ASCIIEncoding Class (System.Text) | Microsoft Docs

using System;
using System.Text;

class ASCIIEncodingExample {
    public static void Main() {
        // The encoding.
        ASCIIEncoding ascii = new ASCIIEncoding();
        
        // A Unicode string with two characters outside the ASCII code range.
        String unicodeString =
            "This Unicode string contains two characters " +
            "with codes outside the ASCII code range, " +
            "Pi (\u03a0) and Sigma (\u03a3).";
        Console.WriteLine("Original string:");
        Console.WriteLine(unicodeString);

        // Save positions of the special characters for later reference.
        int indexOfPi = unicodeString.IndexOf('\u03a0');
        int indexOfSigma = unicodeString.IndexOf('\u03a3');

        // Encode string.
        Byte[] encodedBytes = ascii.GetBytes(unicodeString);
        Console.WriteLine();
        Console.WriteLine("Encoded bytes:");
        foreach (Byte b in encodedBytes) {
            Console.Write("[{0}]", b);
        }
        Console.WriteLine();
        
        // Notice that the special characters have been replaced with
        // the value 63, which is the ASCII character code for '?'.
        Console.WriteLine();
        Console.WriteLine(
            "Value at position of Pi character: {0}",
            encodedBytes[indexOfPi]
        );
        Console.WriteLine(
            "Value at position of Sigma character: {0}",
            encodedBytes[indexOfSigma]
        );

        // Decode bytes back to string.
        // Notice missing Pi and Sigma characters.
        String decodedString = ascii.GetString(encodedBytes);
        Console.WriteLine();
        Console.WriteLine("Decoded bytes:");
        Console.WriteLine(decodedString);
    }
}
// The example displays the following output:
//    Original string:
//    This Unicode string contains two characters with codes outside the ASCII code ra
//    nge, Pi (Π) and Sigma (Σ).
//
//    Encoded bytes:
//    [84][104][105][115][32][85][110][105][99][111][100][101][32][115][116][114][105]
//    [110][103][32][99][111][110][116][97][105][110][115][32][116][119][111][32][99][
//    104][97][114][97][99][116][101][114][115][32][119][105][116][104][32][99][111][1
//    00][101][115][32][111][117][116][115][105][100][101][32][116][104][101][32][65][
//    83][67][73][73][32][99][111][100][101][32][114][97][110][103][101][44][32][80][1
//    05][32][40][63][41][32][97][110][100][32][83][105][103][109][97][32][40][63][41]
//    [46]
//
//    Value at position of Pi character: 63
//    Value at position of Sigma character: 63
//
//    Decoded bytes:
//    This Unicode string contains two characters with codes outside the ASCII code ra
//    nge, Pi (?) and Sigma (?).