can there be 'n' Constructor s in class ?

SUNIL GUTTA

Well as everyone know constructor is must either implicit or explicit one :)

Well i hope there will be only single constructor static or non-static . is there any hope there will be 2 constructors in single class ? If so how it works

Thank you

8 Replies

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Vulpes13y ago
Accepted answer
A class can contain at most one static constructor.

However, it can contain any number of instance constructors.

If you don't provide an instance constructor, then the system provides a default parameterless constructor which does nothing except call the base class's constructor.

Here's an example of a class which contains two instance constructors. Notice how the constructor which takes a single int parameter 'chains' to the other constructor (using the 'this' keyword) which takes int and string parameters, passing the latter a default value:

using System;

class MyClass
{
public int MyInt {get; set;}
public string MyString {get; set;}

public MyClass(int i) : this(i, "Hello")
{
}

public MyClass(int i, string s)
{
MyInt = i;
MyString = s;
}
}

class Test
{
static void Main()
{
MyClass mc = new MyClass(3);
Console.WriteLine(mc.MyInt);
Console.WriteLine(mc.MyString);
MyClass mc2 = new MyClass(4, "Goodbye");
Console.WriteLine(mc2.MyInt);
Console.WriteLine(mc2.MyString);
Console.ReadKey();
}
}

The output of this program is:

3
Hello
4
Goodbye
Vulpes13y ago
I'd deal with that using a variation of Jignesh's method as follows:

// form 1

private void button1_Click(object sender, EventArgs e)
{
Form2 f2 = new Form2(comboBox1);
f2.Show();
}

// form2

public partial class Form2 : Form
{
private ComboBox cb;

public Form2(ComboBox cb)
{
InitializeComponent();
this.cb = cb;
if (cb.SelectedIndex > -1) label1.Text = cb.SelectedItem.ToString();
this.cb.SelectedIndexChanged += cb_SelectedIndexChanged;
}

private void cb_SelectedIndexChanged(object sender, EventArgs e)
{
if (cb.SelectedIndex > -1) label1.Text = cb.SelectedItem.ToString();
}

// any other code
}

When you click the button, the label on form2 should show the currently selected item in the combobox.

Moreover, if you subsequently change the selected item in the combobox on form1, the new item should show automatically in the label on form2.
SUNIL GUTTA13y ago
Well hie ty for your appraach details in detail

i tried like this . I got a doubt did i understood question right ?

I hope i understood question right i.e i used combobox1 & button1 on form 1 and when i click button1 the data selected in combobox1 should get in form2 component like label1 :)

Coming to your approach i tried can you please put me in programming style i can understand much better & debugg to know whats going on

TY
Vulpes13y ago
To access a control on another form you generally need two things:

1. A reference to the Form object.

2. A reference to the control itself.

There are several ways to get #1 but one way which always works without the need for any further code is to use the Application.OpenForms property. As its name suggests, this is a collection of all open forms in the application. You can find a form in this collection either by its index (starting from 0 for the main form) or by its Name property. For example:

Form1 frm1 = (Form1)Application.OpenForms["Form1"];

To get #2, there are again several ways of proceeding.

One way is to simply alter the Modifiers property of the control from private (the default) to public or internal. You can then access the control directly. For example:

ComboBox cb = frm1.comboBox1;

However, some programmers don't like changing a control's access modifier from private because they feel that this breaks encapsulation. A second way to get #2 is therefore to use the Controls property.

Every control has a container which is either the form itself or some container within the form such as a panel or group box. Each container has its own Controls collection.

So, if a control has been placed directly on the form, you can use the form's Controls collection:

ComboBox cb = (ComboBox)frm1.Controls["comboBox1"];

If it were on a panel, you'd need to 'dig down' to the panel's controls collection:

Panel p = (Panel)frm1.Controls["panel1"];
ComboBox cb = (ComboBox)p.Controls["comboBox1"];

A third way (used by Jignesh in the thread you referred to) is to simply pass a reference to the control in the second Form's constructor when this is instantiated and displayed by the first form. To do this you need, of course, to change the constructor to take a parameter of the appropriate type and to save it to a field so that it can be accessed by any method within the other form:

class Form2 : Form
{
private ComboBox cb;

public Form2 (ComboBox cb)
{
this.cb = cb;
}

// rest of code
}

If you use this method, then there is no need to get a reference to the containing form itself.
SUNIL GUTTA13y ago
hi this question not regarding above :) tried to message but i don't have that option

http://www.c-sharpcorner.com/Forums/Thread/201338/

can you explain me this in your simple way so i can understand your type of dealing .

Thank u for burden & i tried to go through ur explanation in that post but i'm confused
Vulpes13y ago
You're right that having multiple instance constructors is known as 'constructor overloading'.

Your own program, which doesn't use constructor chaining, is working fine. Notice that when you use the constructor which takes a single int argument, then 's' is set to its default value of null.

When you have something like this:

public MyClass(int i) : this(i, "Hello")
{
}

it executes as though it were written like this:

public MyClass(int i)
{
MyClass(i, "Hello);
}

though if you try to write it like that you'll get a compiler error.

So, one constructor is calling another which saves you having to copy the second constructor's code into the first. Hence the term : 'constructor chaining'.

If you're using C# 4.0 or later, you can also use a single constructor with an optional parameter to achieve the same result:

public MyClass(int i, string s = "Hello")
{
MyInt = i;
MyString = s;
}

If someone calls this constructor with only a single int parameter, then the second parameter is given its default value of "Hello".

For more details on constructor chaining and constructors generally, I'd check out this link:

http://msdn.microsoft.com/en-us/library/vstudio/ms173115(v=vs.100).aspx
SUNIL GUTTA13y ago
Notice how the constructor which takes a single int parameter 'chains' to the other constructor (using the 'this' keyword) which takes int and string parameters,

Chains ?? i never used 'this' keyword at end of constructor like

public MyClass(int i) : this(i, "Hello")
{
}
can you plz explain it ?
Can you refer me any material or anything to find such different approaches .
SUNIL GUTTA13y ago
Gotcha well the process is called constructor overloading ri8 :)

Ty for help

AM i right please do check below program .! do that justify my question ? i hope it does .! ur view plz

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Accessspecifiers
{
class constructor2
{
int i; string s;
public constructor2(int i)

{
this.i = i;
}

public constructor2(int i, string s)
{
this.i = i;
this.s= s;
}

static void Main()
{
constructor2 mc = new constructor2(3);
Console.WriteLine(mc.i);
constructor2 mc2 = new constructor2(4, "All the best");
Console.WriteLine(mc2.i);
Console.WriteLine(mc2.s);
Console.ReadKey();
}
}
}