Mangling binary data with System.Text.ASCIIEncoding()

This error is very common. Somebody wants to convert non-character binary byte data to a string and tries to do it using System.Text.ASCIIEncoding.GetString. When byte(s) are encountered that have no representation in the character encoding, you’ll get a question mark character in your output string. For us-ascii, all bytes having the most significant bit set (i.e. 8-bit bytes) will be collapsed into one character: ‘?’. Here’s an example in C# that demonstrates the behavior:

	
            byte[] binaryData = new byte[256];
            int i;
            for (i = 0; i < 256; i++)
            {
                binaryData[i] = (byte)i;
            }
            String s0 = "";
            for (i = 0; i < 256; i++)
            {
                s0 = s0 + Convert.ToString(binaryData[i]) + ",";
                if (((i+1) % 10) == 0)
                {
                    s0 = s0 + "\r\n";
                }
            }
            // This is displayed:
            /*
0,1,2,3,4,5,6,7,8,9,
10,11,12,13,14,15,16,17,18,19,
20,21,22,23,24,25,26,27,28,29,
30,31,32,33,34,35,36,37,38,39,
40,41,42,43,44,45,46,47,48,49,
50,51,52,53,54,55,56,57,58,59,
60,61,62,63,64,65,66,67,68,69,
70,71,72,73,74,75,76,77,78,79,
80,81,82,83,84,85,86,87,88,89,
90,91,92,93,94,95,96,97,98,99,
100,101,102,103,104,105,106,107,108,109,
110,111,112,113,114,115,116,117,118,119,
120,121,122,123,124,125,126,127,128,129,
130,131,132,133,134,135,136,137,138,139,
140,141,142,143,144,145,146,147,148,149,
150,151,152,153,154,155,156,157,158,159,
160,161,162,163,164,165,166,167,168,169,
170,171,172,173,174,175,176,177,178,179,
180,181,182,183,184,185,186,187,188,189,
190,191,192,193,194,195,196,197,198,199,
200,201,202,203,204,205,206,207,208,209,
210,211,212,213,214,215,216,217,218,219,
220,221,222,223,224,225,226,227,228,229,
230,231,232,233,234,235,236,237,238,239,
240,241,242,243,244,245,246,247,248,249,
250,251,252,253,254,255,
             * */
            textBox1.Text = s0;
	
            System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding();
	
            // Do a simple round-trip conversion and then check to see if our binary data was mangled:
            String s = enc.GetString(binaryData);
            byte[] binaryData2 = enc.GetBytes(s);
	
            String s2 = "";
            for (i = 0; i < 256; i++)
            {
                s2 = s2 + Convert.ToString(binaryData2[i]) + ",";
                if (((i + 1) % 10) == 0)
                {
                    s2 = s2 + "\r\n";
                }
            }
            // The output:
            /*
0,1,2,3,4,5,6,7,8,9,
10,11,12,13,14,15,16,17,18,19,
20,21,22,23,24,25,26,27,28,29,
30,31,32,33,34,35,36,37,38,39,
40,41,42,43,44,45,46,47,48,49,
50,51,52,53,54,55,56,57,58,59,
60,61,62,63,64,65,66,67,68,69,
70,71,72,73,74,75,76,77,78,79,
80,81,82,83,84,85,86,87,88,89,
90,91,92,93,94,95,96,97,98,99,
100,101,102,103,104,105,106,107,108,109,
110,111,112,113,114,115,116,117,118,119,
120,121,122,123,124,125,126,127,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,63,63,63,63,
63,63,63,63,63,63,
             * */
            textBox2.Text = s2;