Get TextBox value from DataList in edit mode

Question

I want to add a value to a row in a datalist. I have added a textbox and a LinkButton that shows up when the DataList is in edit mode.

The LinkButton has CommandName="Add" and CommandArgument with the id of the row as a value.

I have added a OnItemCommand to the DataList and in the code behind.

But when try to retrieve the text from the TextBox in codebehind it is empty.

Does any one have a clue to what might be wrong?

<asp:DataList ID="dl_courses" runat="server" DataKeyField="CourseId" OnItemCommand="dl_courses_OnItemCommand" OnItemDataBound="dl_courses_ItemDataBound" OnEditCommand="dl_courses_OnEditCommand" OnCancelCommand="dl_courses_OnCancelCommand" OnDeleteCommand="dl_courses_OnDeleteCommand" OnUpdateCommand="dl_courses_OnUpdateCommand"> 
    <ItemTemplate>
        <asp:LinkButton ID="lb_editItem" Text="Editera kursnamnet" CommandName="Edit" runat="server" />&nbsp;<br /><asp:Label ID="lbl_courseId" Text='<%# DataBinder.Eval(Container.DataItem, "CourseId")%>' runat="server" /><br /><asp:Label ID="lbl_coursename" Text='<%# DataBinder.Eval(Container.DataItem, "CourseName")%>' runat="server" /><br />
            <asp:DataList ID="DateList" runat="server">
                <ItemTemplate>
                    &nbsp;&nbsp;&nbsp;<%# Container.DataItem%>
                </ItemTemplate>
            </asp:DataList>
            <asp:TextBox ID="tb_addDate" Visible="false" runat="server" />
    </ItemTemplate>
    <ItemStyle BackColor="#BCBCBC" />
    <AlternatingItemStyle BackColor="#FFFFFF" />
    <EditItemTemplate>
        <asp:TextBox ID="tb_coursename" runat="server" Text='<%# DataBinder.Eval(Container.DataItem, "CourseName")%>' />&nbsp;<asp:LinkButton ID="lb_cancel" Text="Avbryt" CommandName="Cancel" runat="server" />&nbsp;<asp:LinkButton ID="lb_deleteCourse" Text="Radera" CommandName="Delete" runat="server" />&nbsp;<asp:LinkButton ID="lb_updateCourse" Text="Uppdatera" CommandName="Update" runat="server" />
        <br />
        <asp:TextBox ID="tb_addDate" runat="server" /> &nbsp;<asp:LinkButton ID="lb_addNewDate" Text="Lägg till nytt datum" CommandName="Add" runat="server" CommandArgument='<%# DataBinder.Eval(Container.DataItem, "CourseId")%>' />
        <br />
        <asp:DataList ID="DateList" runat="server" OnEditCommand="DateList_OnEditCommand" OnUpdateCommand="DateList_OnUpdateCommand" OnCancelCommand="DateList_OnCancelCommand">
            <ItemTemplate>
                <asp:LinkButton ID="lb_editCourseItem" Text="Editera datum" CommandName="Edit" runat="server" />&nbsp;<%# Container.DataItem%>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="tb_courseDate" Text='<%# Container.DataItem%>' runat="server" /><asp:LinkButton ID="lb_cancel" Text="Avbryt" CommandName="Cancel" runat="server" />&nbsp;<asp:LinkButton ID="lb_updateDate" Text="Uppdatera" CommandName="Update" runat="server" />  <%--CommandArgument='<%# DataBinder.Eval(Container.DataItem, "CourseId")%>'--%>
            </EditItemTemplate>
        </asp:DataList>
        <br />
    </EditItemTemplate>
</asp:DataList>

From CodeBehind

    protected void dl_courses_OnItemCommand(Object sender, DataListCommandEventArgs e)
{
    if (e.CommandName.Equals("Add") && e.Item.ItemType == ListItemType.EditItem)
    {
        TextBox tb = (TextBox)e.Item.FindControl("tb_coursename");
        string courseDate = ((TextBox)e.Item.FindControl("tb_addDate")).Text;
        Guid courseId = new Guid(e.CommandArgument.ToString());
        Course course = RepositoryFactory.CreateCourseRepository().FetchSingle(new Course(null, null, courseId));



        if (IsDate(courseDate))
        {
            //course.CourseDate.Add(Convert.ToDateTime(tb_date.Text));
            course.CourseDate.Add(DateTime.Parse(courseDate));
            updateCourse(course);

            DataList innerList = (DataList)e.Item.FindControl("DateList");
            innerList.EditItemIndex = e.Item.ItemIndex;
            innerList.DataBind();

            //tb_date.Text = "";
            lbl_message.Text = "";
        }
        else
            lbl_message.Text = "Felaktigt datum. Formatet måste vara åååå-mm-dd och datumet måste vara efter dagens datum.<br><br>";
    }
}