To avoid clashing for that validtion is needed using csharp

Question

 design as follows in Data Grid  VIew;


  Date      Session    RK      VK   SK   MK CKK CMK UNNI MS TMS


  7/1/2013    1       REO      PH2  REO
  7/1/2013    2
  7/1/2013    3
  7/1/2013    4
 

In the run time 7th date lst session REO (course) is taken by RK (Staff).then that same course REO is not to be taken by SK (Staff).

suppose SK select the REO course in the data grid view message would come as"RK taken REO Class would you accept again" the message should be the yes or no.

if yes SK allowed to take REO on the 1st Session.if no SK  not allowed to take class.


for that how to do validation for clashing using c sharp in Data Grid View.