Question

If I have:

<asp:TextBox ID="txtField1" runat="server"></asp:TextBox>

Then in my code-behind I can access that web control by its “ClientID”:

txtField1.Text = "value";

That works, but I want an alternate way to access the same web control by reference so I tried doing it this way:

TextBox myTxtField = (TextBox)Page.FindControl("txtField" + 1);
myTxtField.Text = "value";

And I am getting the error: “Object reference not set to an instance of an object.” Any idea if I am doing this right or if there is another way to reference a web control’s ID by passing in a string?

Answer 1

TextBox myTxtField = (TextBox)Page.FindControl("txtField" + 1);

This returns null if the control is sitting on a ContentPage of a MasterPage. Then you should reference it directly (as shown below).

I’ve written something about this issue here: https://stackoverflow.com/a/8781131/284240

Of course it would also return null if the control has another ID or if it’s in another NamingContainer than the page(f.e. in a GridView).

Side note:

Then in my code-behind I can access that web control by its
“ClientID”:

txtField1.Text = "value";

txtField1 is the ID not the ClientID and actually you’re referencing the control directly.