使用LINQ将IEnumerable转换为IList以使用不同的日期填充ListView适配器

Question

I am trying to populate my ListView adapter with unique dates.I am trying to use LINQ to pull the distinct dates from my database. Note: object 'tasks' is declared as IList.

Sample Data:

ID |    Date     | Task
 1    4/10/2015     A
 2    4/10/2015     B
 3    4/9/2015      B

Output:

    Date    
 4/10/2015
 4/9/2015

Here is my code:

protected override void OnResume ()
    {
        base.OnResume ();

        tasks = TaskManager.GetTasks ().GroupBy(x=> x.Created_date).Distinct();

        // create our adapter
        taskList = new Adapters.TaskListAdapter(this, tasks);

        //Hook up our adapter to our ListView
        taskListView.Adapter = taskList;
    }

public class TaskListAdapter : BaseAdapter<Task> {
        Activity context = null;
        IList<Task> tasks = new List<Task>();

        public TaskListAdapter (Activity context, IList<Task> tasks) : base ()
        {
            this.context = context;
            this.tasks = tasks;
        }

Here is my error:

Error CS0266: Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<System.Linq.IGrouping<System.DateTime,Tasky.Core.Task>>' to 'System.Collections.Generic.IList<Tasky.Core.Task>'. An explicit conversion exists (are you missing a cast?) (CS0266) (TaskyAndroid)

Answer 1

Your task object doesn't implement IList, you need to convert it (not cast it, casting won't help, it's not wrongly casted, it's genuinely NOT something that implements ilist as it's just an IEnumerable returned by linq, not backed by an actual physical collection).

The easiest way to do this would be to turn it into a List (which does implement IList) so after your distinct, simply toss a .ToList();

tasks = TaskManager.GetTasks ().GroupBy(x=> x.Created_date).Distinct().ToList();

Make sure to change the definitiion of tasks in your class to be IList else it will still be considered an IEnumerable, or if you can't change that definition make sure you also change your call to TaskListAdapter to include the (now valid) cast.

taskList = new Adapters.TaskListAdapter(this, (IList<Task>)tasks);

Note that this solves your casting issue but it still makes no sense because of the groupby (you have a list of groups, not of tasks) so you'll need to remove it or rework your query first to end up with just an enumeration of tasks.

Answer 2

Please cast your resulting IEnumerable to IList , eg like this:

tasks = TaskManager
        .GetTasks()
        .GroupBy(x=> x.Created_date)
        .Distinct()
        .Cast<Tasky.Core.Task>()
        .ToList();

And by the way, why do you do use GroupBy() clause here? I think .Select(x => x.Created_date).Distinct() should do.