当一个结合中想根据某一个字段做去重方法时使用以下代码
IQueryable 继承自IEnumerable
先举例：
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#region linq to object List<People> peopleList = new List<People>();peopleList.Add(new People { UserName = "zzl", Email = "1" });peopleList.Add(new People { UserName = "zzl", Email = "1" });peopleList.Add(new People { UserName = "lr", Email = "2" });peopleList.Add(new People { UserName = "lr", Email = "2" });Console.WriteLine("用扩展方法可以过滤某个字段，然后把当前实体输出");peopleList.DistinctBy(i => new { i.UserName }).ToList().ForEach(i => Console.WriteLine(i.UserName + i.Email));Console.WriteLine("默认方法，集合中有多个字段，当所有字段发生重复时，distinct生效，这与SQLSERVER相同");peopleList.Select(i => new { UserName = i.UserName, Email = i.Email }).OrderByDescending(k => k.Email).Distinct().ToList().ForEach(i => Console.WriteLine(i.UserName + i.Email));Console.WriteLine("集合中有一个字段，将这个字段重复的过滤，并输出这个字段");peopleList.Select(i => new { i.UserName }).Distinct().ToList().ForEach(i => Console.WriteLine(i.UserName));#endregion |
该扩展方法贴出：
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public static class EnumerableExtensions{ public static IEnumerable<TSource> DistinctBy<TSource, Tkey>(this IEnumerable<TSource> source, Func<TSource, Tkey> keySelector) { HashSet<Tkey> hashSet = new HashSet<Tkey>(); foreach (TSource item in source) { if (hashSet.Add(keySelector(item))) { yield return item; } } }} |
补充知识：c# – .Distinct()调用不过滤
我正在尝试使用AsEnumerable将Entity Framework DbContext查询拉入IEnumerable< SelectListItem>.这将用作填充视图中下拉列表的模型属性.
但是,尽管调用了Distinct(),但每个查询都会返回重复的条目.
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public IEnumerable<SelectListItem> StateCodeList { get; set; }public IEnumerable<SelectListItem> DivCodeList { get; set; } DivCodeList = db.MarketingLookup.AsEnumerable().OrderBy(x => x.Division).Distinct().Select(x => new SelectListItem { Text = x.Division, Value = x.Division }).ToList();StateCodeList = db.MarketingLookup.AsEnumerable().OrderBy(x => x.State).Distinct().Select(x => new SelectListItem { Text = x.State, Value = x.State }).ToList(); |
为了使Distinct生效,如果类型是自定义类型,则序列必须包含实现IEquatable接口的类型的对象.
正如here所述：
Distinct returns distinct elements from a sequence by using the
default equality comparer to compare values.
一个解决方法,为了避免上述情况,因为我可以得出结论,你不需要整个对象而不是它的一个属性,就是将序列的每个元素投影到Division,然后创建OrderBy并调用Distinct ：
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var divisions = db.MarketingLookup.AsEnumerable() .Select(ml=>ml.Division) .OrderBy(division=>division) .Distinct() .Select(division => new SelectListItem { Text = division, Value = division }).ToList(); |
有关此问题的进一步文档，请查看here.
以上这篇完美解决c# distinct不好用的问题就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持服务器之家。
原文链接：https://www.cnblogs.com/daimaxuejia/p/10971869.html
