Dynamic 3D array

Hello everyone! I'm trying to make a 3D infinite array in C# but couldn't find any online. So I came up with this code:

Code (csharp):

1. using System;
2. using System.Collections;
3. using System.Collections.Generic;
4.  
5. public class DArray
6. {
7.     //public ArrayList dynamicData;
8.     public Dictionary<string, Object> dynamicData;
9.  
10.     public DArray ()
11.     {
12.         dynamicData = new Dictionary<string, Object>();
13.         //dynamicData = new ArrayList();
14.     }
15.  
16.     public void Set(int x, int y, int z, Object obj)
17.     {
18.         /*if(dynamicData[x] == null)
19.             dynamicData[x] = new ArrayList();
20.        
21.         if(((ArrayList)dynamicData[x])[y] == null)
22.             dynamicData[x] = new ArrayList();
23.        
24.         ((ArrayList)((ArrayList)dynamicData[x])[y])[z] = obj;*/
25.  
26.         String key = x + ":" + y + ":" + z;
27.         dynamicData[key] = obj;
28.     }
29.  
30.     public Object Get(int x, int y, int z)
31.     {
32.         /*if(dynamicData[x] == null)
33.             return null;
34.        
35.         if(((ArrayList)dynamicData[x])[y] == null)
36.             return null;
37.        
38.         return ((ArrayList)((ArrayList)dynamicData[x])[y])[z];*/
39.  
40.         String key = x + ":" + y + ":" + z;
41.        
42.         if(!dynamicData.ContainsKey(key))
43.             return null;
44.  
45.         return dynamicData[key];
46.     }
47. }

After switching from a non-infinite array to this one, I noticed a huge performance decrease. Is there any way to fix it?

 

Performance loss possibly due to using strings as keys.
Maybe try hashing the xyz values or something

 

I tried to hash it like this:

Code (csharp):

1.  
2.     public int GetHashCode(int x, int y, int z)
3.     {
4.         int hash = unchecked(x + (31 * y) + (31 * 31 * z));
5.         return hash;
6.     }
7.  

(This is for minecraft-like voxel terrain)
But it causes some coordinates to have the same key.

 

Can you just use longs for more bits?