C#

[DllImport("User32.dll",CharSet = CharSet.Auto,SetLastError=true)]
public static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam);

frmMain.txtName.Text = "John";

this.txtName.Text = "John";
// or
txtName.Text = "John";

static void Main()
{
     Application.Run(new frmMain());
}

static void Main()
{
    Application.Run(new frmMain());
}

static void Main()
{
    frmMain frm = new frmMain();
    frm.ShowDialog();
}

public class test : System.Windows.Forms.Form
{
	Button b;
	Label l;	

	public test()
	{
		l = new Label();
		this.Controls.Add(l);

		b = new Button();
		b.Location = new Point(20, 20);
		b.Click += new EventHandler(bClick);	
		this.Controls.Add(b);
	}

	public static void Main()
	{
		Application.Run(new test());
	}

	private void bClick(object sender, EventArgs e)
	{
            l.Text = "Hello";
         }
}

public class test : System.Windows.Forms.Form
{
	Button b;
	Label l;
	Client myClient = new Client();

	public test()
	{
		l = new Label();
		this.Controls.Add(l);
		b = new Button();
		b.Location = new Point(20, 20);
		b.Click += new EventHandler(bClick);
	
		this.Controls.Add(b);
	}
	public static void Main()
	{
		Application.Run(new test());
	}
	private void bLoadClientClick(object sender, EventArgs e)
	{
        	myClient.LoadClient();
		myClient.DisplayClient();
         }
}

public class Client
{
	string strName = "";
	string strAddress = "";
	string strTelephone = "";

	public void DisplayClient()
	{
		txtName.Text = strName;
		txtAddress.Text = strAddress;
		txtTelephone.Text = strTelephone;
	}

	public bool LoadClient()
	{
		...
		...
	}
}