我通过对两个参数进行分组并基于createdate(使用first())在子组列表中选择最新类型来过滤列表.
这消除了x.application和x.externalid属性上的重复项.

var list = ((List<SomeType>)xDic)
            .GroupBy(x => new {x.Application, x.ExternalID})
            .OrderByDescending(z => z.First().CreateDate)
            .Select(y => y.First()).ToList();

我遇到的麻烦是定义了属性的另一种组合(x.application和x.externaldisplayid)进行筛选并分组以采用第一个.

总而言之,我需要通过基于((x.application / x.externalid)或(x.application / x.externaldisplayid))组合过滤掉所有重复项来获得SomeTypes的唯一列表.

Example set:
{ "extID": 1234, "extDspID" : 111, "App" : "Test", "CreateDate": 2/01/2015}
{ "extID": 1234, "extDspID" : 5, "App" : "Test", "CreateDate": 1/01/2015}
{ "extID": 012, "extDspID" : 90, "App" : "Mono", "CreateDate": 6/06/2015}
{ "extID": 999, "extDspID" : 78, "App" : "Epic", "CreateDate": 8/08/2015}
{ "extID": 333, "extDspID" : 78, "App" : "Epic", "CreateDate": 8/12/2015}
{ "extID": 345, "extDspID" : 33, "App" : "Test", "CreateDate": 2/01/2015}
{ "extID": 666, "extDspID" : 33, "App" : "Test", "CreateDate": 1/01/2015}

desired result:
{ "extID": 1234, "extDspID" : 111, "App" : "Test", "CreateDate": 2/01/2015}
{ "extID": 012, "extDspID" : 90, "App" : "Mono", "CreateDate": 6/06/2015}
{ "extID": 333, "extDspID" : 78, "App" : "Epic", "CreateDate": 8/12/2015}
{ "extID": 345, "extDspID" : 33, "App" : "Test", "CreateDate": 2/01/2015}

解决方法:

首先,声明两个相等的comparere来指定您的两个条件,如下所示：

public class MyEqualityComparer1 : IEqualityComparer<SomeType>
{
    public bool Equals(SomeType x, SomeType y)
    {
        return x.Application == y.Application && x.ExternalID == y.ExternalID;
    }

    public int GetHashCode(SomeType obj)
    {
        return (obj.Application + obj.ExternalID).GetHashCode();
    }
}

public class MyEqualityComparer2 : IEqualityComparer<SomeType>
{
    public bool Equals(SomeType x, SomeType y)
    {
        return x.Application == y.Application && x.ExternalDisplayId == y.ExternalDisplayId;
    }

    public int GetHashCode(SomeType obj)
    {
        return (obj.Application + obj.ExternalDisplayId).GetHashCode();
    }
}

然后,按CreatedDate对列表进行排序,然后使用Distinct来过滤列表,如下所示：

var result = xDic
    .OrderByDescending(x => x.CreateDate)
    .Distinct(new MyEqualityComparer1())
    .Distinct(new MyEqualityComparer2());

Distinct方法should remove the later items,因此我们应该能够依赖于使用OrderByDescending的事实,以确保Distinct将删除具有最近CreatedTime的项.

但是,由于Distinct的文档不能保证这一点,因此可以使用如下的自定义独特方法：

public static class Extensions
{
    public static IEnumerable<T> OrderedDistinct<T>(this IEnumerable<T> enumerable, IEqualityComparer<T> comparer)
    {
        HashSet<T> hash_set = new HashSet<T>(comparer);

        foreach(var item in enumerable)
            if (hash_set.Add(item))
                yield return item;
    }
}

并像这样使用它：

var result = xDic
    .OrderByDescending(x => x.CreateDate)
    .OrderedDistinct(new MyEqualityComparer1())
    .OrderedDistinct(new MyEqualityComparer2());