asp.net System.NullReferenceException:

Question

I have an application about article publications. I am trying to make the form to insert an article. I want to make a section for file upload. I am using datalist and want to upload an article. The problem is with code behind, it shows me this error:

System.NullReferenceException: Object reference not set to an instance of an object.

<asp:DataList ID="datalist2" runat="server" 
           onitemcommand="datalist2_ItemCommand">
       <ItemTemplate> 
            <section id="main" class="column" runat="server" style="width:900px;">
                <article class="module width_full" style="width:900px;">
                <header><h3>Post New Article</h3></header>
                <div class="module_content">
                <asp:Label ID="Label1" runat="server">Tema</asp:Label>
                <fieldset>           
                    <asp:TextBox  ID="txtTema" runat="server"></asp:TextBox>
                </fieldset>               
                <asp:Label ID="Label6" runat="server">Abstrakti</asp:Label>
                <fieldset>
                <asp:TextBox  ID="txtAbstrakti" style="height:250px;" Textmode="Multiline" runat="server"></asp:TextBox>                 
                </fieldset>
                <asp:Label ID="Label4" runat="server">Keywords</asp:Label>
                <fieldset>
                    <asp:TextBox  ID="txtKeywords" runat="server"></asp:TextBox>
                    <asp:FileUpload ID="FileUploadArtikull" runat="server" Height="43px" 
                                style="margin-left: 0px" />
                </fieldset>
                <asp:Label ID="Label5" runat="server">Kategoria</asp:Label>
                <fieldset>
                    <asp:TextBox ID="txtKategoria" runat="server"></asp:TextBox>
                </fieldset>                 
                    <asp:Button ID="btnInsert" runat="server" CommandName="Insert" Text="Shto" />
                </div>  
                </article>
        </section> 
        </ItemTemplate>
       </asp:DataList>

     FileUpload FileUploadArtikull = (FileUpload)datalist2.FindControl("FileUploadArtikull");         
       if (FileUploadArtikull.HasFile)
              {
                 int filesize = FileUploadArtikull.PostedFile.ContentLength;
                 if (filesize > 4194304)
                     {
                       ScriptManager.RegisterStartupScript(this, this.GetType(), "popup", "alert('Max 4MB');", true);
                      }
                      else
                      {
 string filename = "artikuj/" + Path.GetFileName(FileUploadArtikull.PostedFile.FileName);
                                //add parameters
                                command.Parameters.AddWithValue("@filename", filename);

                                 conn.Open();
                                 command.ExecuteNonQuery();
                                 conn.Close();
                                 Bind();

                                FileUploadArtikull.SaveAs(Server.MapPath("~/artikuj\\" + FileUploadArtikull.FileName));
                                Response.Redirect("dashboard.aspx");

                            }
                        }
                        else
                        {
                            ScriptManager.RegisterStartupScript(this, this.GetType(), "popup", "alert('ERROR');", true);
                        }

Answer 1

You can not access any control directly from datalist2, you have to loop through each datalist item.

foreach (DataListItem item in datalist2.Items)
{
    FileUpload FileUploadArtikull = (FileUpload)item.FindControl("FileUploadArtikull");

    if (FileUploadArtikull.HasFile)
    {
        int filesize = FileUploadArtikull.PostedFile.ContentLength;
        if (filesize > 4194304)
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "popup", "alert('Max 4MB');", true);
        }
        else
        {
            string filename = "artikuj/" + Path.GetFileName(FileUploadArtikull.PostedFile.FileName);
            //add parameters
            command.Parameters.AddWithValue("@filename", filename);

            conn.Open();
            command.ExecuteNonQuery();
            conn.Close();
            Bind();

            FileUploadArtikull.SaveAs(Server.MapPath("~/artikuj\\" + FileUploadArtikull.FileName));
            Response.Redirect("dashboard.aspx");

        }
    }
    else
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "popup", "alert('ERROR');", true);
    }

}