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Overriding ShowDialog() is not possible in WinForms because there is no overridable method declared for it in Form Class. But there is a way there is will:

 public partial class FormA : Form
 {
 public FormA()
 {
 InitializeComponent();
 }

 public DialogResult ShowDialog(string mes)
 {
 this.textBox1.Text = mes;
 return base.ShowDialog();
 }
 public DialogResult ShowDialog()
 {
 // Your Code
 return base.ShowDialog();
 }
 }