ну готово вроде, почти без багов)

Reconstruction/Functions/LoggedUser/Display.cs

@@ -9,7 +9,7 @@ namespace Reconstruction.Functions
         public static void ShowCompletedTask()
         {
             Console.WriteLine("Tasks completed by " + Service.LoggedUser.Login);
-            ObservableCollection<Task> TaskCollection = new ObservableCollection<Task>(Service.db.Tasks.Where(x => x.AccUserId == Service.LoggedUser.Id && x.StatusId = 3).ToList());
+            ObservableCollection<Task> TaskCollection = new ObservableCollection<Task>(Service.db.Tasks.Where(x => x.AccUserId == Service.LoggedUser.Id && x.StatusId == 3).ToList());
 
             foreach (Task t in TaskCollection)
             {
@@ -29,7 +29,6 @@ namespace Reconstruction.Functions
         public static void ShowUsersTasks()
         {
             ObservableCollection<Task> TaskCollection = new ObservableCollection<Task>(Service.db.Tasks.Where(x => x.AccUserId == Service.LoggedUser.Id).ToList());
-
             foreach (Task t in TaskCollection)
             {
                 User user = (User)Service.db.Users.Where(u => u.Id == t.CreatorUserId).SingleOrDefault();
@@ -40,34 +39,20 @@ namespace Reconstruction.Functions
                                   "\nCreated by " + user.Login +
                                   "\nStatus: " + status.Title +
                                   "\nEnter " + t.Id + " to change its status\n");
+
             }
 
             int id = 0;
             Task task;
 
-            while (true)
-            {
-                try
-                {
-                    id = int.Parse(Console.ReadLine());
-                    task = (Task)Service.db.Tasks.Where(x => x.Id == id).SingleOrDefault();
-                    break;
-                }
-                catch
-                {
-                    Console.WriteLine("You entered a wrong value");
-                }
-            }
-            if(task != null)
+            if(TaskCollection != null)
             {
-                Console.WriteLine("Enter:\n1 - to set task 'incompleted'\n2 - to set task 'in progress'\n3 - to set task 'completed'\n0 - to abolish this operation\n");
-                int statusID;
-
-                while(true)
+                while (true)
                 {
                     try
                     {
-                        statusID = int.Parse(Console.ReadLine());
+                        id = int.Parse(Console.ReadLine());
+                        task = (Task)Service.db.Tasks.Where(x => x.Id == id).SingleOrDefault();
                         break;
                     }
                     catch
@@ -75,18 +60,40 @@ namespace Reconstruction.Functions
                         Console.WriteLine("You entered a wrong value");
                     }
                 }
-                if(statusID != 0 && statusID < 4)
-                {
-                    task.StatusId = statusID;
-                    Service.db.Tasks.Update(task);
-                    Service.db.SaveChanges();
-                    Console.WriteLine("Operation performed successfully");
-                }
-                else
+                if (task != null)
                 {
-                    Console.WriteLine("Operation abolished");
-                }
+                    Console.WriteLine("Enter:\n1 - to set task 'incompleted'\n2 - to set task 'in progress'\n3 - to set task 'completed'\n0 - to abolish this operation\n");
+                    int statusID;
+
+                    while (true)
+                    {
+                        try
+                        {
+                            statusID = int.Parse(Console.ReadLine());
+                            break;
+                        }
+                        catch
+                        {
+                            Console.WriteLine("You entered a wrong value");
+                        }
+                    }
+                    if (statusID != 0 && statusID < 4)
+                    {
+                        task.StatusId = statusID;
+                        Service.db.Tasks.Update(task);
+                        Service.db.SaveChanges();
+                        Console.WriteLine("Operation performed successfully");
+                    }
+                    else
+                    {
+                        Console.WriteLine("Operation abolished");
+                    }
 
+                }
+            }
+            else
+            {
+                Console.WriteLine("User tasks are empty");    
             }
 
             Console.ReadKey();
@@ -126,41 +133,50 @@ namespace Reconstruction.Functions
         {
             ObservableCollection<Task> TaskCollection = new ObservableCollection<Task>(Service.db.Tasks.ToList());
 
+            bool check = false ;
             foreach (Task t in TaskCollection)
             {
                 User user = (User)Service.db.Users.Where(u => u.Id == t.CreatorUserId).SingleOrDefault();
 
-                if(t.StatusId != 1 && t.AccUserId == null)
+                if (t.StatusId != 1 && t.AccUserId == null)
                 {
                     Console.WriteLine("Title: " + t.Title +
                                       "\nDescription: " + t.Description +
-                                      "\nCreated by " + user.Login + 
+                                      "\nCreated by " + user.Login +
                                       "\nEnter " + t.Id + " to take this task\n");
+                    check = true;
                 }
             }
 
             int id = 0;
             Task task;
-
-            while (true)
+            if (check == true)
             {
-                try
-                {
-                    id = int.Parse(Console.ReadLine());
-                    task = (Task)Service.db.Tasks.Where(x => x.Id == id).SingleOrDefault();
-                    break;
-                }
-                catch
+                while (true)
                 {
-                    Console.WriteLine("You entered a wrong value");
+                    try
+                    {
+                        id = int.Parse(Console.ReadLine());
+                        task = (Task)Service.db.Tasks.Where(x => x.Id == id).SingleOrDefault();
+                        break;
+                    }
+                    catch
+                    {
+                        Console.WriteLine("You entered a wrong value");
+                    }
+
                 }
-                
-            }
 
-            task.AccUserId = Service.LoggedUser.Id;
+                task.AccUserId = Service.LoggedUser.Id;
 
-            Service.db.Tasks.Update(task);
-            Service.db.SaveChanges();
+                Service.db.Tasks.Update(task);
+                Service.db.SaveChanges();
+            }
+            else
+            {
+                Console.WriteLine("No available tasks");
+            }
+                
 
             Console.ReadKey();
         }

Reconstruction/Program.cs

@@ -50,6 +50,7 @@ while(Service.IsLoggedIn == true)
             }
         case 4:
             {
+                Display.ShowCompletedTask();
                 break;
             }
         case 5: