You absolutely can convert sbyte[]to byte[]- I can pretty much guarantee you that the Java code will really treat the byte array as unsigned. (Or, to put it another way: base64 is only defined using unsigned bytes ...)
byte[] Convert.ToBase64String. byte[] - # , CLR , #:
sbyte[] x = { -1, 1 };
byte[] y = (byte[]) (object) x;
Console.WriteLine(Convert.ToBase64String(y));
byte[], :
byte[] y = new byte[x.Length];
Buffer.BlockCopy(x, 0, y, 0, y.Length);
.