Passing data between form1.cs and program.cs

A little while ago I've answered an old question which asked how to pass data between two different forms. The answer applies for your situation too. Just pick the one that fits your needs.

1- (Not recommended if there's too many parameters) Passing data through the constructor.

 private void ShowForm(int a, string b, double c)
{
    Form2 frm = new Form2(a, b, c);
    frm.ShowDialog();
}

2- Using public fields of target class. (NOT RECOMMENDED AT ALL)

 private void ShowForm(int a, string b, double c)
{
    Form2 frm = new Form2();
    frm.intval = a;
    frm.strval = b;
    frm.doubleval = c;
    frm.ShowDialog();
} 

3- Using properties.

 private void ShowForm(int a, string b, double c)
{
    Form2 frm = new Form2();
    frm.IntValue = a;
    frm.StringValue = b;
    frm.DoubleValue = c;
    frm.ShowDialog();
} 

4- Using tags.

private void ShowForm(int a, string b, double c)
{
        Form2 frm = new Form2();
        frm.SomeTextBox.Tag = a;
        frm.SomeTextBox2.Tag = b;
        frm.SomeTextBox3.Tag = c;
        frm.ShowDialog();
} 

5- Using delegates. (This one is a little bit tricky).

 //in Form2
public delegate void PassValues(int a, string b, double c);
public PassValues passVals;

private void PassDataThroughDelegate(int a, string b, double c)
{
    if(passVals != null)
        passVals(a,b,c);
}

//in Form1
private void ShowForm(int a, string b, double c)
{
    Form2 frm = new Form2();
    frm.passVals = new Form2.PassValues(UseData);
    frm.ShowDialog();
}

private void UseData(int a, string b, double c)
{
} 

My personal favorite ones are the properties, delegates and in some rare cases constructors.

Alternatively, you can create a static class , put some properties in it, then use it in other forms. This is really helpful if all of your forms need to share some information. Since this is not a way to Pass data between the forms, I did not mention this method in those above.

Update

The above lists the methods to pass data between two classes (in this case forms). However, after reading your question and your comments I can recommend this:

For this particualr case:

1. Open form designer in the visual studio.
2. Double click on the button1. (This will generate an event handler for button1's click event).

3. Fill the function like this:

   private void button1_Click(object sender, EventArgs e)
   {
       textBox2.Text = ChangeString(textBox1.Text);
   }
   
   private string ChangeString(string str)
   {
       string result = "do some stuff to " + str;
       return result;
   }
   

Here the ChangeString function is essentially doing what you want the Program.cs to do. Just add your functionality to it. If what you want it to do is more complex, you could create a class and add the functionality to that class.

What I recommend for you in general:

I think you should take some OO (Object Oriented) classes or read some books. You should get familiar with the concepts of OO and plan what you're supposed to do, before actually doing it. Also, you should read about the inner layers of .Net and C# and know what is the purpose of each part of the programs. I'd go with Console apps first and then go to the Winforms and other UI principles.

That said, I should quickly say that the Program.cs is just behaving as an entry point for your program. You shouldn't try to add logic to that class and try to keep the implementation of each class to itself. Try to separate the things each class can do to avoid complicated designs.

I hope this helps you get started.

static Form1 myForm;
[STAThread]

//we need a holder
//you can access static classes from your forms
public static class myCls {
    public static string myStr;
    static void myFunc(string str) {
        myStr = str; //or whatever
    }
}

static void Main()
{

    //string s1, s2;  //you don't need these
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    myForm = new Form1();
    Application.Run(myForm);
    //s1 = myForm.sendOver(); //you won't use this

}

your form1 must contain:

//change your holder's content whereever you want
void Button1_Click() {
    myCls.myFunc("string");
}

your form2 must contain:

//then call your holder's content whereever you want
void Button1_Click() {
    Text1.Text = myCls.myStr;
}