Introducation:

Explanation:

Let us see how does our Gridview in  .aspx Page look like.

Our .aspx Page; 

<asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource1" AutoGenerateColumns=false
OnRowUpdating="GridView1_RowUpdating" >
<Columns>
<asp:CommandField ShowEditButton="True" ShowSelectButton=true />
<asp:BoundField DataField="id" HeaderText="id" SortExpression="id" />
<asp:BoundField DataField="message" HeaderText="message" SortExpression="message" />
<asp:TemplateField HeaderText="User Name">
<ItemTemplate>
<asp:Label ID="lbl1" runat="server" Text='<%# Bind("username")%>' />
</ItemTemplate>
<EditItemTemplate>
<asp:DropDownList ID="DropDownList1" runat="server" AppendDataBoundItems="true"
DataSourceID="SqlDataSource1" DataValueField=username AutoPostBack="True" SelectedValue='<%# Bind("username")%>'>
<asp:ListItem Value="">Select an Item</asp:ListItem>
</asp:DropDownList>
</EditItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

 Our Dropdownlist is embeded in the "ItemTemplete" of the Gridview. One more thing before going next, Always use Bind() with the Controles in both ItemTemplate and EditItemTemplate. For this Demo we are using SqlDataSource Object as DataSource. Now let us see how does "SqlDataSource" looke like.

<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:PersonalLocal %>"
SelectCommand="SELECT * FROM [ChatMessages]"
UpdateCommand="UPDATE [ChatMessages] SET [username] = @username, [message] = @message WHERE id=@id">
<DeleteParameters>
<asp:Parameter Name="id" Type="Int64" />
</DeleteParameters>
<UpdateParameters>
<asp:Parameter Name="message" Type="String" />
<asp:Parameter Name="online" Type="Boolean" />
<asp:Parameter Name="id" Type="Int64" />
<asp:Parameter Name="username" Type="String" />
</UpdateParameters>
</asp:SqlDataSource>

You noticed that in the Gridview Code we defined an event called "RowUpdating" In the code behind we will show the event code. Now, let us see how does the code behind look like.

Code behind for the Row Update event;

protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
string struName = ((DropDownList)((GridView)sender).Rows[e.RowIndex].FindControl("DropDownList1")).SelectedValue;
e.NewValues["username"] = struName;
e.Cancel = false;

}