What is fastest way to convert bool to byte?

How about:

byte x = value ? (byte) 1 : (byte) 0;

If you're talking about the most efficient way of doing it, there may be some tricks you could do with unsafe code... but is this really a bottleneck for you?

EDIT: I just realized that the conditional operator needs those casts for the operands in order to make the overall expression a byte.

EDIT: Having seen your question, there's a much better way of optimizing it IMO. Currently you'll be performing operations you don't need to either way. Try this instead:

c[i << 1] = k > 9 ? k + 0x37 : k + 0x30;

or

c[i << 1] = k + (k > 9 ? 0x37 : 0x30);

(I suspect it doesn't matter which.)

You only need to perform the comparison and then one addition - instead of two additions and two multiplications after the conversion from bool to byte.

EDIT: Having just tried this, due to potentially branch misses, this can still definitely be slower than the unsafe version... or it can be faster. Picking a random value for k in the range [0, 18), this approach takes twice as long as the unsafe code. Picking a random value for k in the range [0, 1000) (i.e. one branch is picked much more often than the other), this approach is faster than the unconditional one. So what's the pattern for your k value?

Here's some benchmark code:

using System;
using System.Diagnostics;

class Test
{
    static void Main()
    {
        Random rng = new Random();
        int[] ks = new int[100000000];
        for (int i = 0; i < ks.Length; i++)
        {
            ks[i] = rng.Next(1000);
        }

        for (int i = 0; i < 3; i++)
        {
            Console.WriteLine("Iteration {0}", i);
            long sum = 0;
            Stopwatch sw = Stopwatch.StartNew();
            for (int j = 0; j < ks.Length; j++)
            {