How would you make this switch statement as fast as possible?

Question

2009-12-04 UPDATE: For profiling results on a number of the suggestions posted here, see below!

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The Question

Consider the following very

Answer 1

All your strings are at most 2 chars long, and ASCII, so we can use 1 byte per char. Furthermore, more likely than not, they also never can have \0 appear in them (.NET string allows for embedded null characters, but many other things don't). With that assumption, we can null-pad all your strings to be exactly 2 bytes each, or an ushort:

""   -> (byte) 0 , (byte) 0   -> (ushort)0x0000
"A"  -> (byte)'A', (byte) 0   -> (ushort)0x0041
"B"  -> (byte)'B', (byte) 0   -> (ushort)0x0042
"BT" -> (byte)'B', (byte)'T'  -> (ushort)0x5442

Now that we have a single integer in a relatively (64K) short range, we can use a lookup table:

MarketDataExchange[] lookup = {
    MarketDataExchange.NBBO, 
    MarketDataExchange.NONE, 
    MarketDataExchange.NONE, 
    ...
    /* at index 0x041 */
    MarketDataExchange.AMEX,
    MarketDataExchange.BSE,
    MarketDataExchange.NSE,
    ...
};

Now, obtaining the value given a string is:

public static unsafe MarketDataExchange GetMarketDataExchange(string s)
{
   // Assume valid input
   if (s.Length == 0) return MarketDataExchange.NBBO;

   // .NET strings always have '\0' after end of data - abuse that
   // to avoid extra checks for 1-char strings. Skip index checks as well.
   ushort hash;
   fixed (char* data = s)
   {
       hash = (ushort)data[0] | ((ushort)data[1] << 8);
   }

   return lookup[hash];
}