Escape invalid XML characters in C#

As the way to remove invalid XML characters I suggest you to use XmlConvert.IsXmlChar method. It was added since .NET Framework 4 and is presented in Silverlight too. Here is the small sample:

void Main() {
    string content = "\v\f\0";
    Console.WriteLine(IsValidXmlString(content)); // False

    content = RemoveInvalidXmlChars(content);
    Console.WriteLine(IsValidXmlString(content)); // True
}

static string RemoveInvalidXmlChars(string text) {
    var validXmlChars = text.Where(ch => XmlConvert.IsXmlChar(ch)).ToArray();
    return new string(validXmlChars);
}

static bool IsValidXmlString(string text) {
    try {
        XmlConvert.VerifyXmlChars(text);
        return true;
    } catch {
        return false;
    }
}

And as the way to escape invalid XML characters I suggest you to use XmlConvert.EncodeName method. Here is the small sample:

void Main() {
    const string content = "\v\f\0";
    Console.WriteLine(IsValidXmlString(content)); // False

    string encoded = XmlConvert.EncodeName(content);
    Console.WriteLine(IsValidXmlString(encoded)); // True

    string decoded = XmlConvert.DecodeName(encoded);
    Console.WriteLine(content == decoded); // True
}

static bool IsValidXmlString(string text) {
    try {
        XmlConvert.VerifyXmlChars(text);
        return true;
    } catch {
        return false;
    }
}

Update: It should be mentioned that the encoding operation produces a string with a length which is greater or equal than a length of a source string. It might be important when you store a encoded string in a database in a string column with length limitation and validate source string length in your app to fit data column limitation.

Use SecurityElement.Escape

using System;
using System.Security;

class Sample {
  static void Main() {
    string text = "Escape characters ： < > & \" \'";
    string xmlText = SecurityElement.Escape(text);
//output:
//Escape characters ： &lt; &gt; &amp; &quot; &apos;
    Console.WriteLine(xmlText);
  }
}

If you are writing xml, just use the classes provided by the framework to create the xml. You won't have to bother with escaping or anything.

Console.Write(new XElement("Data", "< > &"));

Will output

<Data>&lt; &gt; &amp;</Data>

If you need to read an XML file that is malformed, do not use regular expression. Instead, use the Html Agility Pack.