using System;

using System.Text;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main(string[] args)

    {

        var base64Str = Convert.ToBase64String(Encoding.UTF8.GetBytes("this is a Base64 str"));

        var notBase64Str = "this is not a Base64 str";

        Console.WriteLine(await IsBase64String(base64Str)); // True

        Console.WriteLine(await IsBase64String(notBase64Str)); // False

    }

​

    public static async Task<bool> IsBase64String(string base64)

    {

        return await Task.FromResult(CheckIfIsBase64String());

        // Note the use of a local non-async function so you can use `Span<T>`

        bool CheckIfIsBase64String()

        {

            Span<byte> buffer = stackalloc byte[base64.Length]; // avoid array heap allocation

            return Convert.TryFromBase64String(base64, buffer, out int bytesParsed);

        }

    }

}