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C# Dictionary<string, string>

Posted on 2011-03-06
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Last Modified: 2013-12-17
I'm testing this sample code I got from www.dotnetperls.com .

Question:
1. What is the default hash algorithm being used for the key strings?

I see:
"string1" hashed to 0x2163a1a6
"string2" hashed to 0x440b1641

Is this a good hash algorithm for long strings?

using System;
using System.Collections.Generic;

namespace dicstring
{
    class Program
    {
        static void Main(string[] args)
        {
            // A. Create a new Dictionary with two keys and two values.
            Dictionary<string, string> example = new Dictionary<string, string>();
            example.Add("string1", "C#");
            example.Add("string2", "Perl");

            // B. Lookup a string in the Dictionary.
            string value;
            if (example.TryGetValue("string2", out value))
            {
                Console.WriteLine("Found {0}", value);
            }

            // C. See if it contains this key.
            Console.WriteLine(example.ContainsKey("string3"));

            // D. Add this key if it isn't there.
            if (!example.ContainsKey("string3"))
            {
                example.Add("string3", "VB.NET");
            }

            // E. Enumerate the keys
            foreach (var pair in example)
            {
                Console.WriteLine("Key = {0}, Value = {1}", pair.Key, pair.Value);
            }
        }
    }
}

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0
Comment
Question by:deleyd
5 Comments
 
LVL 1

Expert Comment

by:lalitgada
ID: 35053604
what u want to know?
0
 
LVL 44

Expert Comment

by:AndyAinscow
ID: 35053962
>>Is this a good hash algorithm for long strings?

Yes, because it works.  The algorithm MUST result in different values for different entries.  Do not attempt to write your own algorithm unless you know what you are doing.
0
 

Author Comment

by:deleyd
ID: 35057971
How can I find out what the hash algorithm is? Is it documented somewhere? Or do I just have to trace the code internals to find out the hard way?
0
 
LVL 23

Accepted Solution

by:
wdosanjos earned 500 total points
ID: 35060319
I used .NET Reflector to disassemble String.GetHashCode() (.NET Framework 4).  Here is the result:

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail), SecuritySafeCritical]
public override unsafe int GetHashCode()
{
    fixed (char* str = ((char*) this))
    {
        char* chPtr = str;
        int num = 0x15051505;
        int num2 = num;
        int* numPtr = (int*) chPtr;
        for (int i = this.Length; i > 0; i -= 4)
        {
            num = (((num << 5) + num) + (num >> 0x1b)) ^ numPtr[0];
            if (i <= 2)
            {
                break;
            }
            num2 = (((num2 << 5) + num2) + (num2 >> 0x1b)) ^ numPtr[1];
            numPtr += 2;
        }
        return (num + (num2 * 0x5d588b65));
    }
}

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Here is a note from the String.GetHashCode() documentation:
http://msdn.microsoft.com/en-us/library/system.string.gethashcode.aspx

The behavior of GetHashCode is dependent on its implementation, which might change from one version of the common language runtime to another. A reason why this might happen is to improve the performance of GetHashCode.
Note Note

Notes to Callers

The value returned by GetHashCode is platform-dependent. It differs on the 32-bit and 64-bit versions of the .NET Framework.
0
 

Author Closing Comment

by:deleyd
ID: 35064278
Thank you for alerting me to .NET Reflector
0

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