public static void ClearForm(System.Windows.Forms.Control parent)
{
    foreach (System.Windows.Forms.Control ctrControl in parent.Controls)
    {
         //Loop through all controls 
         if (object.ReferenceEquals(ctrControl.GetType(), typeof(System.Windows.Forms.TextBox)))
         {
              //Check to see if it's a textbox 
              ((System.Windows.Forms.TextBox)ctrControl).Text = string.Empty;
                    //If it is then set the text to String.Empty (empty textbox) 
          }
          else if (object.ReferenceEquals(ctrControl.GetType(), typeof(System.Windows.Forms.RichTextBox)))
          {
               //If its a RichTextBox clear the text
               ((System.Windows.Forms.RichTextBox)ctrControl).Text = string.Empty;
          }
          else if (object.ReferenceEquals(ctrControl.GetType(), typeof(System.Windows.Forms.ComboBox)))
          {
               //Next check if it's a dropdown list 
               ((System.Windows.Forms.ComboBox)ctrControl).SelectedIndex = -1;
                    //If it is then set its SelectedIndex to 0 
          }
          else if (object.ReferenceEquals(ctrControl.GetType(), typeof(System.Windows.Forms.CheckBox)))
          {
               //Next uncheck all checkboxes
               ((System.Windows.Forms.CheckBox)ctrControl).Checked = false;
          }
          else if (object.ReferenceEquals(ctrControl.GetType(), typeof(System.Windows.Forms.RadioButton)))
          {
               //Unselect all RadioButtons
               ((System.Windows.Forms.RadioButton)ctrControl).Checked = false;
          }
          if (ctrControl.Controls.Count > 0)
          {
              //Call itself to get all other controls in other containers 
              ClearForm(ctrControl);
          }
     }
}

//Example call
ClearForm(this);