How to transform from string containing XML and XSLT to excel in C#?

ds.WriteXml(MyXmlPath);

  XPathDocument xmlDoc = new XPathDocument(MyXmlPath);
  XslCompiledTransform XSLTransform = new XslCompiledTransform();
  XSLTransform.Load(AppBasePath + @"\Master\XSLT\" + strSelectedXSLT.ToString() + ".xslt");
  XSLTransform.Transform(MyXmlPath, MyExcelPath);

From the above coding i am write xml into disk for the given path by using dataset. And read from the disk path in order to transform xls file.BECAUSE ITS TAKE HEAVY TIME TO WRITE EXCEL

**PROBLEM : Instead of writing & Reading the xml content , why should i write the xml content into string and convert .

What I have tried:

so i tried below coding . But its not working . 

StringWriter sw = new StringWriter();
    ds.WriteXml(sw, XmlWriteMode.IgnoreSchema);
    string xmlcontent = sw.ToString();

XslCompiledTransform XSLTransform = new XslCompiledTransform();
 XSLTransform.Load(AppBasePath + @"\Master\XSLT\" + strSelectedXSLT.ToString() + ".xslt");
 XSLTransform.Transform(xmlcontent, MyExcelPath);

Any suggestions ?