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已知有类

class Item{
       public int id {
    get; set; }    public string result {
    get; set; }    public override string ToString()    {
           return $"{result}{id}";    }}

兩個實例：

List
   
     oldSet = new List
    
     {
       new Item {
   id = 1, result ="old"},    new Item {
   id = 2, result ="old"},    new Item {
   id = 3, result ="old"},};List
     
       newSet = new List
      
       {
       new Item {
   id = 2, result ="new"},    new Item {
   id = 3, result ="new"},    new Item {
   id = 4, result ="new"},};
      
     
    
   

現想合并两个列表，结果如下：

List mergedSet = [{old1, null}, {old2, new2}, {old3, new3}, {null, new4}];

實現方法1：兩次左联接，加上1次Union

var mergedSet = oldSet.Select(o =>    new {
   OldSet = o, NewSet = newSet.FirstOrDefault(n => n.id == o.id)})    .Union(newSet.Where(n => oldSet.All(o => o.id != n.id)).Select(n =>        new {
   OldSet = (Item) null, NewSet = n}));

方法2:兩次GroupJoin，加1次Union。DefaultIfEmpty，左联接時，即外联接不存在對應的元素，也会列出左聯接的元素。

var mergedSet = (from o in oldSet                     join n in newSet on o.id equals n.id into ns                     from n in ns.DefaultIfEmpty()                     select new {
    OldSet = o, NewSet = n })                    .Union(from n in newSet                           join o in oldSet on n.id equals o.id into os                           from o in os.DefaultIfEmpty()                           select new {
    OldSet = o, NewSet = n })  .Distinct();

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