how to bind a dropdownlist in gridview?

If you are using template column then you can bind your drop-down from mark-up using data-binding expressions. For example,

<asp:TemplateField HeaderText="XYZ">
  <ItemTemplate>
    <asp:DropDownList runat="server" ID="MyDD" DataSourceId="MyDataSource" />
  </ItemTemplate> 
</asp:TemplateField>

Above is assuming that your drop-down data in constant across rows. If it is changing then you can use data-binding expression such as

<asp:DropDownList runat="server" DataSource='<%# GetDropDownData(Container) %>' DataTextField="Text" DataValueField="Value"  />

GetDropDownData will be a protected method in code-behind that will return the data (data-table, list, array) for the given row.

You can use GridView.RowDataBound event (or RowCreated event) in code-behind to fill drop-downs. For example,

  protected void GridView_RowDataBound(Object sender, GridViewRowEventArgs e)
  {

    if(e.Row.RowType == DataControlRowType.DataRow)
    {
      // Find the drop-down (say in 3rd column)
      var dd = e.Row.Cells[2].Controls[0] as DropDownList;
      if (null != dd) {
         // bind it
      }

      /*
      // In case of template fields, use FindControl
      dd = e.Row.Cells[2].FindControl("MyDD") as DropDownList;
      */
    }

  }

In addition to the proposed methods, you may also bind your controls within your markup, in this way:

<asp:GridView ID="MyGrid" runat="server" DataSourceID="MyDataSource1">
    <Columns>
        <asp:TemplateField>
            <EditItemTemplate>
                <asp:DropDownList ID="DropDownList1" runat="server" SelectedValue='<%# Bind ("CustomerId") %>' DataSourceID="CustomersDataSource" DataTextField="CustomerName" DataValueField="CustomerId" >
                </asp:DropDownList>
            </EditItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>