Fast C# IBAN Checksum Verifier / Validation

IBANChecksum.cs

//Copyright (C) 2013 DvdKhl
//Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"),
//to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
//and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
//The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
//THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
//WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

//static bool IsIbanChecksumValid(ReadOnlySpan<char> iban) { //Use this line if you target a runtime which supports Spans
static bool IsIbanChecksumValid(string iban) {
	if(iban.Length < 4 || iban[0] == ' ' || iban[1] == ' ' || iban[2] == ' ' || iban[3] == ' ') throw new InvalidOperationException();

	var checksum = 0;
	var ibanLength = iban.Length;
	for(int charIndex = 0; charIndex < ibanLength; charIndex++) {
		var c = iban[(charIndex + 4) % ibanLength];
		if(c == ' ') continue;

		int value;
		if(c >= '0' && c <= '9') {
			value = c - '0';
		} else if(c >= 'A' && c <= 'Z') {
			value = c - 'A';
			checksum = (checksum * 10 + value / 10 + 1) % 97;
			value %= 10;
		} else if(c >= 'a' && c <= 'z') {
			value = c - 'a';
			checksum = (checksum * 10 + value / 10 + 1) % 97;
			value %= 10;
		} else throw new InvalidOperationException();

		checksum = (checksum * 10 + value) % 97;
	}
	return checksum == 1;
}

Nice work, but fails when IBAN contains spaces, e.g. 'GB82 WEST 1234 5698 7654 32'. In my fork I fixed this the 'brute force' way, by replacing all spaces with empty string, probably a faster method is possible

The better fix is to increment the shift (initially 4) when a space is encountered.

Needed this one again.
I've fixed the error mentioned by jhgbrt.