Fastest way to convert a possibly-null-terminated ascii byte[] to a string?

Oneliner (assuming the buffer actually contains ONE well formatted null terminated string):

String MyString = Encoding.ASCII.GetString(MyByteBuffer).TrimEnd((Char)0);

Any reason not to use the String(sbyte*, int, int) constructor? If you've worked out which portion of the buffer you need, the rest should be simple:

public static string UnsafeAsciiBytesToString(byte[] buffer, int offset, int length)
{
    unsafe
    {
       fixed (byte* pAscii = buffer)
       { 
           return new String((sbyte*)pAscii, offset, length);
       }
    }
}

If you need to look first:

public static string UnsafeAsciiBytesToString(byte[] buffer, int offset)
{
    int end = offset;
    while (end < buffer.Length && buffer[end] != 0)
    {
        end++;
    }
    unsafe
    {
       fixed (byte* pAscii = buffer)
       { 
           return new String((sbyte*)pAscii, offset, end - offset);
       }
    }
}

If this truly is an ASCII string (i.e. all bytes are less than 128) then the codepage problem shouldn't be an issue unless you've got a particularly strange default codepage which isn't based on ASCII.

Out of interest, have you actually profiled your application to make sure that this is really the bottleneck? Do you definitely need the absolute fastest conversion, instead of one which is more readable (e.g. using Encoding.GetString for the appropriate encoding)?

I'm not sure of the speed, but I found it easiest to use LINQ to remove the nulls before encoding:

string s = myEncoding.GetString(bytes.TakeWhile(b => !b.Equals(0)).ToArray());