using stringbuilder to build ContentPlaceHolder

Question

hello.. i'm using stringbuilder to create contentPlaceholder in nestedMasterpage but i get this error. -->

 

Cannot find ContentPlaceHolder 'MainContent' in the master page '/Masterpage/leftMenuMaster.Master', verify content control's ContentPlaceHolderID attribute in the content page.

 

how do i fix this?..My codes are as below :

 

stringBuilder.Append("</td>");

stringBuilder.AppendFormat("<td>fsdfsfsdfs</td>");

stringBuilder.AppendFormat("<td style=width: 99%; background-color:white; vertical-align:top><asp:ContentPlaceHolder ID=MainContent runat=server><p>&nbsp;MainContent</p></asp:ContentPlaceHolder></td>");

stringBuilder.Append("</tr>");

stringBuilder.Append("</table>");

stringBuilder.Append("</div>");

 

TQ.