How to get the innerHTML(source) of a web page using c#?

You'll need to create a web request, and make a request out to the internet to fetch the website.  This is done by calling "GetResponse()", which returns a WebResponse object, which you will cast into an HttpWebResponse, and then call GetResponseStream() on.  The result of GetResponseStream(), will be a stream object, which you can then use to instantiate a StreamReader, and read the text. 

To write the text out to a file, call File.WriteAllText.

Here's an example:

static void WriteSourceToFile(string url, string filename)

{

    WebRequest request = HttpWebRequest.Create(url);

    HttpWebResponse response = (HttpWebResponse)request.GetResponse();

    Stream responseStream = response.GetResponseStream();

    StreamReader reader = new StreamReader(responseStream);

    string documentText = reader.ReadToEnd();

    reader.Close();

    File.WriteAllText(filename, documentText);

}