關于Enumerable distinct 的學習筆記-白红宇

關于Enumerable distinct 的學習筆記

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发布时间：2019-05-24

本文共 4389 字，大约阅读时间需要 14 分钟。

**Linq 的distinct 常用于對數組、集合的去重操作。**   **一、對于數值型數組或集合去重直接Distinct()就可以。**  new List
   
    () {
    1, 2, 3, 4, 3, 2 }.Distinct().ToList().ForEach(o => Console.WriteLine(o));    **二、對于自定義類型的集合，直接Distinct往往得不到我們想要的結果**    //例子：     public class Food     {
           public int foodId {
    get; set; }        public string foodName {
    get; set; }        public override string ToString()        {
               return string.Format("{0}\t{1}", foodId.ToString(), foodName.ToString());        }    }      private List
    
      FoodInit()    {
           List
     
       fl = new List
      
       ();        fl.Add(new Food {
    foodId = 1, foodName = "rice" });        fl.Add(new Food {
    foodId = 2, foodName = "noodle" });        fl.Add(new Food {
    foodId = 1, foodName = "rice" });        return fl;    }    //調用：        FoodInit().Distinct().ToList().ForEach(o => Console.WriteLine(o));        //結果：        //1   rice        //2   noodle        //1   rice      	  ******//解決方法：******    **//方法1. 使用匿名類**       FoodInit().Select(o => new {
    id = o.foodId, name = o.foodName }).Distinct().ToList().ForEach(o => Console.WriteLine(o));        //結果：        //{ id = 1, name = rice }        //{ id = 2, name = noodle }            **//方法2. 重寫自定義類的Equals,GetHashCode方法****         public class Food : IEquatable
       
                {
               public int foodId {
    get; set; }            public string foodName {
    get; set; }            public override string ToString()            {
                   return string.Format("{0}\t{1}", foodId.ToString(), foodName.ToString());            }                    public  bool Equals(Food obj)            {
                   if (obj == null || GetType() != obj.GetType())                    return false;                Food foodItem = obj as Food;                return (foodItem.foodId == this.foodId); //規則可以按具體需要，例如 ID  一樣則視為相等            }            public override int GetHashCode()            {
                   return foodId.GetHashCode(); //注意：按自定義的規則，HashCode相同，才會再去比較 Equals            }        }				        //調用            FoodInit().Distinct().ToList().ForEach(o => Console.WriteLine(o));            //結果：            //1   rice            //2   noodle						   **//我們可參考 MSDN 上的例子**    public class Product : IEquatable
        
          { public string Name { get; set; } public int Code { get; set; } public bool Equals(Product other) { //Check whether the compared object is null. if (Object.ReferenceEquals(other, null)) return false; //Check whether the compared object references the same data. if (Object.ReferenceEquals(this, other)) return true; //Check whether the products' properties are equal. return Code.Equals(other.Code) && Name.Equals(other.Name); } // If Equals() returns true for a pair of objects // then GetHashCode() must return the same value for these objects. public override int GetHashCode() { //Get hash code for the Name field if it is not null. int hashProductName = Name == null ? 0 : Name.GetHashCode(); //Get hash code for the Code field. int hashProductCode = Code.GetHashCode(); //Calculate the hash code for the product. return hashProductName ^ hashProductCode; } } **// 方法3： 實現類的IEqualityComparer接口** public class Compare : IEqualityComparer
         
           { public bool Equals(Food x, Food y) { return x.foodId == y.foodId;//可以自定义去重规则，此处将FoodId相同的就相等 } public int GetHashCode(Food obj) { return obj.foodId.GetHashCode();//注意，按自定義去重規則 } } //調用： FoodInit().Distinct(new Compare()).ToList().ForEach(s => Console.WriteLine(s.ToString())); //結果： //1 rice //2 noodle **//但這樣仍不方便，每個自定義類都需要去寫。我們可以寫一個通用的DISTINCT擴展方法**** public static class DistinctExtensions { public class CommonEqualityComparer
          
            : IEqualityComparer
           
             { private Func
            
              keySelector; public CommonEqualityComparer(Func
             
               keySelector) { this.keySelector = keySelector; } public bool Equals(T x, T y) { return EqualityComparer
              
               .Default.Equals(keySelector(x), keySelector(y)); } public int GetHashCode(T obj) { return EqualityComparer
               
                .Default.GetHashCode(keySelector(obj)); } } public static IEnumerable
                
                  Distinct
                 
                  (this IEnumerable
                  
                    source, Func
                   
                     keySelector) { return source.Distinct(new CommonEqualityComparer
                    
                     (keySelector)); } } //調用 FoodInit().Distinct(p => p.foodId).ToList().ForEach(o => Console.WriteLine(o)); 或 FoodInit().Distinct(p => p.foodName).ToList().ForEach(o => Console.WriteLine(o)); //結果 //1 rice //2 noodle