Fisrt try

CommandParser.cs

@@ -0,0 +1,59 @@
+namespace SectTask;
+
+public class CommandParser
+{
+    public void Commands(string?[] args)
+    {
+        TaskManager td = new TaskManager();
+        for (int i = 0; i < args.Length - 1; ++i)
+        {
+            if (args[i] is null)
+            { 
+                return;
+            } 
+            switch (args[i]) 
+            { 
+                case "/add": 
+                    td.AddTask(Convert.ToInt32(args[i + 4]), Convert.ToDateTime(args[i + 5]),Convert.ToBoolean(args[i + 6])); 
+                    break;
+                case "/all": 
+                    td.ShowTasks(); 
+                    break;
+                case "/delete": 
+                    td.DeleteTask(Convert.ToInt32(args[i + 1])); 
+                    break;
+                case "/save": 
+                    td.toFile(args[i + 1]); 
+                    break;
+                case "/load": 
+                    td.fromFile(args[i + 1]); 
+                    break;
+                case "/completed": 
+                    td.ChangeTaskStatus(Convert.ToInt32(args[i + 1]), Convert.ToBoolean(args[i + 2])); 
+                    break;
+                case "/create-group": 
+                    td.CreateGroup(Convert.ToInt32(args[i + 1])); 
+                    break;
+                case "/delete-group": 
+                    td.DeleteGroup(Convert.ToInt32(args[i + 1])); 
+                    break;
+                case "/add-to-group": 
+                    td.AddToGroup(Convert.ToInt32(args[i + 1]), Convert.ToInt32(args[i + 2])); 
+                    break;
+                case "/delete-from-group": 
+                    td.DeleteTaskFromGroup(Convert.ToInt32(args[i + 1]), Convert.ToInt32(args[i + 2])); 
+                    break;
+                case "/add-subtask": 
+                    SubTask sub1 = new SubTask(Convert.ToBoolean(args[i + 1]), Convert.ToInt32(args[i + 2]), args[i + 3], args [i + 4]); 
+                    td.AddSubTask(Convert.ToInt32(args[i + 5]),sub1); 
+                    break;
+                case "/change-sub-status": 
+                    td.ChangeSubtaskStatus(Convert.ToInt32(args[i + 1]), Convert.ToInt32(args[i + 1])); 
+                    break;
+                case "/today":
+                    td.PrintTodaysDeadline(); 
+                    break;
+            }
+        }
+    }
+}

Group.cs

@@ -0,0 +1,13 @@
+namespace SectTask;
+
+public class Group
+{    
+    public List<Task> Tasks;
+    public int? GroupId { get; set; }
+
+    public Group(int Id)
+    {
+        Tasks = new List<Task>();
+        GroupId = Id;
+    }
+}

Methods.cs

@@ -0,0 +1,189 @@
+using System.Data;
+using System.Numerics;
+
+namespace SectTask;
+
+using System.Text.Json;
+
+public class TaskManager
+{
+    private readonly List<Task> _taskData;
+    private readonly List<Group> _groupData;
+
+    public TaskManager()
+    {
+        _groupData = new List<Group>();
+        _taskData = new List<Task>();
+    }
+
+    public void AddTask(int id, DateTime deadline, bool complete)
+    {
+        _taskData.Add(new Task(id, deadline, complete));
+        Console.WriteLine("Task Created!");
+    }
+
+    public void AddSubTask(int taskId, SubTask subTask)
+    {
+
+        var adder = _taskData
+            .FirstOrDefault(task => task.TaskId == taskId)
+            ?.SubTasks;
+        if (adder != null)
+        {
+            adder.Add(subTask);
+            Console.WriteLine("SubTask Created!");
+        }
+        else
+        {
+            Console.WriteLine("Try another Id");
+        }
+
+    }
+
+    public void ChangeSubtaskStatus(int taskId, int subTaskId)
+    {
+        _taskData
+            .FirstOrDefault(task => task.TaskId == taskId)?
+            .SubTasks
+            .FirstOrDefault(subtask => subtask.SubTaskId == subTaskId)?
+            .ChangeStatus();
+        Console.WriteLine("Status Changed!");
+    }
+
+    public void ShowTasks()
+    {
+        foreach (var task in _taskData)
+        {
+            Console.WriteLine(task!.TaskId);
+            Console.WriteLine(task!.TaskStatus);
+            foreach (var task1 in task.SubTasks)
+            {
+                var json = JsonSerializer.Serialize(task1);
+                Console.WriteLine(json);
+            }
+
+            var json1 = JsonSerializer.Serialize(task.DeadLine);
+            Console.WriteLine(json1);
+        }
+    }
+
+    public void DeleteTask(int id)
+    {
+        for (var i = 0; i < _taskData.Count; ++i)
+        {
+            if (_taskData[i]!.TaskId == id)
+            {
+                _taskData.Remove(_taskData[i]);
+            }
+        }
+    }
+
+    public void ChangeTaskStatus(int id, bool status)
+    {
+        foreach (var t in _taskData)
+        {
+            t!.TaskStatus = status;
+        }
+
+        Console.WriteLine("Status Changed!");
+    }
+
+    public void toFile(string? ph)
+    {
+        var str = new StreamWriter(ph!);
+        foreach (var t in _taskData)
+        {
+            str.WriteLine(t!.TaskId);
+            str.WriteLine(t!.TaskStatus);
+            var json = JsonSerializer.Serialize(t?.SubTasks);
+            var json1 = JsonSerializer.Serialize(t?.DeadLine);
+            str.WriteLine(json);
+            str.WriteLine(json1);
+        }
+
+        str.Close();
+    }
+
+    public void fromFile(string? ph)
+    {
+        if (!File.Exists(ph))
+        {
+            throw new Exception("File not found");
+        }
+
+        using var reader = new StreamReader(ph);
+        _taskData.Clear();
+        for (var i = 0; i < System.IO.File.ReadAllLines(ph).Length; ++i)
+        {
+            var json = reader.ReadLine();
+            if (json != null) _taskData.Add(JsonSerializer.Deserialize<Task>(json)!);
+        }
+    }
+
+    public void PrintTodaysDeadline()
+    {
+        foreach (var task in _taskData)
+        {
+            if (task.DeadLine.Day == DateTime.Now.Day)
+            {
+                var json = JsonSerializer.Serialize(task);
+                Console.WriteLine(json);
+            }
+        }
+    }
+
+    public void CreateGroup(int Id)
+    {
+        _groupData.Add(new Group(Id));
+    }
+
+    public void AddToGroup(int taskId, int groupId)
+    {
+        foreach (var t in _taskData)
+        {
+            if (t.TaskId == taskId)
+            {
+                foreach (var t1 in _groupData)
+                {
+                    if (t1.GroupId == groupId)
+                    {
+                        t1.Tasks.Add(t);
+                    }
+                }
+            }
+        }
+    }
+
+    public void ShowGroup()
+    {
+        foreach (var t in _groupData)
+        {
+            var json = JsonSerializer.Serialize(t!.GroupId);
+            var json1 = JsonSerializer.Serialize(t!.Tasks);
+            Console.WriteLine(json);
+            Console.WriteLine(json1);
+        }
+    }
+
+    public void DeleteGroup(int Id)
+    {
+        var group = _groupData.Single(group => group.GroupId == Id);
+        _groupData.Remove(group);
+    }
+
+    public void DeleteTaskFromGroup(int groupId, int taskId)
+    {
+        foreach (var group in _groupData.Where(t => t.GroupId == groupId))
+        {
+            for (var j = 0; j < _taskData.Count; ++j)
+            {
+                if (_taskData[j].TaskId == taskId)
+                {
+                    group.Tasks.Remove(group.Tasks[j]);
+                }
+            }
+        }
+    }
+
+
+}

