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());
}