Question

how to decode a base 64 encoded string encoded using Base64Encode with ATL_BASE64_FLAG_NOPAD flag.

Answer 1

Assuming that just means “don’t put the = at the end” you can add the padding directly:

public static string Base64PadEnd(string unpadded)
{
    switch(unpadded.Length % 4)
    {
        case 0: return unpadded;
        case 2: return unpadded + "==";
        case 3: return unpadded + "=";
        default: throw new ArgumentException("Invalid unpadded base64");
    }
}

(The way base64 works, you should never end up with an unpadded value with a final block of 1 character.)

After adding the padding, just use Convert.FromBase64String as usual.

EDIT: As noted in comments, if your base64 string contains whitespace, you should remove that first before adding the padding. A simple text = Regex.Replace(text, @"\s", "") should do the trick.