Question

Consider a page, when the page loads, nothing shows up.

It works when i pass the querystring on the browser as this:

http://localhost:51765/foo/foo.aspx?ID=c516f4f4-36a9-40a7-baad-d2419ea631b9

I want it to work when the page load not when i pass the querystring on the browser.

Can someone help me with this?

<asp:SqlDataSource ID="categoriesDataSource" runat="server" 
     connectionString="<%$ ConnectionStrings:ConnectionString %>"
     SelectCommand="SELECT [CategoryID], [Name] FROM [Categories] WHERE ([UserId] = @UserId) ORDER BY [Name]">
     <SelectParameters>
          <asp:QueryStringParameter Name="UserId" QueryStringField="ID" />
     </SelectParameters>
</asp:SqlDataSource>

<asp:DropDownList ID="categories" runat="server" AutoPostBack="True"
       DataSourceID="categoriesDataSource" DataTextField="Name" 
         AppendDataBoundItems="True" DataValueField="CategoryID">
        <asp:ListItem  Value="">-- All Albums --</asp:ListItem>
</asp:DropDownList>

 <asp:SqlDataSource ID="picturesDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
      SelectCommand="SELECT [PictureID], [Title], [UploadedOn] FROM [Pictures] WHERE UserId = @UserId AND 
       (CategoryID = @CategoryID Or @CategoryID IS NULL) ORDER BY UploadedOn DESC" 
        CancelSelectOnNullParameter="False">
  <SelectParameters>
      <asp:ControlParameter ControlID="categories" Name="CategoryID" PropertyName="SelectedValue"/>
      <asp:QueryStringParameter Name="UserId" QueryStringField="ID" />
  </SelectParameters>
</asp:SqlDataSource> 


<asp:GridView ID="GridView1" runat="server" DataSourceID="picturesDataSource">
</asp:GridView>

Answer 1

It is difficult to answer your question without showing the code of the page or at least explaining what it does. From the url it seems that the page relies on the ID parameter and tries to parse it to a Guid. You need to test whether the ID parameter is passed and use it only in this case:

string id = Request["ID"];
if (!string.IsNullOrEmpty(id))
{
    // The ID parameter has been passed => use its value here
}