I want to write a program that do this:
It should receive the URL (Wikipedia 's URL) from input and download that web page
And read the content of that page and save it into a file
please help me

This is an option. It is widely used.
I happened to use it a day ago..

WebRequest request = WebRequest.Create("url");
WebResponse response = request.GetResponse();
Stream current_data = response.GetResponseStream();
string content = "";
using (StreamReader sr = new StreamReader(response))
    content = sr.ReadToEnd();
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