Use byte[] as key in dictionary

Question

I need to use a byte[] as a key in a Dictionary. Since byte[] doesn't override the default GetHashCode method, two separate byte[] objects that contain the same data will use two separate slots in the dictionary. Basically what I want is this:

Dictionary<byte[], string> dict = new Dictionary<byte[], string>(); dict[new byte[] {1,2,3}] = "my string"; string str = dict[new byte[] {1,2,3}]; // I'd like str to be set to "my string" at this point 

Is there a simple way to do this? The only thing I can think of would be to build a wrapper class that just contains a byte[] and override GetHashCode based on the contents of the byte[], but this seems error prone.

Answer 1

By default byte[] will be compared by reference which is not what you want in this case. What you need to do is specify a custom IEqualityComparer<byte[]> and do the comparison you want.

For example

public class ByteArrayComparer : IEqualityComparer<byte[]> {   public bool Equals(byte[] left, byte[] right) {     if ( left == null || right == null ) {       return left == right;     }     return left.SequenceEqual(right);   }   public int GetHashCode(byte[] key) {     if (key == null)       throw new ArgumentNullException("key");     return key.Sum(b => b);   } } 

Then you can do

var dict = new Dictionary<byte[], string>(new ByteArrayComparer()); 

Solution for 2.0

public class ByteArrayComparer : IEqualityComparer<byte[]> {   public bool Equals(byte[] left, byte[] right) {     if ( left == null || right == null ) {       return left == right;     }     if ( left.Length != right.Length ) {       return false;     }     for ( int i= 0; i < left.Length; i++) {       if ( left[i] != right[i] ) {         return false;       }     }     return true;   }   public int GetHashCode(byte[] key) {     if (key == null)       throw new ArgumentNullException("key");     int sum = 0;     foreach ( byte cur in key ) {       sum += cur;     }     return sum;   } }