Nullable annotation added

WD7UVN_HFT_2023241.Logic/LogicServices.cs

@@ -26,7 +26,7 @@ namespace WD7UVN_HFT_2023241.Logic
             this.CRUDOperations = CRUDOperations;
         }
 
-		public IQueryable<Employee> WhoMaintainsService(int serviceId)
+		public IQueryable<Employee>? WhoMaintainsService(int serviceId)
 		{
 			IQueryable<Employee> query =
 				from employee in CRUDOperations.ReadAllEmployees()
@@ -34,10 +34,15 @@ namespace WD7UVN_HFT_2023241.Logic
 				where service.ID == serviceId
 				select employee;
 
+			if (query.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+			
 			return query;
 		}
 
-		public Employee WhoIsResponsibleForService(int serviceId)
+		public Employee? WhoIsResponsibleForService(int serviceId)
 		{
 			var query =
 				from service in CRUDOperations.ReadAllServices()
@@ -46,28 +51,54 @@ namespace WD7UVN_HFT_2023241.Logic
 				where service.ID == serviceId
 				select employee;
 
+			if (query.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+            
 			return query.FirstOrDefault();
 		}
 
-public IQueryable<Employee> WhoWorksInMaintainerTeam(int maintainerTeamId)
+        public IQueryable<Employee>? WhoWorksInMaintainerTeam(int maintainerTeamId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllEmployees()
             .Where(e => e.MAINTAINER_ID == maintainerTeamId);
+
+			if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
 
-        public IQueryable<Employee> GetSubordinates(int managerId)
+        public IQueryable<Employee>? GetSubordinates(int managerId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllEmployees()
             .Where(e => e.MANAGER_ID == managerId);
+
+			if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
 
-        public IQueryable<Customer> WhoUsesService(int serviceId)
+        public IQueryable<Customer>? WhoUsesService(int serviceId)
         {
-            return CRUDOperations
+            var res = CRUDOperations
             .ReadAllCustomers()
             .Where(c => c.SERVICE_ID == serviceId);
+
+            if (res.ToList().Count == 0)
+            {
+                throw new NullReferenceException("No such database entry");
+            }
+
+            return res;
         }
     }
 }