C#去重併合並數據

以下為一個合併學生分數的示例數據，依據主鍵統計各個學生的總分數

        IList<Student> students=new List<Student>();
        students.Add(new Student() { Id = 3, Score = 99, Name = "語文" });
        students.Add(new Student() { Id = 5, Score = 87, Name = "語文" });
        students.Add(new Student() { Id = 3, Score = 95, Name = "數學" });
        students.Add(new Student() { Id = 3, Score = 97, Name = "外語" });
        students.Add(new Student() { Id = 5, Score = 90, Name = "外語" });
        students.Add(new Student() { Id = 2, Score = 96, Name = "語文" });
        students.Add(new Student() { Id = 5, Score = 88, Name = "數學" });
        students.Add(new Student() { Id = 2, Score = 97, Name = "數學" });

拿到這種數據，如果要將數據合併，常規的做法就是通過遍歷實現，代碼簡單也容易理解

    Dictionary<int,int> query=new Dictionary<int, int>();
    foreach (var element in students) {
        if (query.ContainsKey(element.Id)) {
            query[element.Id]+=element.Score;
        }else{
            query.Add(element.Id,element.Score);
            }
        }

不考慮代碼行數為工作績效的情況下，可以通過Linq或擴展方法一行優雅的搞定

    var query=(from s in students 
        group s by s.Id into ng
        select ng).ToDictionary(d=>d.Key,d=>d.Sum(s=>s.Score));

    var query=students.GroupBy(g=>g.Id).ToDictionary(d=>d.Key,d=>d.Sum(s=>s.Score));

通過GroupBy過濾主鍵去掉重覆項，避免ToDictionary時重覆主鍵異常,當然上面都是簡單的轉換，下麵可以實現稍微點複雜的去重合併

var query= students.GroupBy(g=>g.Id).Select(s=>new Student{Id=s.Key,Score=s.Sum(n=>n.Score),Name=string.Join(",",s.Select(t=>t.Name))});

當然太複雜的合併其實不推薦純擴展方法來實現，很容易造成語法太過複雜維護成本高，對於複雜的合併還是推薦通過Linq和擴展方法組合自定義函數來實現。