如何在异步存储库方法中返回空的IQueryable [英] How to return empty IQueryable in an async repository method

问题描述

假设我有一个简单的存储库类，其中有一个GetByNames方法

Lets say I have a simple repository class, with one GetByNames method

public class MyRepo
{
    private readonly MyDbContext _db;

    public MyRepo(MyDbContext db)
    {
        _db = db;
    }

    public IQueryable<MyObject> GetByNames(IList<string> names)
    {
        if (names== null || !names.Any())
        {
            return Enumerable.Empty<MyObject>().AsQueryable();
        }

        return _db.MyObjects.Where(a => names.Contains(a.Name));
    }
}

现在，当我将其与异步EntityFramework ToListAsync()扩展名一起使用

Now when I use it with async EntityFramework ToListAsync() extension

var myObjects = awawit new MyRepo(_db).GetByNames(names).ToListAsync();

如果我传入空列表或null，因为Enumerable.Empty<MyObject>().AsQueryable()未实现IDbAsyncEnumerable<MyObject>接口，它将崩溃.

It will blow up if I pass in empty list or null because Enumerable.Empty<MyObject>().AsQueryable() does not implement IDbAsyncEnumerable<MyObject> interface.

源IQueryable没有实现IDbAsyncEnumerable.只有实现IDbAsyncEnumerable的源才能用于Entity Framework异步操作.有关更多详细信息，请参见 http://go.microsoft.com/fwlink/?LinkId=287068.

所以我的问题是，如何在不访问数据库的情况下返回实现IDbAsyncEnumerable的空IQueryable<>?

So my question is, how can I return an empty IQueryable<> that implements IDbAsyncEnumerable, without hitting the database?

推荐答案

如果您不想访问数据库，则很可能必须提供自己的实现IDbAsyncEnumerable的空IQuerable实现.但我认为这并不难.在所有枚举器中，对于Current仅返回null，对于MoveNext仅返回false.在Dispose中什么也不做.试试吧. Enumerable.Empty<MyObject>().AsQueryable()与数据库无关，它绝对不实现IDbAsyncEnumerable.根据此.

If you don't want to hit the DB, you'll most likely have to provide your own implementation of empty IQuerable that implements IDbAsyncEnumerable. But I don't think it is too hard. In all the enumerators just return null for Current and false for MoveNext. In Dispose just do nothing. Try it. Enumerable.Empty<MyObject>().AsQueryable() has nothing to do with database, it definitely does not implement IDbAsyncEnumerable. You need an implementation that does, according to this.

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