How to check for a valid Base64 encoded string

Use Convert.TryFromBase64String from C# 7.2

public static bool IsBase64String(string base64)
{
   Span<byte> buffer = new Span<byte>(new byte[base64.Length]);
   return Convert.TryFromBase64String(base64, buffer , out int bytesParsed);
}

Update: For newer versions of C#, there's a much better alternative, please refer to the answer by Tomas here: https://stackoverflow.com/a/54143400/125981.

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It's pretty easy to recognize a Base64 string, as it will only be composed of characters 'A'..'Z', 'a'..'z', '0'..'9', '+', '/' and it is often padded at the end with up to three '=', to make the length a multiple of 4. But instead of comparing these, you'd be better off ignoring the exception, if it occurs.