How I can pass values from a Windows Form to Another (from Child Window to Parent Window)

In your child form, declare a publicly visible variable. Then from the parent form, access it by child.variable.

public class childform
{
public int variable;
}

in parent, you can read/pass the value by
child = new childform();
child.variable = 1;

You may also wish to declare it as
static public int variable
if you do not wish to create an instance of the childform first.

You could create a Class that holds the values you want to store. Pass the Class as a parameter to your child form.

MyClass cls = new MyClass();
MyForm frm = new MyForm(cls);
frm.Show(); // Or ShowDialog();
string something = cls.Value; // Although it probably won't have a value here yet...

All of this is of course pretty basic .NET stuff... You should consider reading some books and/or articles[^] :thumbsup:

on the child form add a button and on the click event if certain conditions are met use

string something = "some value"; this.DialogResult=DialogResult.OK;

and if the conditions are not met then

this.DialogResult=DialogResult.Cancel;

Now in the parent form set if the child form's DialogResult is DialogResult.OK then set child form's string to textBox field on parent form like this.

ChildForm childform = new ChildForm();                                                   if (cildform.ShowDialog() == DialogResult.Cancel)                                      return;                                                       textBox1.Text=childform.something;