Using byte[] as a key in a concurrent dictionary.

Question

Hi All,
I have created a Concurrent Dictionary with key as a object containing byte array using IEqualityComparer. I was wondering how do I create a hash code for each such object when adding to my dictionay. I tried below creating hash code using sum of all
the bytes present in the byte array of the object, but the performance is worst when we try to compare with normal dictionary with some existing data type as a key. Please suggest how do I improve performance of my dictionary with custom key(object with byte
array).
Thanks and Regards,
Lucas

Answer 1

Calculating the hash of a large byte array can be more expensive than handling hash collisions.  So perhaps use your special knowledge about the data to extract a few bytes from the array to use in the hash.
eg
public int GetHashCode(byte[] obj)
if (obj == null)
return -1;
if (obj.Length == 0)
return 0;
if (obj.Length < 64)
return obj.Length * obj[1];
var fp = new byte[4];
fp[0] = obj[0];
fp[1] = obj[16];
fp[2] = obj[27];
fp[3] = obj[55];
return obj.Length ^ BitConverter.ToInt32(fp, 0);
David
David http://blogs.msdn.com/b/dbrowne/