C# TextBox.text 返回 null

Question

I am trying to make an editor for a webpage. However, whenever I try and access the text from my textboxes in a gridview, I receive the error "object reference not set to an instance of an object." My code is as follows.

 SqlConnection con = new SqlConnection(DatabaseClient.ConnectionString);
            try
            {
            int id = (int)CompoundTable.DataKeys[e.RowIndex].Value;



            TextBox name = CompoundTable.Rows[e.RowIndex].FindControl("name") as TextBox;
                TextBox cat = (TextBox)CompoundTable.Rows[e.RowIndex].FindControl("cation_quantity");
                TextBox catname = (TextBox)CompoundTable.Rows[e.RowIndex].FindControl("cation");
                TextBox an = (TextBox)CompoundTable.Rows[e.RowIndex].FindControl("anion");
                TextBox anName = (TextBox)CompoundTable.Rows[e.RowIndex].FindControl("anion_quantity");
                TextBox diff = (TextBox)CompoundTable.Rows[e.RowIndex].FindControl("difficulty_level");

                con.Open();
                SqlCommand command = new SqlCommand("UPDATE Compound SET compound_name='" + name.Text  + "' WHERE compound_name = @compound",con); //The line that the error is thrown.
                command.Parameters.Add("@compound", SqlDbType.NVarChar, CompoundName.Text.Trim().Length).Value = CompoundName.Text.Trim();
                command.ExecuteNonQuery();
                con.Close();


                CompoundTable.EditIndex = -1;
                BindGridView();
            }

<asp:GridView ID="CompoundTable" runat="server" AllowSorting="True" AllowPaging="True" AutoGenerateColumns="False" PageSize="20" OnSorting="CompoundTable_Sorting" OnPageIndexChanging="CompoundTable_PageIndexChanging" OnRowCancelingEdit="CompoundTable_CancelEdit" OnRowEditing="CompoundTable_Edit" OnRowUpdating="CompoundTable_Update" CellPadding="4">
                <Columns>
                    <asp:BoundField DataField="id" HeaderText="Compound" SortExpression="name" />
                    <asp:BoundField DataField="name" HeaderText="Compound" SortExpression="name" />
                    <asp:BoundField DataField="cation" HeaderText="Cation" SortExpression="cation" />
                    <asp:BoundField DataField="cation_quantity" HeaderText="Cation Quantity" SortExpression="cation" />
                    <asp:BoundField DataField="anion" HeaderText="Anion" SortExpression="anion" />
                    <asp:BoundField DataField="ANION_QUANTITY" HeaderText="Anion Quantity" SortExpression="anion" />
                    <asp:BoundField DataField="difficulty_level" HeaderText="Difficulty" SortExpression="difficulty_level" />
                    <asp:HyperLinkField DataNavigateUrlFields="compound_id" DataNavigateUrlFormatString="DeleteCompound.aspx?compound={0}" Text="Delete Compound" />
                     <asp:TemplateField>
            <ItemTemplate>
                <asp:LinkButton ID="LBEdit" runat="server"  CommandName="Edit" >Edit</asp:LinkButton>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:LinkButton ID="LBCancel" runat="server" CommandName="Cancel">Cancel</asp:LinkButton>
                <asp:LinkButton ID="LBUpdate" runat="server" CommandName="Update">Update</asp:LinkButton>
            </EditItemTemplate>
        </asp:TemplateField>
                </Columns>
            </asp:GridView>

Answer 1

Look at what CompoundTable.Rows[e.RowIndex].FindControl("name") is returning. I don't know webforms too well, but a quick look at the docs shows that BoundField does not inherit from TextBox .

When you cast using as , if the cast fails, no exception is thrown, but the returned object is null. You are getting the exception when trying to access the property on the null object.

Answer 2

Because your name.Text is null.

If your target is updating the object by clicking the button update, you could do it by doing the following simple way:

                protected void YourGridIdHere_RowCommand(object sender, GridViewCommandEventArgs e)
                {
                    // If multiple buttons are used in a GridView control, use the
                    // CommandName property to determine which button was clicked.
                    if(e.CommandName=="Your command in CommandName")
                    {
                      // Convert the row index stored in the CommandArgument
                      // property to an Integer.
                      int index = Convert.ToInt32(e.CommandArgument);

                      // Retrieve the row that contains the button clicked 
                      // by the user from the Rows collection.
                      GridViewRow row = YourGridIdHere.Rows[index];

                      // Get data from current selected row.    
                      int ID_COL = 0; 
                      TableCell wantedData = row.Cells[ID_COL];
                      string str = wantedData.Text;

                      // So on....
                }

In form html, you could run it by adding:

OnRowCommand="YourGridIdHere_RowCommand"

to your:

<asp:GridView ID="CompoundTable" runat="server" AllowSorting="True" AllowPaging="True" AutoGenerateColumns="False" PageSize="20" OnSorting="CompoundTable_Sorting" OnPageIndexChanging="CompoundTable_PageIndexChanging" OnRowCancelingEdit="CompoundTable_CancelEdit" OnRowEditing="CompoundTable_Edit" OnRowUpdating="CompoundTable_Update" CellPadding="4">