Program.cs

@@ -0,0 +1,42 @@
+namespace SectTask;
+
+static class Program
+{
+    static void Main(string?[] args)
+    {
+        CommandParser input = new CommandParser();
+        input.Commands(args);
+
+        /* TaskManager td = new TaskManager();
+        SubTask subTask = new SubTask(false, 1,"sport", "beerpong") ;
+        SubTask subTask1 = new SubTask(false, 2,"sprt", "beerpong") ;
+        SubTask subTask2 = new SubTask(false, 3,"port", "beerpong") ;
+
+        td.AddTask(1,DateTime.Now, true);
+        td.AddTask(2,DateTime.Now, true);
+        td.AddTask(3,DateTime.Now, true);
+
+        td.ShowTasks();
+        Console.WriteLine();
+        td.ChangeTaskStatus(1, false);
+        td.ShowTasks();
+        Console.WriteLine();
+        td.ChangeSubtaskStatus(1, 1);
+        td.ShowTasks();
+        td.AddSubTask(1,subTask);
+        td.AddSubTask(1,subTask1);
+        td.AddSubTask(5,subTask2);
+        td.CreateGroup(1);
+        td.AddToGroup(1,1);
+        td.AddToGroup(2,1);
+        Console.WriteLine();
+        //td.ShowTasks();
+
+        td.ShowGroup();
+        td.DeleteTaskFromGroup(1,2);
+        td.ShowGroup();
+        td.DeleteGroup(1);
+        td.ShowGroup();
+    */
+    }
+}

SubTask.cs

@@ -0,0 +1,22 @@
+namespace SectTask;
+
+public class SubTask
+{
+    public bool Status { get; set; }
+
+    public string? Name { get; set; }
+
+    public string? Description { get; set; }
+
+    public int SubTaskId { get; set; }
+
+    public SubTask(bool complete, int id, string? name, string? description)
+    {
+        Status = complete;
+        Name = name;
+        Description = description;
+        SubTaskId = id;
+    }
+
+    public void ChangeStatus() => Status = !Status;
+}

Task.cs

@@ -0,0 +1,16 @@
+namespace SectTask;
+public class Task
+{
+    public List<SubTask> SubTasks;
+    public int TaskId { get; set; }
+    public bool TaskStatus { get; set; }
+    public DateTime DeadLine { get; set; } // 01.01.0001 0:00:00
+
+    public Task(int id, DateTime deadline, bool status)
+    {
+        TaskId = id;
+        SubTasks = new List<SubTask>();
+        DeadLine = deadline;
+        TaskStatus = status;
+    }
+}

sectreborn.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net6.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+</Project>

sectreborn.sln

@@ -0,0 +1,16 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "sectreborn", "sectreborn\sectreborn.csproj", "{68D18D10-A682-4D7E-BB67-3815EA1336E7}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{68D18D10-A682-4D7E-BB67-3815EA1336E7}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{68D18D10-A682-4D7E-BB67-3815EA1336E7}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{68D18D10-A682-4D7E-BB67-3815EA1336E7}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{68D18D10-A682-4D7E-BB67-3815EA1336E7}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal