当密钥本身是字典（C＃）时，如何检查字典中是否存在密钥？

Question

I have a dictionary [mapData] as below and check key is a [Dictionary< string, DateTime >] exists in [mapData] , but it did not work:

        var mapData = new Dictionary<Dictionary<string, DateTime>, List<Student>>();

        foreach (var st in listStudent)
        {
            // Create Key
            var dicKey = new Dictionary<string, DateTime>();
            dicKey.Add(st.Name, st.Birthday);

            // Get mapData
            if (!mapData.ContainsKey(dicKey))       // ===> Can not check key exists
            {
                mapData.Add(dicKey, new List<Student>());
            }
            mapData[dicKey].Add(st);
        }

I tried with extension method as the below, but also not work:

        public static bool Contains<Tkey>(this Dictionary<Tkey, List<Student>> dic, Tkey key)
        {
            if (dic.ContainsKey(key))
                return true;
            return false;
        }

Any tips on these will be great help. Thanks in advance.

Answer 1

You don't want to go there. Complex objects are not meant to be keys in a dictionary. I would suggest to move the dictionary to the value and create a more tree-like structure.

There is one possibility to get this working, but I would advice against it for above reasons: the implementation of a custom IEqualityComparer which compares the keys in the dictionary against another dictionary.

Answer 2

this is because you try to find the key based on object reference not the key content. Your dictionnary key is a composite of Key + value (string + DateTime) reference.

this wont work unless you rewrite and IEqualityComparer.

var objectA = new ObjectA();
var objectB = new ObjectA();

objectA != objectB unless you rewritte the equals.

EDIT

This sample is comming from MSDN https://msdn.microsoft.com/en-us/library/ms132151(v=vs.110).aspx

using System;
using System.Collections.Generic;

class Example
{
   static void Main()
   {
      BoxEqualityComparer boxEqC = new BoxEqualityComparer();

      var boxes = new Dictionary<Box, string>(boxEqC);

      var redBox = new Box(4, 3, 4);
      AddBox(boxes, redBox, "red");

      var blueBox = new Box(4, 3, 4);
      AddBox(boxes, blueBox, "blue");

      var greenBox = new Box(3, 4, 3);
      AddBox(boxes, greenBox, "green");
      Console.WriteLine();

      Console.WriteLine("The dictionary contains {0} Box objects.",
                        boxes.Count);
   }

   private static void AddBox(Dictionary<Box, String> dict, Box box, String name)
   {
      try {
         dict.Add(box, name);
      }
      catch (ArgumentException e) {
         Console.WriteLine("Unable to add {0}: {1}", box, e.Message);
      }
   }
}

public class Box
{
    public Box(int h,  int l, int w)
    {
        this.Height = h;
        this.Length = l;
        this.Width = w;
    }

    public int Height { get; set; }
    public int Length { get; set; }
    public int Width { get; set; }

    public override String ToString()
    {
       return String.Format("({0}, {1}, {2})", Height, Length, Width);
    }
}


class BoxEqualityComparer : IEqualityComparer<Box>
{
    public bool Equals(Box b1, Box b2)
    {
        if (b2 == null && b1 == null)
           return true;
        else if (b1 == null | b2 == null)
           return false;
        else if(b1.Height == b2.Height && b1.Length == b2.Length
                            && b1.Width == b2.Width)
            return true;
        else
            return false;
    }

    public int GetHashCode(Box bx)
    {
        int hCode = bx.Height ^ bx.Length ^ bx.Width;
        return hCode.GetHashCode();
    }
}
// The example displays the following output:
//    Unable to add (4, 3, 4): An item with the same key has already been added.
//
//    The dictionary contains 2 Box objects.