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Pass value from parent form to child form?

I'm trying to pass a value set in my parent form to a second form. I created a property with a get part in the parent form.
I don't want to do something like:
Form2 secondForm = new Form2(value);
It is an already exiting form and I don't want to keep creating a new form every time I want to pass a value.

See this example.
1-Create a window form application,Declare a public string global variable in Form1 , using this variable we can pass value from Form1 to Form2.
2-Now in form2 ,Create a object for Form1 and Get the value using this object.
See image

You have some possibilities here:
Give a Reference from your first Form as value
Form2 secondForm = new Form2(yourForm1);
So you can access via the getter in your first Form. yourForm1.MyValue;
This seems to be a bit ugly. Better is you create a Interface, which hold your Property and is implemented from you first Form.
public interface IValueHolder
{
public int MyValue {get;}
}
public class FirstForm : Form, IValueHolder
{
public int MyValue{get;}
//Do your form stuff
Form2 form = new Form2(this);
}
So your Form2 just take the Interface and stays independent from Form1. Further you can create a Property on Form2 which you access from Form1. For example if your Property in Form1 changes you set the value from Form2 as well.
public class Form2 : Form
{
public int MyValue{get;set;}
}
public class Form1 : Form
{
private int _myValue;
public int MyValue
{
set
{
if (_myValue != value)
{
form2.MyValue = value;
}
}
}
}
At least you can use a Event maybe. Further you can create a Property on Form2 which holds an Form1 Reference or a IValueHolder as described above.
Hope this helps.

I'm not sure about how you are going to use the Form2 in the parent(let it be frmParent). anyway you can follow any of the steps below:
Define the property in the child form as static so that you can access that by using Form2.frmProperty.
Or define the property as public and then access through the class instance, so that you can access the variable through that instance as long as the instance existed. something like the following:
Form2 secondFormInstance = new Form2();
secondFormInstance.frmProperty = 10;
// at some later points
int childValue = secondFormInstance.frmProperty; // take value from that variable
secondFormInstance.frmProperty++; // update the value
Or you can use like what you specified in the question.

Passing data between forms (and back)

On Form1 I have a label with my total points. In Form2 I made a store were you can purchase things with those points. However I am able to pass the points value to Form2 but I can't figure out how I can send the points value back to Form1.
The code I use to send the value from Form1 to Form2.
Inventory inventory = new Inventory();
inventory.points = points.
I used the search function but since I just started writing code I find most of the given answers too confusing.

Code a constructor for form2 class as below:
public Form2(string strTextBox)
{
InitializeComponent();
label1.Text=strTextBox;
}
On Form1 where you want to call Form2 and pass a value
Form2 frm=new Form2(textBox1.Text);
frm.Show();
Add a property in Form1 to retrieve value from textbox:
public string _textBox1
{
get{return textBox1.Text;}
}
On Form2:
public string _textBox
{
set{label1.Text=value;}
}

Pass the Form1 instance to Form2 when setting the points.
This requires a Form1 public variable in Form2.
e.g
Inventory inventory = new Inventory();
inventory.points = points;
inventory.form1 = this;
and when sending back in Form2
this.form1.points = this.points;
I hope this helps!

Change Properties of Form form1 with "this" but not Form1

Inside this scope,
namespace Project1
{
public partial class Form1 : Form
{
public Form1()
{
//Change Form1 properties here
}
}
}
If change Properties DoubleBuffered via either
Project1.Form1.DoubleBuffered = false;
or
Form1.DoubleBuffered = false;
then happen error
An object reference is required for the non-static field, method, or property 'System.Windows.Forms.Control.DoubleBuffered.get'
But, if change via
this.DoubleBuffered = false;
then the error is solved. I mouse hovered the this and it showed
class Project1.Form1
Can anyone explain why 'this' keyword represent the Project1.Form1 but I have error if I replace 'this' with Project1.Form1 or Form1? thanks

