Object reference not set to an instance of an object.

Michael YoeH · August 22nd, 2009, 09:28 PM

I try to bind the dropdownlist in the GridView,but always get the result said:Object reference not set to an instance of an object.
I set the breakpoint to follow the code,it showed I did get the data from the database,but when I follow the next code,the error showed.
My code:

Code:

cs code:
DropDownList ddlUpdateCategory = (DropDownList)this.GridView1.Rows[Convert.ToInt32(e.CommandArgument)].Cells[1].FindControl("ddlUpdateCategory");
            using (NorthwindEntities ctx = new NorthwindEntities())
            {
                ddlUpdateCategory.DataTextField = "CategoryName";
                ddlUpdateCategory.DataValueField = "CategoryID";
                ddlUpdateCategory.DataSource = ctx.Categories;
                ddlUpdateCategory.DataBind();
            }

aspx code:
<asp:TemplateField HeaderText="CategoryName">
<EditItemTemplate>
<asp:DropDownList ID="ddlUpdateCategory" runat="server" DataTextField="CategoryName" DataValueField="CategoryID">
</asp:DropDownList>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Bind("CategoryName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

Thanks in advance!

Lee Dumond · August 22nd, 2009, 10:12 PM

It's probably not finding the Dropdownlist.

First, what event is this code attached to? (I assume it is the RowDataBound event of the GridView)

And second, which cell in the row is the Dropdown list supposed to be in?

Michael YoeH · August 22nd, 2009, 10:31 PM

First,thanks for your reply in time.
Yes,you are right.

Code:

  protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "upload")
        {
            GridViewRow gvr = ((GridViewRow)(((Button)(e.CommandSource)).Parent.Parent));//此得出的值是表示那行被选中的索引值
            int Key = Convert.ToInt32(GridView1.DataKeys[gvr.RowIndex].Value);
            FileUpload fu = (FileUpload)this.GridView1.Rows[gvr.RowIndex].Cells[1].FindControl("fuUpload");
            int updateImagelength = fu.PostedFile.ContentLength;
            byte[] updateimage = new byte[updateImagelength];
            fu.PostedFile.InputStream.Read(updateimage, 0, updateImagelength);
            using (NorthwindEntities ctx = new NorthwindEntities())
            {
                var category = ctx.Categories.First(p => p.CategoryID == Key);
                category.Picture = updateimage;
                ctx.SaveChanges();
            }
        }
        if (e.CommandName == "Edit")
        {
            DropDownList ddlUpdateCategory = (DropDownList)this.GridView1.Rows[Convert.ToInt32(e.CommandArgument)].Cells[1].FindControl("ddlUpdateCategory");
            using (NorthwindEntities ctx = new NorthwindEntities())
            {
                ddlUpdateCategory.DataTextField = "CategoryName";
                ddlUpdateCategory.DataValueField = "CategoryID";
                ddlUpdateCategory.DataSource = ctx.Categories;
                ddlUpdateCategory.DataBind();
            }
        }
    }

And I need to click the Edit button in the Gridview,then shows the binded dropdownlist in the second column.

Lee Dumond · August 23rd, 2009, 02:55 AM

When a GridView RowCommand event is raised with a CommandName == "Edit", this automatically in turn raises the RowEditing event. The RowEditing event occurs before the GridView control enters edit mode.

Because the ddlUpdateCategories control is contained in the row's EditItemTemplate, it is not rendered until the GridView enters edit mode. When your event handler is invoked, the GridView has not yet entered edit mode, and therefore the ddlUpdateCategories control does not yet exist.

I suggest you find a different way to bind the ddlUpdateCategories to your list of categories. You can do that declaratively directly inside the EditItemTemplate.

If you want to do it at runtime, you can handle the RowDataBound event, which will fire for each row, then use the RowType property to determine if the current row is in edit mode. If it is, you should be able to get a reference to the ddlUpdateCategories at that time.

Michael YoeH · August 23rd, 2009, 05:24 AM

Hi,
I really appreciate your help.It works like you said.Thaks,anyway!

Lee Dumond · August 23rd, 2009, 02:07 PM

Quote:

Originally Posted by Michael YoeH

Hi,
I really appreciate your help.It works like you said.Thaks,anyway!

You can use the Thanks button at the bottom of my post to thank me.