How to access a form control for another form?

It's easy, first you can access the other form like this: (let's say your other form is Form2)

//in Form 1
Form2 F2 = new Form2();
foreach (Control c in F2.Controls)
         if(c.Name == "TextBox1")
            c.Text = "hello from Form1";

That's it, you just write in TextBox1 in Form2 from Form1.

Access the form's controls like this:

formname.controls[Index]

You can cast as appropriate control type, Example:

DataGridView dgv = (DataGridView) formname.Controls[Index];

Making them Singleton is not a completely bad idea, but personally I would not prefer to do it that way. I'd rather pass the reference of one to another form. Here's an example.

Form1 triggers Form2 to open. Form2 has overloaded constructor which takes calling form as argument and provides its reference to Form2 members. This solves the communication problem. For example I've exposed Label Property as public in Form1 which is modified in Form2.

With this approach you can do communication in different ways.

Download Link for Sample Project

//Your Form1

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

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

    public string LabelText
    {
        get { return Lbl.Text; }
        set { Lbl.Text = value; }
    }
}

//Your Form2

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private Form1 mainForm = null;
    public Form2(Form callingForm)
    {
        mainForm = callingForm as Form1; 
        InitializeComponent();
    }

    private void Form2_Load(object sender, EventArgs e)
    {

    }

    private void button1_Click(object sender, EventArgs e)
    {
        this.mainForm.LabelText = txtMessage.Text;
    }
}

_{(source: ruchitsurati.net)}

_{(source: ruchitsurati.net)}

I usually use the Singleton Design Pattern for something like this http://en.wikipedia.org/wiki/Singleton_pattern . I'll make the main form that the application is running under the singleton, and then create accessors to forms and controls I want to touch in other areas. The other forms can then either get a pointer to the control they want to modify, or the data in the main part of the application they wish to change.

Another approach is to setup events on the different forms for communicating, and use the main form as a hub of sorts to pass the event messages from one form to another within the application.