In the Ok button event handler iterate through all the controls on the form
setting focus on them and then calling Form.Validate() that will
automatically call the validating event handlers for the controls

Check out this code from Windows Form Programming in C# by Chris Sells

// Retrieve all controls and all child controls etc.
// Make sure to send controls back at lowest depth first
// so that most child controls are checked for things before
// container controls, e.g., a TextBox is checked before a
// GroupBox control
Control[] GetAllControls() {
 ArrayList allControls = new ArrayList();
 Queue queue = new Queue();
 queue.Enqueue(this.Controls);

 while( queue.Count > 0 ) {
   Control.ControlCollection
     controls = (Control.ControlCollection)queue.Dequeue();
   if( controls == null || controls.Count == 0 ) continue;

   foreach( Control control in controls ) {
     allControls.Add(control);
     queue.Enqueue(control.Controls);
   }
 }

 return (Control[])allControls.ToArray(typeof(Control));
}

void okButton_Click(object sender, EventArgs e) {
 // Validate each control manually
 foreach( Control control in GetAllControls() ) {
   // Validate this control
   control.Focus();
   if( !this.Validate() ) {
     this.DialogResult = DialogResult.None;
     break;
   }
 }
}