REPLACE

The following code snippet should give you the idea.

The idea is go through the string and see if it ascii character value in in range of values you consider valid.
You may need to post more details before you get the answer you are after.

byte[] bScrubbed = new byte[(int)dwBytesRead];
int nPos = 0;

// Scrub the non-ascii characters
for (int i = 0; i < (int)dwBytesRead; i ++)
{
   if (((int)bData[i] > 19) && ((int)bData[i] < 125))
   {
      bScrubbed[nPos] = bData[i];
      nPos++;
   }
}

you can do this using the ASCII encoding class ... I can give you specific code to handle the case if you can tell me what encoding you are coming from (i.e. UTF 16) ...

Gregory,

Yep. Its UTF 16.

suggest the data you receive is correct data but in UTF16 and it saved in s[]

char [] s;

//some code to receive data in s

Encoder UTFEncoder = Encoding.UTF16.GetEncoder();

int byteCount = UTFEncoder.GetByteCount(s, 0, s.Length, true);
Byte[] bytes = new Byte[byteCount];
int bytesEncodedCount = UTFEncoder.GetBytes(s, 0,s.Length, bytes, 0, true);
//ok it change to UTF16
// bytesEncodedCount is the real changed bytes number
 Console.WriteLine("{0} bytes used to encode characters.", bytesEncodedCount );
//show the encoded bytes
Console.Write("Encoded bytes: ");
  foreach (Byte b in s) {
        Console.Write("[{0}]", b);
   }





UTFEncoder

once you have them in a byte array as displayed by tzxie2000 you can then get them into an ASCII string by using the ASCII encoder object .GetString() method.
note that null characters cause issues with strings in .net so they should be removed prior to this process (C style strings)

ptmcomp,

You are good. How about to replace using regex any special character with \\special character ?
Example :  $    to   \\$

Thank you.

I'm not sure if I understand your question.
If you want to replace "$" by "\\$" and let's say "@" by "\\@" then you could use this:  
string result = Regex.Replace(input, @"[$@]", @"\\$0");
Note: "$0" stands for the string matched by the expression