C# EF去除重复列DistinctBy方式
C# EF去除重复列DistinctBy
在网上看了LinQ有DistinctBy方法,实际在用的时候并没有找到,后来参照了该网站才发现写的是拓展方法
https://www.jb51.net/article/273355.htm
1.添加一个扩展方法
public static class DistinctByClass { public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) { HashSet<TKey> seenKeys = new HashSet<TKey>(); foreach (TSource element in source) { if (seenKeys.Add(keySelector(element))) { yield return element; } } } }
2.使用方法如下(针对ID,和Name进行Distinct)
var query = people.DistinctBy(p => new { p.Id, p.Name });
3.若仅仅针对ID进行distinct:
var query = people.DistinctBy(p => p.Id);
C#集合用Distinct去掉重复的元素,IEqualityComparer<T>原理
json测试数据
[ { "Type": "ROLETYPE_USER", "Value": "9846cdac-ae21-4be4-a284-50158dd19606" }, { "Type": "ROLETYPE_USER", "Value": "4f6665fc-b0b9-4865-bf12-eff06810efa3" }, { "Type": "ROLETYPE_USER", "Value": "ca4dbf59-d248-4538-9fe8-62e1cafcde6c" }, { "Type": "ROLETYPE_USER", "Value": "7c4852c2-b2c2-4688-92a3-3dd1b00bacf8" }, { "Type": "ROLETYPE_USER", "Value": "982a2f9d-a079-4613-825f-c2ef9801eb3e" }, { "Type": "ROLETYPE_USER", "Value": "ca4dbf59-d248-4538-9fe8-62e1cafcde6c" }, { "Type": "ROLETYPE_USER", "Value": "ca4dbf59-d248-4538-9fe8-62e1cafcde6c" }, { "Type": "ROLETYPE_ROLE", "Value": "76f62bf8-bf24-48c1-b70e-7628ff08c3fb" }, { "Type": "ROLETYPE_USER", "Value": "982a2f9d-a079-4613-825f-c2ef9801eb3e" } ]
实体类型
public class TempSetting { public string Type { get; set; } public Guid Value { get; set; } }
创建比较类,继承自IEqualityComparer<TSource>,一般要选择特征值字段来设置hashCode值返回才有效果,或者直接设置哈希值返回1(这样每次都会调用Equals方法,但这样性能低)。
IEqualityComparer比较对象是否相等是优先调用GetHashCode()比较哈希值是否相等,不相等的就不会调用Equals方法,如果哈希值相等的才会调用调用Equals方法再比较是否相等最终确认,所以 GetHashCode方法里面要设置特征值字段来返回hashcode,这样速度才快,性能高
public class CompareSetting : IEqualityComparer<TempSetting> { public bool Equals(TempSetting x, TempSetting y) { return x.Value == y.Value; } public int GetHashCode(TempSetting obj) { if (obj == null) { return 0; } //return obj.GetHashCode();//比较不出来重复的 //string km = obj.Type + obj.Value;//比较重复值成功 //return km.GetHashCode(); //GetHashCode推荐选择自己看中的特征值字段来进行比较,否则有时候可能比较不成功 return obj.Value.GetHashCode(); } } //测试比较 string funs = "[{\"Type\":\"ROLETYPE_USER\",\"Value\":\"9846cdac-ae21-4be4-a284-50158dd19606\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"4f6665fc-b0b9-4865-bf12-eff06810efa3\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"ca4dbf59-d248-4538-9fe8-62e1cafcde6c\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"7c4852c2-b2c2-4688-92a3-3dd1b00bacf8\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"982a2f9d-a079-4613-825f-c2ef9801eb3e\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"ca4dbf59-d248-4538-9fe8-62e1cafcde6c\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"ca4dbf59-d248-4538-9fe8-62e1cafcde6c\"},{\"Type\":\"ROLETYPE_ROLE\",\"Value\":\"76f62bf8-bf24-48c1-b70e-7628ff08c3fb\"},{\"Type\":\"ROLETYPE_USER\",\"Value\":\"982a2f9d-a079-4613-825f-c2ef9801eb3e\"}]"; List<TempSetting> list = Newtonsoft.Json.JsonConvert.DeserializeObject<List<TempSetting>>(funs); var list2 = list.Distinct(new CompareSetting());
总结
以上为个人经验,希望能给大家一个参考,也希望大家多多支持脚本之家。
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