Performance loss in VB.net equivalent of light weight conversion from hex to byte - vb.net

Question

I have read through the answers here https://stackoverflow.com/a/14332574/44080
I've also tried to produce equivalent VB.net code:
Option Strict ON
Public Function ParseHex(hexString As String) As Byte()
If (hexString.Length And 1) <> 0 Then
Throw New ArgumentException("Input must have even number of characters")
End If
Dim length As Integer = hexString.Length \ 2
Dim ret(length - 1) As Byte
Dim i As Integer = 0
Dim j As Integer = 0
Do While i < length
Dim high As Integer = ParseNybble(hexString.Chars(j))
j += 1
Dim low As Integer = ParseNybble(hexString.Chars(j))
j += 1
ret(i) = CByte((high << 4) Or low)
i += 1
Loop
Return ret
End Function
Private Function ParseNybble(c As Char) As Integer
If c >= "0"C AndAlso c <= "9"C Then
Return c - "0"C
End If
c = ChrW(c And Not &H20)
If c >= "A"C AndAlso c <= "F"C Then
Return c - ("A"C - 10)
End If
Throw New ArgumentException("Invalid nybble: " & c)
End Function
Can we remove the compile errors in ParseNybble without introducing data conversions?
Return c - "0"c Operator '-' is not defined for types 'Char' and 'Char'
c = ChrW(c And Not &H20) Operator 'And' is not defined for types 'Char' and 'Integer'

Answer 1

As it stands, no.
However, you could change ParseNybble to take an integer and pass AscW(hexString.Chars(j)) to it, so that the data conversion takes place outside of ParseNybble.

Answer 2

This solution is much much faster than all the alternative i have tried. And it avoids any ParseNybble lookup.
Function hex2byte(s As String) As Byte()
Dim l = s.Length \ 2
Dim hi, lo As Integer
Dim b(l - 1) As Byte
For i = 0 To l - 1
hi = AscW(s(i + i))
lo = AscW(s(i + i + 1))
hi = (hi And 15) + ((hi And 64) >> 6) * 9
lo = (lo And 15) + ((lo And 64) >> 6) * 9
b(i) = CByte((hi << 4) Or lo)
Next
Return b
End Function