Question

i wrote a function for download a webpage : function like:

public string GetWebPage(string sURL)
    {
        System.Net.WebResponse objResponse = null;
        System.Net.WebRequest objRequest = null;
        System.IO.StreamReader objStreamReader = null;
        string sResultPage = null;
        try
        {
            objRequest = System.Net.HttpWebRequest.Create(sURL);
            objResponse = objRequest.GetResponse();
            objStreamReader = new System.IO.StreamReader(objResponse.GetResponseStream());
            sResultPage = objStreamReader.ReadToEnd();
            return sResultPage;
        }
        catch (Exception ex)
        {
            return "";
        }
    }

But my problem is that. when this function working at that time application goto freeze (not response) and that time my can’t not do any thing. How can i solve this problem. when downloading at time user can do other thing in my application.

Answer 1

Welcome to the world of blocking IO.

Consider the following:

You want your program to download a web page and then return the first 10 letters it finds in the source html. Your code might look like this:

...
string page = GetWebPage("http://example.com"); // download web page
page = page.Substring(0, 10);
Console.WriteLine(page);
....

When your program calls GetWebPage(), it must WAIT for the web page to be fully downloaded before it can possibly try to call Substring() – else it may try to get the substring before it actually downloads the letters.

Now consider your program. You’ve got lots of code – maybe a GUI interface running – and it’s all executing line by line one instruction at a time. When your code calls GetWebPage(), it can’t possibly continue executing additional code until that request is fully finished. Your entire program is waiting on that request to finish.

The problem can be solved in a few different ways, and the best solution depends on exactly what you’re doing with your code. Ideally, your code needs to execute asynchronously. c# has methods that can handle a lot of this for you, but one way or another, you’re going to want to start some work – downloading the web page in your case – and then continue executing code until your main thread is notified that the webpage is fully downloaded. Then your main thread can begin parsing the return value.

I’m assuming that since you’ve asked this question, you are very new to threads and concurrency in general. You have a lot of work to do. Here are some resources for you to read up about threading and implementing concurrency in c#:

C# Thread Introduction

.NET Asynchronous IO Design