convert double to string

i am working in forms and in database its type is double and i want convert in text.

1: I have a double value as below

 public double? Area {get; set;}

2:  <asp:TextBox ID="txtArea" runat="server" placeholder="Area" CssClass="form-control"></asp:TextBox>

 string v = txtTarget.ToString();
 Form.Area = v;

I am using this way but not works