Entity Framework Core CompileAsyncQuery syntax to do a query returning a list? - async-await

Question

Documentation and examples online about compiled async queries are kinda sparse, so I might as well ask for guidance here.
Let's say I have a repository pattern method like this to query all entries in a table:
public async Task<List<ProgramSchedule>> GetAllProgramsScheduledList()
{
using (var context = new MyDataContext(_dbOptions))
{
context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
return await context.ProgramsScheduledLists.ToListAsync();
}
}
This works fine.
Now I want to do the same, but with an async compiled query.
One way I managed to get it to compile is with this syntax:
static readonly Func<MyDataContext, Task<List<ProgramSchedule>>> GetAllProgramsScheduledListQuery;
static ProgramsScheduledListRepository()
{
GetAllProgramsScheduledListQuery = EF.CompileAsyncQuery<MyDataContext, List<ProgramSchedule>>(t => t.ProgramsScheduledLists.ToList());
}
public async Task<List<ProgramSchedule>> GetAllProgramsScheduledList()
{
using (var context = new MyDataContext(_dbOptions))
{
context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
return await GetAllProgramsScheduledListQuery(context);
}
}
But then on runtime this exception get thrown:
System.ArgumentException: Expression of type 'System.Collections.Generic.List`1[Model.Scheduling.ProgramSchedule]' cannot be used for return type 'System.Threading.Tasks.Task`1[System.Collections.Generic.List`1[Model.Scheduling.ProgramSchedule]]'
The weird part is that if I use any other operator (for example SingleOrDefault), it works fine. It only have problem returning List.
Why?

Answer 1

EF.CompileAsync for set of records, returns IAsyncEnumrable<T>. To get List from such query you have to enumerate IAsyncEnumrable and fill List,
private static Func<MyDataContext, IAsyncEnumerable<ProgramSchedule>> compiledQuery =
EF.CompileAsyncQuery((MyDataContext ctx) =>
ctx.ProgramsScheduledLists);
public static async Task<List<ProgramSchedule>> GetAllProgramsScheduledList(CancellationToken ct = default)
{
using (var context = new MyDataContext(_dbOptions))
{
context.ChangeTracker.QueryTrackingBehavior = QueryTrackingBehavior.NoTracking;
var result = new List<ProgramSchedule>();
await foreach (var s in compiledQuery(context).WithCancellation(ct))
{
result.Add(s);
}
return result;
}
}