adding two texboxes of values different forms and displaying in another textbox in second form

First change the Modifiers property in the Properties Box of the TextBox (say textBox1) on the first form (Form1 say) from private to internal.

It's assumed that the TextBoxes on the second form are called textBox1 and textBox2.

Now put this code in textbox1's Leave event handler on the second form (add the skeleton handler from the Properties Box):

private button1_Click (object sender, EventArgs e)

{

Form1 f1 = (Form1)Appplication.OpenForms["Form1"];

int maximumLeaves, takenLeaves;

int.TryParse(f1.textBox1, out maximumLeaves);

int.TryParse(this.textBox1, out takenLeaves);

int remainingLeaves = maximumLeaves - takenLeaves;

if (remainingLeaves < 0) remainingLeaves = 0;

this.textBox2.Text = remainingLeaves.ToString();

}