Details View and Drop Down List

protected void DropDownListState_DataBound(object sender, System.EventArgs e)
{
    try {
        DropDownList ddlState = (DropDownList)sender;
        ListItem listItem = new ListItem("--Select--", "");
        ddlState.Items.Insert(0, listItem);
        Data.DataRowView View = DetailsViewPrimary.DataItem;
        ddlState.SelectedValue = View.Item("StateProvinceID").ToString();
        ObjectDataSourceCounty.FilterExpression = "StateProvinceID='" + ddlState.SelectedValue + "'";
    }
    catch (Exception ex) {
    }
    try {
        DropDownList ddlState = (DropDownList)sender;
        Data.DataRowView View = DetailsViewMailing.DataItem;
        ddlState.SelectedValue = View.Item("StateProvinceID").ToString();
        ObjectDataSourceCounty.FilterExpression = "StateProvinceID='" + ddlState.SelectedValue + "'";
    }
    catch (Exception ex) {
    }
}

I faced the same problem (based on selected State, County has to be filtered in Dropdownlist in DetailView Template). so add a FilterExpression to the datasource of the Parent DropDown (in my case State) on its DataBound Event.

<asp:TemplateField HeaderText="State:">
<EditItemTemplate>
<asp:DropDownList ID="DropDownListState" runat="server" DataSourceID="ObjectDataSourceState" DataTextField="StateProvince" DataValueField="StateProvinceID" OnDataBound="DropDownListState_DataBound" OnSelectedIndexChanged="DropDownListState_SelectedIndexChanged" AutoPostBack="true">
asp:DropDownList>
<asp:RequiredFieldValidator ControlToValidate="DropDownListState" runat='server' ID="RequiredFieldValidatorTextBoxState" ErrorMessage="*">asp:RequiredFieldValidator>
EditItemTemplate>
<InsertItemTemplate>
<asp:DropDownList ID="DropDownListState" runat="server" DataSourceID="ObjectDataSourceState" DataTextField="StateProvince" DataValueField="StateProvinceID" OnDataBound="DropDownListState_DataBound" OnSelectedIndexChanged="DropDownListState_SelectedIndexChanged" AutoPostBack="true">
asp:DropDownList>
<asp:RequiredFieldValidator ControlToValidate="DropDownListState" runat='server' ID="RequiredFieldValidatorTextBoxState" ErrorMessage="*">asp:RequiredFieldValidator>
InsertItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Bind("StateProvince") %>'>asp:Label>
ItemTemplate>
asp:TemplateField>
<asp:TemplateField HeaderText="County:">
<EditItemTemplate>
<asp:DropDownList ID="DropDownListCounty" runat="server" DataSourceID="ObjectDataSourceCounty" DataTextField="County" DataValueField="CountyID" OnDataBound="DropDownListCounty_DataBound">
asp:DropDownList>
EditItemTemplate>
<InsertItemTemplate>
<asp:DropDownList ID="DropDownListCounty" runat="server" DataSourceID="ObjectDataSourceCounty" DataTextField="County" DataValueField="CountyID" OnDataBound="DropDownListCounty_DataBound">
asp:DropDownList>
InsertItemTemplate>
<ItemTemplate>
<asp:Label ID="Label2" runat="server" Text='<%# Bind("County") %>'>asp:Label>
ItemTemplate>
asp:TemplateField>

Protected Sub DropDownListState_DataBound(ByVal sender As Object, ByVal e As System.EventArgs)
Try
Dim ddlState As DropDownList = CType(sender, DropDownList)
Dim listItem As ListItem = New ListItem("--Select--", "")
ddlState.Items.Insert(0, listItem)
Dim View As Data.DataRowView = DetailsViewPrimary.DataItem
ddlState.SelectedValue = View.Item("StateProvinceID").ToString()ObjectDataSourceCounty.FilterExpression = "StateProvinceID='" + ddlState.SelectedValue + "'"
Catch ex As Exception
End Try
Try
Dim ddlState As DropDownList = CType(sender, DropDownList)
Dim View As Data.DataRowView = DetailsViewMailing.DataItem
ddlState.SelectedValue = View.Item("StateProvinceID").ToString()ObjectDataSourceCounty.FilterExpression = "StateProvinceID='" + ddlState.SelectedValue + "'"
Catch ex As Exception
End Try
End Sub