. just pass the value 1 in the first call

public class Test
{    
    private int result, other;
    public Test() : this(1)        
    {        
       // Do some other stuff.        
       other = 2;    
    }    
    public Test(int number)    
    {        
        // Assign inner variables.        
        result = number;    
    }

}

And why can't you prepare the argument?

public class Test
{    
    private int result, other;
    public Test() : this(PrepareArgument())
    {        
       // Do some other stuff.        
       other = 2;    
    }    
    public Test(int number)    
    {        
        // Assign inner variables.        
        result = number;    
    }
    private int PrepareArgument()
    {
         int argument;
         // whatever you want to do to prepare the argument
         return argument;
    }
}

or... if the prepareArgument is to be called from the default constructor, then it cannot be dependent on any passed in arguments. If it is a constant, just make it a constant ...

public class Test
{   
    const int result = 1;
    private int result, other;
    public Test() 
    {        
       // Do some other stuff.        
       other = 2;    
    }    
}

if it's value is dependent on other external state, then you might rethink your design... Why not calculate it before calling the constructor in the first place?

Din't think of putting the preperation in a seperate method, it's not ideal but it will work, thanks. — MrFox, Nov 14 '10 at 22:28
And depending on how complex it is, it could also be expressed inside the this() cpnstruction — Charles Bretana, Nov 14 '10 at 22:33
Just a small note: `PrepareArgument` must be `static` in the second approach. — Saeb Amini, Jan 28 '15 at 10:14

score 1 · Answer 2 · answered Nov 14 '10 at 22:16

Sounds like a little refactoring will help. While you can't do exactly what you want to do, there are always alternatives. I realize your actual code might be more complicated than the code you gave, so take this for an example refactoring of your problem. Extracting all common code into a new SetVariables method.

public class Test
{
    private int result, other;

        // The other constructor can be called from header,
        // but then the argument cannot be prepared.
        public Test() : this(0)
        {
        // Prepare arguments.
        int calculations = 1;

        // Call constructor with argument.
        SetVariables(calculations);

        // Do some other stuff.
        other = 2;
    }

    public Test(int number)
    {
        // Assign inner variables.
        SetVariables(number);
    }

    private void SetVariables(int number)
    {
        result = number;
    }
}

score 0 · Answer 3 · answered Nov 14 '10 at 22:16

0

If you want to call a constructor, but can't do it from the header, the only way is to define a private Initialize method. It appears you are doing "real work" in your constructor, which is not the task of a constructor. You should think of a different way to achieve your goal as your way severely prohibits debugging and others from reading your code.

answered Nov 14 '10 at 22:16

Femaref

60,705
7
138
176

Why are we talking about headers and C# in the same topic? (not you so much as the OP) – Ed S. Nov 14 '10 at 22:17
I meant the header of the constructor, ie. constructor chaining, and that should be the same he was talking about as well. – Femaref Nov 14 '10 at 22:36

How call constructor inside other constructor?

3 Answers3

Linked