@inject AssignmentDragContainer dragContainer
@inject CoursePlanner planner

@code {
  [Parameter]
  [EditorRequired]
  public LocalAssignment Assignment { get; set; } = new();

  private void HandleDragStart()
  {
    dragContainer.DropCallback = (DateTime dropDate) => {
      var module = planner
        .LocalCourse?
        .Modules
        .First(m => 
          m.Assignments
            .Select(a => a.id)
            .Contains(Assignment.id)
        );
      if (module == null)
      {
        Console.WriteLine("module is null");
        return;
      }
      if (planner.LocalCourse != null)
      {
        var newCourse = planner.LocalCourse with 
        {
          Modules = planner.LocalCourse.Modules.Select(m => 
            m.Name != module.Name
              ? m
              : m with 
              {
                Assignments = module.Assignments.Select(a => 
                  a.id != Assignment.id
                  ? a
                  : a with 
                  {
                    due_at=dropDate
                  }
                )
              }
          )
        };
        planner.LocalCourse = newCourse;
      }
    };
  }

  private void HandleDragEnd()
  {
    dragContainer.DropCallback = null;
  }
}

<div
  draggable="true"
  @ondragstart="HandleDragStart"
  @ondragend="HandleDragEnd"
  role="button"
>
  @Assignment.name
</div>