DropDownList in GridView

It is necessary to convert bound field to a template field, then set your DropDownList up. In the example it is bound to an ObjectDataSource. It is possible to use statement like (SelectedValue=<%# Bind("Field") %>), but this way is good only for read-only gridview.
Any postback can throw an exception connected to the Bind/Eval way of binding -
'Databinding methods such as Eval(), XPath(), and Bind() can only be used in the context of a databound control'.

The better way is by using customized code during processing the ItemUpdating event:

<asp:TemplateField HeaderText="Header text">
  <ItemTemplate>
    <asp:DropDownList ID="ddlItem"
        runat="server" OnDataBound="ddlItems_DataBound"
        DataSourceID="ObjectDataSource1"    AppendDataBoundItems="false"
        DataTextField="Name" DataValueField="Id" >
    </asp:DropDownList>
  </ItemTemplate>
</asp:TemplateField>

protected void ddlItems_DataBound(object sender, EventArgs e)
{
  DropDownList ddl = (DropDownList)sender;
  GridViewRow gvr = (GridViewRow)ddl.NamingContainer;
  if (gvr.DataItem != null)
  {
      string id = ((DataRowView)gvr.DataItem)["Id"].ToString();
      ddl.ClearSelection();
      ListItem li = ddl.Items.FindByValue(id);
      if (li != null) li.Selected = true;
  }     
}

It is important to use cast of sender, not direct calling to the GridView1 control. Every GridViewRow is actually DetailsView object.

protected void GridView1_RowUpdating
 (object sender, GridViewUpdateEventArgs e)
{     
 DropDownList ddl =
   (DropDownList)(((GridView)sender).Rows[e.RowIndex].FindControl("ddlItems"));
 e.NewValues["Id"] = ddl.SelectedValue;
}