Instead using a TextBox in the EditTemplate of a GridView, if needed you can use a DropdownList , here is a code in HTML page :
<EditItemTemplate>
<asp:DropDownList ID="type_edit" runat="server" AutoPostBack="true" SelectedValue='<%# Bind("transaction_type") %>' >
<asp:ListItem Value="" ></asp:ListItem>
<asp:ListItem Value="LOAN">LOAN</asp:ListItem>
<asp:ListItem Value="PRINCIPAL PAYMENT">PRINCIPAL PAYMENT</asp:ListItem>
<asp:ListItem Value="INTEREST PAYMENT">INTEREST PAYMENT</asp:ListItem>
</asp:DropDownList>
</EditItemTemplate>
You do not have to write anything in code behind page.😀😀