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Posted 26 Oct 2011

Consistent Hash function for SQL Server and .NET

, 26 Oct 2011
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Provides a couple of hash functions (string to int) that will return the same value in C# and T-SQL
Sometimes, you need to do some hashing in T-SQL on the server side and use it in C# or vice-versa.
The algorithm used by the native 'checksum' is not divulged.
Here is a simple string to hash 32 bit function that can be used to get a number from a string.
An optional parameter is available if you want to reduce the result to a shorter number.
Both functions return the same.

T-SQL Function Source
CREATE FUNCTION FnMD5Hash(@SourceString varchar(8000), @Modulo INT = 0)
	IF @Modulo = 0
		RETURN CAST(HashBytes( 'MD5', @SourceString) AS INTEGER)
		RETURN ABS(CAST(HashBytes( 'MD5', @SourceString) AS INTEGER)) % @Modulo

C# Function Source
static System.Security.Cryptography.MD5CryptoServiceProvider md5Provider =
   new System.Security.Cryptography.MD5CryptoServiceProvider();
// the database is usually set to Latin1_General_CI_AS which is codepage 1252
static System.Text.Encoding encoding = 
static Int32 SQLStringToHash(string sourceString, int modulo = 0)
   var md5Bytes = md5Provider.ComputeHash(encoding.GetBytes(sourceString));
   var result = BitConverter.ToInt32(new byte[] { 
      md5Bytes[15], md5Bytes[14], md5Bytes[13], md5Bytes[12] }, 0);
   if (modulo == 0) return result;
   else return Math.Abs(result) % modulo;

T-SQL Test Source
DECLARE @SourceString AS VARCHAR(255) = 'Joyeux Noël'
PRINT 'Source: ' + @SourceString
PRINT 'Hash:'
PRINT dbo.FnMD5Hash(@SourceString, default)
PRINT 'Hash (0..999):'
PRINT dbo.FnMD5Hash(@SourceString, 1000)

C# Test Source
string sourceString = "Joyeux Noël";
Console.WriteLine("C# Test");
Console.WriteLine("Source: "+sourceString);
Console.WriteLine("Hash: " + SQLStringToHash(sourceString));
Console.WriteLine("Hash (0..999): " + SQLStringToHash(sourceString, 1000));

T-SQL Test Output
T-SQL Test
Source: Joyeux Noël
Hash: -92694766
Hash (0..999): 766

C# Test Output
C# Test
Source: Joyeux Noël
Hash: -92694766
Hash (0..999): 766


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Pascal Ganaye
Software Developer (Senior)
France France
I am a French programmer.
These days I spend most of my time with the .NET framework, JavaScript and html.

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