Access roles in nested group

Question

I want to see if the current user is in the group "B_Group" (which this Group is in the Group "A_Group").

I used the code below:

if(Roles.IsUserInRole(@"A_Group\B_Group"))

but it does not find the user and returns false. also when I enumerate the roles of the user there is no such role for the user:

foreach( var role in Roles.GetRolesForUser())

it returns groups which are not in another group but does not return groups like B_Group.

Is there any solution for it?

tnx.