Inspired by philcruz's Regular Expression solution, I've made a pure LINQ solution
public static string PureAscii(this string source, char nil = ' ')
{
var min = '\u0000';
var max = '\u007F';
return source.Select(c => c < min ? nil : c > max ? nil : c).ToText();
}
public static string ToText(this IEnumerable<char> source)
{
var buffer = new StringBuilder();
foreach (var c in source)
buffer.Append(c);
return buffer.ToString();
}
This is untested code.