Binding value to textbox inside of gridview - asp.net

Question

I have problems with binding a value of a field inside a gridview to a textbox which is inside the gridview as well. I'm intending to do this for editing the table.
I tried to do this with eval and bind, but the textbox won't show the values and I have absolutely no clue why.
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:GridView ID="gvBS" runat="server" AutoGenerateColumns="false" DataKeyNames="ID" SkinID="gvWithoutWidth">
<Columns>
<asp:CommandField ShowEditButton="true" EditImageUrl="~/Images/GridView/gv_edit.png" ButtonType="Image"
CancelImageUrl="~/Images/GridView/gv_cancel.png" UpdateImageUrl="~/Images/GridView/gv_update.png"/>
<asp:TemplateField HeaderText="Sollmonat" HeaderStyle-HorizontalAlign="Left">
<EditItemTemplate>
<asp:TextBox ID="tbSollMonat" runat="server" Text='<%# Eval("SollMonat") %>'></asp:TextBox>
<asp:RequiredFieldValidator ID="rfvSollMonat" ValidationGroup="Update" runat="server"
ControlToValidate="tbSollMonat" ErrorMessage="Bitte Sollmonat (dd.mm.yyyy) angeben"
SetFocusOnError="true">*</asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="revSollMonat" ValidationGroup="Update" runat="server"
ValidationExpression="^\d+$" ControlToValidate="tbSollMonat">*</asp:RegularExpressionValidator>
</EditItemTemplate>
<ItemTemplate>
<%# Eval("SollMonat")%>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
The thing is that it works fine inside the ItemTemplate, but doesn't inside the EditItemTemplate-element. Really no clue what the problem is.
Code behind:
Sub gvBS_RowEditing(ByVal sender As Object, ByVal e As GridViewEditEventArgs) Handles gvBS.RowEditing
gvBS.EditIndex = e.NewEditIndex
End Sub
Sub gvBS_RowCancelingEdit() Handles gvBS.RowCancelingEdit
Me.gvBS.EditIndex = -1
gvBS_DataBind()
End Sub

Answer 1

I assume that the GridView enters never edit-mode since you aren't handling the RowEditing event or you didn't DataBind it after you've set gvBS.EditIndex = e.NewEditIndex;.
<asp:GridView
OnRowEditing="gvBS_RowEditing" OnRowCancelingEdit="gvBS_RowCancelingEdit"
ID="gvBS" runat="server" AutoGenerateColumns="false"
DataKeyNames="ID" SkinID="gvWithoutWidth">
codebehind (BindGrid is the method which databinds your grid):
protected void gvBS_RowEditing(object sender, GridViewEditEventArgs e)
{
gvBS.EditIndex = e.NewEditIndex;
BindGrid();
}
protected void gvBS_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
{
gvBS.EditIndex = -1;
BindGrid();
}
You should also remember to databind it only on the first load, not on consecutive postbacks when ViewState is enabled(default). Therefore you can check the page's IsPostBack property:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
BindGrid();
}
}

Answer 2

Try the Bind instead of Eval in EditItemTemplate like this
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:GridView ID="gvBS" runat="server" AutoGenerateColumns="false" DataKeyNames="ID" SkinID="gvWithoutWidth">
<Columns>
<asp:CommandField ShowEditButton="true" EditImageUrl="~/Images/GridView/gv_edit.png" ButtonType="Image"
CancelImageUrl="~/Images/GridView/gv_cancel.png" UpdateImageUrl="~/Images/GridView/gv_update.png"/>
<asp:TemplateField HeaderText="Sollmonat" HeaderStyle-HorizontalAlign="Left">
<EditItemTemplate>
<asp:TextBox ID="tbSollMonat" runat="server" Text='<%# Bind("SollMonat") %>'></asp:TextBox>
<asp:RequiredFieldValidator ID="rfvSollMonat" ValidationGroup="Update" runat="server"
ControlToValidate="tbSollMonat" ErrorMessage="Bitte Sollmonat (dd.mm.yyyy) angeben"
SetFocusOnError="true">*</asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="revSollMonat" ValidationGroup="Update" runat="server"
ValidationExpression="^\d+$" ControlToValidate="tbSollMonat">*</asp:RegularExpressionValidator>
</EditItemTemplate>
<ItemTemplate>
<%# Eval("SollMonat")%>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</ContentTemplate>