To Set Permission Level Depending upon Roles

SPRoleDefinitionCollection collRoleDefinitions = web.RoleDefinitions;
SPRoleAssignmentCollection collRoleAssignments = web.RoleAssignments;
SPRoleAssignment oRoleAssignment = new SPRoleAssignment(strLogin,"","","");
SPRoleDefinitionBindingCollection collRoleDefinitionBindings = oRoleAssignment.RoleDefinitionBindings;
collRoleDefinitionBindings.Add(collRoleDefinitions["Read"]);
web.AllowUnsafeUpdates=true;