c# - 将键值对分组到字典

Question

我有以下代码:

using System.Collections.Generic;


public class Test
{
    static void Main()
    {

        var items = new List<KeyValuePair<int, User>>
                        {
                            new KeyValuePair<int, User>(1, new User {FirstName = "Name1"}),
                            new KeyValuePair<int, User>(1, new User {FirstName = "Name2"}),
                            new KeyValuePair<int, User>(2, new User {FirstName = "Name3"}),
                            new KeyValuePair<int, User>(2, new User {FirstName = "Name4"})
                        };

    }
}
public class User
{
    public string FirstName { get; set; }
}

如您所见，同一个 key 有多个用户。现在我想将它们分组并将列表对象转换为字典，其中键相同(1,2 如上所示)但值将是集合。像这样:

 var outputNeed = new Dictionary<int, Collection<User>>();
        //Output:
        //1,Collection<User>
        //2,Collection<User>

即他们现在分组了。

我怎样才能做到这一点？

Answer 1

我建议您使用 Lookup<TKey, TElement> 反而。此数据结构专门用作从键到值集合的映射。

//uses Enumerable.ToLookup: the Id is the key, and the User object the value
var outputNeeded = items.ToLookup(kvp => kvp.Key, kvp => kvp.Value);

当然，如果您确实需要字典(可能允许可变性)，您可以这样做:

var outputNeeded = new Dictionary<int, Collection<User>>();

foreach(var kvp in list)
{
   Collection<User> userBucketForId;

   if(!outputNeeded.TryGetValue(kvp.Key, out userBucketForId)) 
   {
       // bucket doesn't exist, create a new bucket for the Id, containing the user
       outputNeeded.Add(kvp.Key, new Collection<User> { kvp.Value });
   }
   else 
   {   // bucket already exists, append user to it.
       userBucketForId.Add(kvp.Value);
   }
}

另一方面，Collection<T>除非您打算对其进行子类化，否则类并不是那么有用。你确定你不只是需要一个 List<User> ？