EF独特（IEqualityComparer）错误 [英] EF Distinct (IEqualityComparer) Error

问题描述

早安！

给定：

public class FooClass
{
    public void FooMethod()
    {
        using (var myEntity = new MyEntity)
        {
            var result = myEntity.MyDomainEntity.Where(myDomainEntity => myDomainEntity.MySpecialID > default(int)).Distinct(new FooComparer);
        }
    }

}

public class FooComparer : IEqualityComparer<MyEntity.MyDomainEntity>
{
    public bool Equals(MyEntity.MyDomainEntity x, MyEntity.MyDomainEntity y)
    {
        return x.MySpecialID == y.MySpecialID;
    }

    public int GetHashCode(MyEntity.MyDomainEntity obj)
    {
        return obj.MySpecialID.GetHashCode();
    }
}

这将编译，但在运行时我会得到一个

This will compile, but on runtime I will get an Linq to Entity could not translate Comparer-Exception.
Any suggestions?

推荐答案

如果您提供自己的比较，您需要执行.NET代码中的 Distinct 调用。为了确保发生这种情况，请使用 AsEnumerable 将 IQueryable< T> 转换为 IEnumerable< T& ; ：

If you're providing your own comparisons, you'll need to execute the Distinct call in .NET code. To make sure that happens, use AsEnumerable to turn IQueryable<T> into IEnumerable<T>:

var result = myEntity.MyDomainEntity
        .Where(myDomainEntity => myDomainEntity.MySpecialID > default(int))
        .AsEnumerable()
        .Distinct(new FooComparer());

当然这时你会从数据库中拉出更多的数据。另一种方法是对数据进行分组：

Of course at that point you'll be pulling more data across from the database. An alternative is to group the data instead:

var result = from entity in myEntity.MyDomainEntity
             where entity.MySpecialID > 0
             group entity by entity.MySpecialID into groups
             select groups.FirstOrDefault();

这将让你成为每个ID遇到的第一个实体（假设我的查询不是失败的我）。这基本上是什么Distinct，反正，但它都在数据库。

That will get you the first entity encountered with each ID (assuming my query-fu isn't failing me). That's basically what Distinct does anyway, but it's all at the database.

（注意未来的读者：调用 First（）比 FirstOrDefault（）更有意义，但显然不起作用。）

(Note to future readers: calling First() makes more sense than FirstOrDefault(), but apparently that doesn't work.)

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