In my article Data Formatting in Grid View, I discussed how you can set DataFormatString property on BoundField column to format the value displayed in that column. But trouble is that this approach only works if your data type in that column is of type like numeric, datetime. This approach does not work if you have data type as string. For example in one of cases we had phone numbers in table stored as "NVarChar" and my client wanted to display it in format (xxx) xxx-xxxx. When I tried to set DataFormatString value to {0:(###) ### ####}, it did not work.

Now your option is converting the data type of that column to something that can be formatted using the format string. For example in our case the phone numbers could be parsed as Int64. Then we could easily format them to by applying custom formatting string {0:(###) ### ####}. You will handle OnRowDatabound event and take care of the conversion there. I tried to handle it in OnRowCreated, unfortunately it does not work. It seems that GridView implementation overrides any test you set in the value at this stage.

Following is the GridView definition I have in the demo project.

<asp:GridView ID="ctlGridView" runat="server" autogeneratecolumns=False 
    gridlines=Horizontal onrowdatabound="OnRowDataBound">
  <columns>
      <asp:boundfield datafield="ID" sortexpression="ProductID" />
      <asp:boundfield datafield="FirstName" headertext="First Name" sortexpression="Name" />
      <asp:boundfield datafield="LastName" headertext="Last Name" />
      <asp:BoundField DataField="DOB" HeaderText="Date Of Birth" HtmlEncode="False" DataFormatString="{0:d}" />
      <asp:BoundField DataField="Phone" HeaderText="Phone Number" HtmlEncode="False" />
  </columns>
</asp:GridView>							
							

Here is the implementation I have in RowDataBound event handler.

 protected void OnRowDataBound(object sender, EventArgs e)
 {
     GridViewRowEventArgs ea = e as GridViewRowEventArgs;
     if (ea.Row.RowType == DataControlRowType.DataRow)
     {
         DataRowView drv = ea.Row.DataItem as DataRowView;
         Object ob = drv["Phone"];
         if (!Convert.IsDBNull(ob))
         {
             Int64 iParsedValue = 0;
             if (Int64.TryParse(ob.ToString(), out iParsedValue))
             {

                 TableCell cell = ea.Row.Cells[4];
                 cell.Text = String.Format(System.Globalization.CultureInfo.CurrentCulture, 
                    "{0:(###) ###-####}", new object[] { iParsedValue });
             }
         }
     }
 }