using System;

using System.Text;

using System.Threading.Tasks;

​

public class Program

{

    private static System.Buffers.SearchValues<char> base64SearchValues =

        System.Buffers.SearchValues.Create(@"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=");

    public async static 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

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

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

    }

​

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

    {

        return await Task.FromResult(

            base64?.Length > 0 &&

            base64?.Length % 4 == 0 &&

            base64.AsSpan().IndexOfAnyExcept(base64SearchValues) < 0

        );

    }

}