// The following is a validation condition that prevents users to enter more than 3 characters								
private void textBox1_Validating(object sender, CancelEventArgs e)
{
    if (textBox1.Text.Length > 3)
    {   MessageBox.Show("Length exceeds 3 characters");
         e.Cancel = true;
    }
}

//  You can surpass the above validation condition on assigning a calculated value for textBox1.

textBox1.Enabled = false;
this.ValidateChildren(ValidationConstraints.Enabled);
textBox1.Text = "abcd" + "1234";