Dataset - assingnment of value to cell without effect

Question

Hi,
why does the following not work?

 DataSet ds = new DataSet();
 ds.ReadXml("D:\\test.xml");
 ds.Tables[8].Rows[0].ItemArray[0] = "test";  // this assingment does not work.

Before assingnment the value was "XXXXX".
After assingment the value was still "XXXXX", but it shold be "test"