C#

if (TabControl.TabPages.Contains(TabControl.TabPages[index]))
ITabControl.TabPages.Add(TabControl.TabPages[index]);

public static char Chr(int i) 
{
    if (0 <= i && i <= 127) return (char)i;
    if (i < -0x8000 || i > 0xFFFF) throw new ArgumentOutOfRangeException();
    
    System.Text.Encoding enc = System.Text.Encoding.Default;
    if (enc.GetMaxByteCount(1) == 1 && (i < 0 || i > 0xFF)) throw new ArgumentOutOfRangeException();
    
    System.Text.Decoder dec = enc.GetDecoder();
    byte[] b;
    char[] c;
    if (0 <= i && i <= 0xFF) 
    {
        b = new byte[1];
        b[0] = checked((byte) (i & 0xFF));
        c = new char[1];
        dec.GetChars(b, 0, 1, c, 0);
    }
    else 
    {
        b = new byte[2];
        b[0] = checked((byte) ((i >> 8) & 0xFF));
        b[1] = checked((byte) (i & 0xFF));
        c = new char[1];
        dec.GetChars(b, 0, 2, c, 0);
    }
    return c[0];
}
 
public static int Asc(char ch) 
{
    int i = (int)ch;
    if (i < 128) return i;
    
    System.Text.Encoding enc = System.Text.Encoding.Default;
    char[] c = new char[] {ch};
    byte[] b = enc.GetBytes(c);
    if (b.Length == 2 && BitConverter.IsLittleEndian) 
    {
        byte t = b[0];
        b[0] = b[1];
        b[1] = t;
    }
    if (b.Length == 2)
        return BitConverter.ToInt16(b,0);
    else
        return b[0];
}