如何在Base64算法中使用中文(中文和base64代码的转换)

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我的代码如下:
private string aa(string base64string)
{
System.Byte[] b=Convert.FromBase64String(base64string);
string s="";
for (int i=0;i<b.Length;i++)
{
s+=(char)b[i];
}
return s;
}
private void button1_Click(object sender, System.EventArgs e)
{
rtb1.Text=aa(rtb2.Text);
}

private void button2_Click(object sender, System.EventArgs e)
{

System.IO.MemoryStream memoryStream=new System.IO.MemoryStream();
System.Byte[] b;

rtb1.SaveFile(memoryStream,System.Windows.Forms.RichTextBoxStreamType.TextTextOleObjs );
b=memoryStream.GetBuffer();
rtb2.Text=Convert.ToBase64String(b);
memoryStream.Close();
}
从中文转换成base64代码可以,但是从base64转换成中文就出现了乱码