How to check for a valid Base64 encoded string

Question

Is there a way in C# to see if a string is Base 64 encoded other than just trying to convert it and see if there is an error? I have code code like this:

// Convert base64-encoded hash value into a byte array.
byte[] HashBytes = Convert.FromBase64String(Value);

I want to avoid the "Invalid character in a Base-64 string" exception that happens if the value is not valid base 64 string. I want to just check and return false instead of handling an exception because I expect that sometimes this value is not going to be a base 64 string. Is there some way to check before using the Convert.FromBase64String function?

Thanks!

Update:
Thanks for all of your answers. Here is an extension method you can all use so far it seems to make sure your string will pass Convert.FromBase64String without an exception. .NET seems to ignore all trailing and ending spaces when converting to base 64 so "1234" is valid and so is " 1234 "

public static bool IsBase64String(this string s)
{
    s = s.Trim();
    return (s.Length % 4 == 0) && Regex.IsMatch(s, @"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None);

}

For those wondering about performance of testing vs catching and exception, in most cases for this base 64 thing it is faster to check than to catch the exception until you reach a certain length. The smaller the length faster it is

In my very unscientific testing: For 10000 iterations for character length 100,000 - 110000 it was 2.7 times faster to test first.

For 1000 iterations for characters length 1 - 16 characters for total of 16,000 tests it was 10.9 times faster.

I am sure there is a point where it becomes better to test with the exception based method. I just don't know at what point that is.

Answer 1

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);
}