Read an integer at C# at a WindowsFormsApplication

The TextBox's Text property returns a string. You therefore have to convert it to an integer:

   int i = int.Parse(textBox1.Text);

It's also possible to make sure that what's in the textbox is a valid integer and, if it isn't, to ask the user to re-input it:

   int i;

   bool isValid = int.TryParse(textBox1.Text, out i);

   if (!isValid)
   {
      MessageBox.Show("Not a valid integer. Please re-enter");
      textBox1.Focus();
      return;
   }


  // otherwise 'i' contains the integer value of what's in the textbox