How to convert a string to ASCII

.NET stores all strings as a sequence of UTF-16 code units. (This is close enough to "Unicode characters" for most purposes.)

Fortunately for you, Unicode was designed such that ASCII values map to the same number in Unicode, so after you've converted each character to an integer, you can just check whether it's in the ASCII range. Note that you can use an implicit conversion from char to int - there's no need to call a conversion method:

string text = "Here's some text including a \u00ff non-ASCII character";
foreach (char c in text)
{
    int unicode = c;
    Console.WriteLine(unicode < 128 ? "ASCII: {0}" : "Non-ASCII: {0}", unicode);
}

For Any String try this:

string s = Console.ReadLine();
foreach( char c in s)
{
    Console.WriteLine(System.Convert.ToInt32(c));
}
Console.ReadKey();

Try Linq:

Result = string.Join("", input.ToCharArray().Where(x=> ((int)x) < 127));

This will filter out all non ascii characters. Now if you want an equivalent, try the following:

Result = string.Join("", System.Text.Encoding.ASCII.GetChars(System.Text.Encoding.ASCII.GetBytes(input.ToCharArray())));