insert all the textbox of windowform in list

Question

hello my question is about how can i add all the values of textbox in a list.

i write this code but it show me an error

 

List<frmgerant> listgerant = new List<frmgerant>();

listgerant.Add(txt_nom_gerant.Text.ToString());

listgerant.Add(txt_prenom_gerant.Text.ToString());

 

 

 

how can i solve that?