I can't add an object to a list

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ASP.NET

Hi. i want Add one item from GridGroup to GridProf Group but when I click btnAdd don't happen any thing. in my page i have these control:

It shows courses in dropdownList and when I select an item from dpdCourse it loads course's groups in GridGroup:

<asp:SqlDataSource ID="SqlCourse" runat="server" 
        ConnectionString="<%$ ConnectionStrings:HomeWorkSystemConnectionString %>" 
        SelectCommand="SELECT [Name], [Id] FROM [Course]"></asp:SqlDataSource>

     <asp:DropDownList ID="dpdCourse" CssClass= "txt" runat="server" 
               DataSourceID="SqlCourse" DataTextField="Name" DataValueField="Id" 
               AppendDataBoundItems="True" AutoPostBack="True">
               <asp:ListItem Text="select" Value="0"></asp:ListItem>
           </asp:DropDownList>

  <asp:SqlDataSource ID="SqlGroup" runat="server" 
        ConnectionString="<%$ ConnectionStrings:HomeWorkSystemConnectionString %>" 
        SelectCommand="SELECT [code], [Day], [Hour], [Capacity], [NumberOfStd], [Id] FROM [CourseGroup] WHERE ([CursId] = @CursId)">
        <SelectParameters>
            <asp:ControlParameter ControlID="dpdCourse" Name="CursId" 
                PropertyName="SelectedValue" Type="Int32" />
        </SelectParameters>
    </asp:SqlDataSource>

 <asp:GridView ID="GridCrsGroup" runat="server" AutoGenerateColumns="False" 
        CssClass ="GridStyle" DataKeyNames="Id" DataSourceID="SqlGroup">
        <HeaderStyle BackColor="#006699" Font-Bold="True" ForeColor="White" />
        <RowStyle BackColor="#99CCFF" ForeColor="#333333" />
        <AlternatingRowStyle BackColor="White" />
        <Columns>
            <asp:ButtonField ButtonType="Button" CommandName="Select" DataTextField="Id" 
                Text="Button">
                <ControlStyle BackColor="#006699" />
            </asp:ButtonField>
            <asp:BoundField DataField="code" HeaderText="code" SortExpression="code" />
            <asp:BoundField DataField="Day" HeaderText="Day" SortExpression="Day" />
            <asp:BoundField DataField="Hour" HeaderText="Hour" SortExpression="Hour" />
            <asp:BoundField DataField="Capacity" HeaderText="Capacity" 
                SortExpression="Capacity" />
            <asp:BoundField DataField="NumberOfStd" HeaderText="NumberOfStd" 
                SortExpression="NumberOfStd" />
        </Columns>
        
    </asp:GridView>

And i have one dropdownList to show Professor when select an item it shows professor's group in GridProfGroup:

<asp:SqlDataSource ID="SqlProf" runat="server"
       ConnectionString="<%$ ConnectionStrings:HomeWorkSystemConnectionString %>"
       SelectCommand="SELECT [Id], [Family] FROM [User] WHERE ([UserType] = @UserType)">
       <SelectParameters>
           <asp:Parameter DefaultValue="P" Name="UserType" Type="String" />
       </SelectParameters>
   </asp:SqlDataSource>

 <asp:DropDownList ID="dpdProf" CssClass ="txt" runat="server"
              AppendDataBoundItems="True" DataSourceID="SqlProf" DataTextField="Family"
              DataValueField="Id" AutoPostBack="True">
              <asp:ListItem Text ="انتخاب کنید" Value= "0"></asp:ListItem>
          </asp:DropDownList>

<asp:SqlDataSource ID="SqlProfGroup" runat="server"
       ConnectionString="<%$ ConnectionStrings:HomeWorkSystemConnectionString %>"
       SelectCommand="SELECT [code], [Day], [Hour], [Capacity], [NumberOfStd], [Id] FROM [CourseGroup] WHERE ([PrfId] = @PrfId)">
       <SelectParameters>
           <asp:ControlParameter ControlID="dpdProf" Name="PrfId"
               PropertyName="SelectedValue" Type="Int32" />
       </SelectParameters>
   </asp:SqlDataSource>

<asp:GridView ID="GridProfGroup" runat="server" CssClass ="GridStyle"
       AutoGenerateColumns="False" DataKeyNames="Id" DataSourceID="SqlProfGroup">
       <Columns>
           <asp:ButtonField ButtonType="Button" CommandName="Select" DataTextField="Id"
               Text="Button">
               <ControlStyle BackColor="#006699" />
           </asp:ButtonField>
           <asp:BoundField DataField="code" HeaderText="کد" SortExpression="code" />
           <asp:BoundField DataField="Day" HeaderText="روز" SortExpression="Day" />
           <asp:BoundField DataField="Hour" HeaderText="ساعت" SortExpression="Hour" />
           <asp:BoundField DataField="Capacity" HeaderText="ظرفیت"
               SortExpression="Capacity" />
           <asp:BoundField DataField="NumberOfStd" HeaderText="تعداد دانشجویان"
               SortExpression="NumberOfStd" />
       </Columns>

and I have a button that add selected item from GridGroup to GridProfGroup and add it to professor's list<group>:

In code behind:

protected void btnAdd_Click(object sender, EventArgs e)
  {
      int index = (int)GridCrsGroup.SelectedDataKey.Value;
      var group = DB.CourseGroups.Where(g => g.Id == index).First();
      int profId = Convert.ToInt32( dpdProf.SelectedItem.Value);
      Professor selectedProf = (Professor)DB.Users.Where(p => p.Id == profId).First();
      selectedProf.AddGroup(group);
      group.AssignProfessor(selectedProf);
      GridProfGroup.DataBind();

  }

And in CourseGroup tabel DB ProfId(professor's Id) is still null. If it helps I Change the ProfId to : Allow null = Yes, because where I were defining a group I were not assign its professor I assign it in this Page.

thank you.