Todo application implementation
As a very basic application this is fine, however, if the program becomes more complicated and you want to add a data base and asynchronous programming you will run into conflicts with some of the class names. For instance the Task class is supplied by the C# library for asynchronous programming.
If I was going to implement a task list application I would have a few more properties for the task class, such as:

Priority
Status (not started, started, in progress, completed).
Due date
Name
Description

These are the minimum I would use, some additional properties might be

Assigned to
Assigned by

if this was a multi-user task list.