C#

int num = Directory.GetFiles("directorypath", "*.xls").Length;

form2 x = new form2();
x.FormClosed += new FormClosedEventHandler(this.Form_FormClosed);
this.Visible = false;
x.Show();
...
void Form_FormClosed(object sender, FormClosedEventArgs e)
{
this.Visible = true;
}

if ((textBox1.Text == "admin") && (textBox2.Text == "xxx")) 
{
Form2 mehmet = new Form2();
mehmet.FormClosed += new FormClosedEventHandler(this.Form_FormClosed);
mehmet.Show();
this.Visible = false;
}
...
void Form_FormClosed(object sender, FormClosedEventArgs e)
{
this.Visible = true;
}

Form1 form1 = null;
public Form2(Form1 form1)
{
    this.form1 = form1;
}

private void form2_Closed(object sender, EventArgs e)
{
    this.form1.Visible = true;
}

Form2 form2 = new Form2(this);  // "this" is important
form2.ShowDialog();

// Form1.cs //
public string MyTextbox
{
   get { return this.textbox.Text; }
   set { this.textbox.Text = value; }
}

// Form2.cs //
private void ChangeTextInForm1()
{
   this.form1.MyTextbox = "Hello World Again";
}

for (int count = 0;count < thing.Length; count++)
{
FieldInfo info = this.GetType().GetField(thing[count], BindingFlags.NonPublic);
info.SetValue(this, input.readline());
}

MethodInvoker mInvoker = new MethodInvoker(ShowMyDialog);
Invoke(mInvoker);

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Threading;

namespace WindowsApplication5
{
	public class Form1 : System.Windows.Forms.Form
	{
		private System.Windows.Forms.Button button1;
		private System.ComponentModel.Container components = null;

		public Form1()
		{	InitializeComponent(); }

		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{ components.Dispose(); }
			}
			base.Dispose( disposing );
		}

		private void InitializeComponent()
		{
			this.button1 = new System.Windows.Forms.Button();
			this.SuspendLayout();
			this.button1.Location = new System.Drawing.Point(48, 72);
			this.button1.Name = "button1";
			this.button1.TabIndex = 1;
			this.button1.Text = "Change text";
			this.button1.Click += new System.EventHandler(this.button1_Click);
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(208, 189);
			this.Controls.Add(this.button1);
			this.Name = "Form1";
			this.Text = "Form1";
			this.ResumeLayout(false);
		}

		[STAThread]
		static void Main() 
		{	Application.Run(new Form1()); }

		private void button1_Click(object sender, System.EventArgs e)
		{
			/***** WRONG *****/
//			Thread t = new Thread(new ThreadStart(Run));
//			t.Start();

			/***** OK ******/
			Thread t = new Thread(new ThreadStart(CreateNewLabel));
			t.Start();
		}

		private void CreateNewLabel()
		{
			MethodInvoker minvoker = new MethodInvoker(Run);
			Invoke(minvoker);
		}

		private void Run()
		{
			try
			{
				Label label = new Label();
				label.Name = "MyLabel";
				label.Text = "MyLabel";
				label.Location = new Point(50, 50);
				this.Controls.Add(label);
			}
			catch(Exception ex)
			{	MessageBox.Show(ex.ToString());	}
		}
	}
}