In your case, this keyword refer to current instance of Form1. And explicitly mentioning Form1 means you are referring to Form1 as class name.
All properties that is not declared as static associated to instance of the class, not the class it self. Hence, you can only access the property from the class instance.
That behavior is make sense because you can have more than one instance of Form1 each can have different value of DoubleBuffered property :
var instance1 = new Form1();
var instance2 = new Form1();
instance1.DoubleBuffered = true;
instance2.DoubleBuffered = false;

request for explaining code on form's reference

How can the lines mark with * in Form2 be referred back to Form1?
I mean when Form1 is instantiated, it can only be referenced by
the name Form1. But in the lines with *, Form1 is used as a type
not a object. However, you can use the m_parent as reference for
Form1 in the form called Form2
(I hope someone may get what I was trying to ask.)
public partial class Form2 : Form
{
* private Form1 m_parent;
* public Form2(Form1 frm1)
{
InitializeComponent();
* m_parent = frm1;
}
}
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
}

What you have there is a constructor for Form2 which takes a reference to a Form1 instance. On Form1 there may be code which looks something like:
var form = new Form2(this);
form.Show();
What is happening is you create Form2 with a reference to the current form and show it. It now has access to public members of Form1

Both Form1 and Form2 are normal classes that can be instantiated.
In the Form2 class there is a constructor wich takes an instance of Form1 object as parameter. In the constructor's body the parameter (the reference to the Form1 object) is then storen in m_parent variable.

getting textbox value from another form

I have an TextBox named pass in Form1 that I need to get the value of in form2. I tried this:
public partial class Form1 : Form {
public string GetPass() {
return pass.Text;
}
}
public partial class form2 : Form {
//...
MessageBox.Show(new Form1().GetPass());
}
The above code returns an empty string, why?

You are not showing your actual code as evidenced by the syntax errors etc. - the only logical explanation for your problem is that you are not passing the reference to Form1 correctly to Form2, but create a new form instead - that new form would have the empty textbox.
To further help you, please show how you pass the reference to your Form1 in your actual code.
Edit:
Is see your edit now and above is exactly the problem. You have to pass a Form1 instance to form2 instead of creating a new one, i.e.:
public partial class form2 : Form
{
private Form1 form1;
public form2(Form1 otherForm)
{
form1 = otherForm;
}
public void Foo()
{
MessageBox.Show(form1.GetPass());
}
}

Define one string variable as Public in declaration section
for ex. we have a form with name "frmOne"
public string strVar = string.Empty;
Now, assign the value of TextBox of "frmOne" to that variable from where you are getting the value of Textbox.
for ex.
strVar = Textbox1.Text.ToString();
Now in another form say "frmTwo", you will get access the value of that textbox of "frmOne" something like that (where you want to get the value) :
frmOne frm = new frmOne();
string strValue = frm.strVar;
So, finally strValue local variable of frmTwo contains the value of Textbox of frmOne.

You are creating a NEW form1 where the textbox is likely to be blank, and calling GetPass() on that empty form. You need an instance of the already-opened form1 where the textbox might have a value.

Because you are creating a new instance of Form1 each time you call GetPass().
You need to get the instance of the opened form1 one way or another, and call GetPass on it:
form1.GetPass();
If there is no specifics on the order of creation of form1 and form2, you can use the following to get the instance of form1:
foreach (Form openedForm in Application.OpenForms) {
if (openedForm.GetType() == Form1) {
MessageBox.Show(openedForm.GetPass());
}
}

It's returning empty because you're creating a new instance of the form. Assuming that Form1 is already open somewhere, you need to retrieve the existing instance of Form1 and pull the value from there.

hi you can write this :
public partial class Form1: Form
{
public Form1()
{
InitializeComponent();
}
internal Form2 F2=new form2();
private void CommandBarButton1_Click(object sender, EventArgs e)
{
MessageBox.Show(f2.TextBox1.Text);
}
}