C# Dictionary<string, string>

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);
            }
        }
    }
}

Open in new window

deleydAsked:
Who is Participating?
 
wdosanjosCommented:
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));
    }
}

Open in new window


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
 
lalitgadaCommented:
what u want to know?
0
 
AndyAinscowFreelance programmer / ConsultantCommented:
>>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
 
deleydAuthor Commented:
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
 
deleydAuthor Commented:
Thank you for alerting me to .NET Reflector
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.