Change TextBox data in Gridview (asp net c #)

Question

I created a gridview containing textbox that are populated in the backend:

<asp:GridView ID="grvMyTest" runat="server" AutoGenerateColumns="False"
EnableModelValidation="True" Height="30%" TabIndex="9" 
AllowSorting="True" Width="100%" Visible="true" AllowPaging="True" 
PageSize="20" CssClass="mGrid" PagerStyle-CssClass="pgr">
  <Columns>
     <asp:TemplateField HeaderText="Jan">
        <ItemTemplate>
           <asp:TextBox ID="tbjan" runat="server" Text='<%# Eval("mJan") %>' 
              Width="50px" style="text-align: center"></asp:TextBox>
        </ItemTemplate>
      <HeaderStyle BackColor="Control" HorizontalAlign="Center" />
      <ItemStyle HorizontalAlign="Center" />
   </asp:TemplateField>

In the backend I would like that when the user clicks the button I would like to retrieve the value of the TextBox to update in the database:

<asp:Button runat="server" Text="Alter values" id="testButton" OnClick="clickedButton" />

Back-end code:

protected void clickedButton(object sender, System.EventArgs e)
{
  foreach (GridViewRow row in grvMyTest.Rows) //Running all lines of grid
  {
      TextBox value = ((TextBox)(row.Cells[0].FindControl("mJan")));
   }
}

But the value is always null even having given in the database that appear on the grid.

When the page loads the values appear: Grid But, the value is null when the button as clicked (clickedButton method).

Answer 1

A very quick and simple solution would be to add a Label to the GridView and set it's Visiblity to false.

<asp:TextBox ID="tbjan" runat="server" Text='<%# Eval("mJan") %>'></asp:TextBox>
<asp:Label ID="tbjanLabel" runat="server" Text='<%# Eval("mJan") %>' Visible="false"></asp:Label>

Then you can compare those values in code behind

 TextBox value = (TextBox)(row.FindControl("tbjan"));
 Label lbl = (Label)(row.FindControl("tbjanLabel"));

 if (lbl.Text == value.Text)
 {
     //no change
 }

Answer 2

作为对VDWWD响应的补充，请确保页面未调用回发并从gridview中丢失了引用。