我有一些使用 Entity Framework Core 中可用的异步扩展方法的代码:
public async Task<MyResult> DoQuery<T>(IQueryable<T> queryable)
{
var count = await queryable.CountAsync();
var firstItems = await queryable
.Take(5)
.ToArrayAsync();
return new MyResult(count, firstItems);
}
当我提供的函数直接来自 EF 的 IQueryable
时,这非常有效。我还想重用此代码来对 LINQ 到对象“查询”执行一些逻辑:
var evens = Enumerable.Range(0, 10).Where(i => i % 2 == 0);
var result = DoQuery(evens.AsQueryable());
失败了(这并不奇怪):
System.InvalidOperationException: The provider for the source IQueryable doesn't implement IAsyncQueryProvider. Only providers that implement IEntityQueryProvider can be used for Entity Framework asynchronous operations.
看起来重构是有序的,但我很好奇:有什么方法可以将普通可枚举变成“虚拟”AsyncEnumerable
或 AsyncQueryable
同步处理CountAsync
?
最佳答案
您需要创建一个内存中的DbAsyncQueryProvider
来处理异步查询。关于如何做到这一点有详细的解释here 。滚动到关于使用异步查询进行测试的部分。以下是从该链接复制和粘贴的代码:
using System.Collections.Generic;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Linq.Expressions;
using System.Threading;
using System.Threading.Tasks;
namespace TestingDemo
{
internal class TestDbAsyncQueryProvider<TEntity> : IDbAsyncQueryProvider
{
private readonly IQueryProvider _inner;
internal TestDbAsyncQueryProvider(IQueryProvider inner)
{
_inner = inner;
}
public IQueryable CreateQuery(Expression expression)
{
return new TestDbAsyncEnumerable<TEntity>(expression);
}
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
{
return new TestDbAsyncEnumerable<TElement>(expression);
}
public object Execute(Expression expression)
{
return _inner.Execute(expression);
}
public TResult Execute<TResult>(Expression expression)
{
return _inner.Execute<TResult>(expression);
}
public Task<object> ExecuteAsync(Expression expression, CancellationToken cancellationToken)
{
return Task.FromResult(Execute(expression));
}
public Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken)
{
return Task.FromResult(Execute<TResult>(expression));
}
}
internal class TestDbAsyncEnumerable<T> : EnumerableQuery<T>, IDbAsyncEnumerable<T>, IQueryable<T>
{
public TestDbAsyncEnumerable(IEnumerable<T> enumerable)
: base(enumerable)
{ }
public TestDbAsyncEnumerable(Expression expression)
: base(expression)
{ }
public IDbAsyncEnumerator<T> GetAsyncEnumerator()
{
return new TestDbAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
}
IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator()
{
return GetAsyncEnumerator();
}
IQueryProvider IQueryable.Provider
{
get { return new TestDbAsyncQueryProvider<T>(this); }
}
}
internal class TestDbAsyncEnumerator<T> : IDbAsyncEnumerator<T>
{
private readonly IEnumerator<T> _inner;
public TestDbAsyncEnumerator(IEnumerator<T> inner)
{
_inner = inner;
}
public void Dispose()
{
_inner.Dispose();
}
public Task<bool> MoveNextAsync(CancellationToken cancellationToken)
{
return Task.FromResult(_inner.MoveNext());
}
public T Current
{
get { return _inner.Current; }
}
object IDbAsyncEnumerator.Current
{
get { return Current; }
}
}
}
关于c# - 如何将可枚举类型转换为 AsyncEnumerable 类型?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/43036884/