SQLite EF6异步API引发InvalidCastException

Question

I'm new to entity framework and try to use it with SQLite. I've a working setup if I don't use the async API.

In my simplified szenario are only two tables containing a one-to-many relationship. DB setup and insertion of values works fine. Query is a problem.

Here is the simplified code:

var connectionStringBuilder = new SQLiteConnectionStringBuilder { DataSource = dbFile, ForeignKeys = true };
using (var connection = new SQLiteConnection { ConnectionString = connectionStringBuilder.ConnectionString })
{
    await connection.OpenAsync();
    // query data
    using (var context = new TestContext(connection, contextOwnsConnection: false))
    {
        var xQuery = context.Xs.Include(item => item.Ys);

        var syncX = xQuery.First(); // works fine
        var asyncX = await xQuery.FirstAsync(); // throws InvalidCastException
    }
}

Why does the call to FirstAsync() throws an exception while First() doesn't?

The same happens using SingleAsync() and ToListAsync() . Currently I'm of the opinion that this happens on all async calls.

Removing the Include(...) clause from query will fix the problem but forces a second database query on access to the property.

To avoid hints like "you're calling First and FirstAsync on the same query object...": NO. The problem still occurs if I only call ...Async() without calling a synchronous method first. That's just for clearification.

I'm using a WinForms application .Net 4.7.1, EF6 (by adding System.Data.SQLite v1.0.108 via Nuget).

The complete code to reproduce the problem:

private async void OnClick(object sender, EventArgs e)
{
    var dbFile = "test.sqlite";
    if (File.Exists(dbFile)) File.Delete(dbFile);
    SQLiteConnection.CreateFile(dbFile);

    var connectionStringBuilder = new SQLiteConnectionStringBuilder { DataSource = dbFile, ForeignKeys = true };
    using (var connection = new SQLiteConnection { ConnectionString = connectionStringBuilder.ConnectionString })
    {
        await connection.OpenAsync();

        // setup database
        using (var context = new TestContext(connection, contextOwnsConnection: false))
        {
            await context.Database.ExecuteSqlCommandAsync("CREATE TABLE X (Id VARCHAR2 PRIMARY KEY);");
            await context.Database.ExecuteSqlCommandAsync("CREATE TABLE Y (Id VARCHAR2 PRIMARY KEY, XId VARCHAR2 NOT NULL, FOREIGN KEY (XId) REFERENCES X(Id));");

            var x0 = new X { Id = "X0" };
            var y0 = new Y { Id = "Y0", XId = x0.Id }; // Currently I don't know why using the navigation property 'X = x0' isn't working but the XId will work.
            var y1 = new Y { Id = "Y1", XId = x0.Id }; // Currently I don't know why using the navigation property 'X = x0' isn't working but the XId will work.

            x0.Ys.Add(y0);
            x0.Ys.Add(y1);

            context.Xs.Add(x0);
            context.Ys.Add(y0); // not needed but for safety :-)
            context.Ys.Add(y1); // not needed but for safety :-)

            await context.SaveChangesAsync();
        }

        // query data
        using (var context = new TestContext(connection, contextOwnsConnection: false))
        {
            var xQuery = context.Xs.Include(item => item.Ys);

            var syncX = xQuery.First(); // works fine
            var asyncX = await xQuery.FirstAsync(); // throws InvalidCastException
        }
    }
}

Using the following model classes:

public class TestContext : DbContext
{
    public TestContext(DbConnection existingConnection, bool contextOwnsConnection = true) :
        base(existingConnection, contextOwnsConnection)
    {
    }

    public DbSet<X> Xs { get; set; }

    public DbSet<Y> Ys { get; set; }
}

[Table("X")]
public class X
{
    public X()
    {
        // ReSharper disable once VirtualMemberCallInConstructor
        this.Ys = new HashSet<Y>();
    }

    [Column("Id")]
    [Key, Required]
    public string Id { get; set; }

    public virtual ICollection<Y> Ys { get; set; }
}

[Table("Y")]
public class Y
{
    [Column("Id")]
    [Key, Required]
    public string Id { get; set; }

    [Column("XId")]
    [Required]
    public string XId { get; set; }

    [ForeignKey("XId")]
    public virtual X X { get; set; }
}

Answer 1

I'm sorry, I can't write comments.

Your problem is very interesting, the first thought I had was that the types do not match, because I remembered exactly what FirstAsync takes IQueryable, and if I do include something the type of request is IIncludeableQueryable, but I tested my guess and looked at the implementation through dot-peek, alas, it work in both cases, and by itself IIncludeableQueryableis inherited from IQueryable

        var testQuery = DbContext.Practices.Include(x => x.Facility);
        var testQuery2 = DbContext.Practices.Select(x => x);

        var asyncPracticeCorrectType = await testQuery2.FirstAsync();
        var asyncPractice = await testQuery.FirstAsync();

i got entity Practice in both ways:(

I think your problem in area SQLite, because your code look correctly

Answer 2

I tried entity framework core instead of entity framework 6 which works perfectly according to that problem.

In my option it's a bug in entitiy framework 6 or in the sqlite provider.