Many-to-Many DATARELATION :Couldnt' figure it at all

One way to do this is to capture the CurrentCellChanged event on DataGrid1 if the CurrentRow is different then previously, then get the t3-id and loop through table 4 getting all records that have a mathcing t3-id and add those records to DataGrid3...

pls can u show me in code...what will happen in

mydatagrid_currentcellchanged(sender, e)
{
  //   ? mydatagrid.currenrowIndex ?

   //how to check if current row is different
   // how to get the column field i.e t3-ID
  // how to loop through the table4

 
  // mydatagrid3.bind();
}

before the 'foreach' statement
i did this

DataTable dt_new = new DataTable();
         
     dt_new  = mydataSet1.t4 ;

and then used that in the 'foreach'

-----  foreach ( DataRow dr in  dt_new ) {  createrows and bindgrid} ------

but its giving me this error
--------------------------------------------------------------------------------------------------------
"foreach statement cannot operate on variables of type 'System.Data.DataTable' because 'System.Data.DataTable' does not contain a definition for 'GetEnumerator', or it is inaccessible"
--------------------------------------------------------------------------------------------------------


and Should  this "private int lastRowIndex = 0;"
 be  "static int lastRowIndex;"

oh..

it has to be

foreach ( DataRow dr in  dt_new.Rows)..

---> dt_new.Rows

And when i run the program

it gives an exception : System.Argument exception at the "foreach( DataRow dr in  dt_new.Rows).."

additional information : "this row is already in another table"

Question : WE are trying to get the rows from a different table in the dataset using the DataRow object., when the currentcellchanged event is fired. is that possible?