I was looking into setting proxy for WebView for Windows Store applications. Unfortunately most of the methods I tested did not pan out.
Method #1: Using .NET APIs
The MSFT Employee said WebView will use IE proxy settings. However you would be able to set proxy for custom HTTP requests with HTTPClient and HTTPWebRequest. (More on this later).
Method #2: Reflection
I attempted to use reflection to see if there are any hidden methods that I could call inside the WebView class. Unfortunately this method did not pan out also. I should probably say right now that none of the methods panned out except the last one (kind of).
So using reflection, I wasn't able to view private members. After a bit of researching I found out that BindingFlags had to be set to Private or class variables (forget the actual syntax). But since I was doing this in a Windows Store application I was unable to set the binding flags (the windows store apis did not support this).
So then I had to create a PCL (portable class library) and then use reflection with the binding flags set to private variables and see if I was able to find any set proxy members. The only thing I found was a hasProxyImplementation method. But no way to set the actual proxy for WebView.
Method #3: 3rd Party Components
Method #4: App Level Proxy
There isn't a way to set app level proxy programmatically. I believe it was Windows 8.1 where you can set proxy for metro apps through the metro settings menu.
Method #5: Setting IE Proxy
And as far as I know, there is no way of setting proxy for IE.
Method 6: Intercepting HTTP Requests from WebView
Intercepting Requests & Proxy
After looking into it, I was able to get one method to work. By listening into a socket using HttpService and point the WebView source to "http://locahost:[port#]" I am able to intercept the initial request that the WebView makes to the socket.
I make my own request using HttpClient (after the initial request comes in). Take a look how to add a proxy to Http Request Messages: Proxy with HTTP Requests
When I get the response back, I write the headers and the content back to the WebView. The WebView then display the page.
Handling External Links
Now for images/external resources that have absolute links (http:// google.com/logo.png), the WebView would handle this by itself and you would not receive a request through the socket. To make this go through the socket (and proxy) you would need to change the actual link on the page. For example, when the initial request comes through for http://localhost:[port#], you would make your own request with HttpClient to http:// google.com (as an example). When i get the response back, the content of that response would have the content of index.html. You would need to replace all absolute links (like http:// google.com/logo.png) with your own relative links or with some other identifier, http://locahost:[port#]/?q=[url]. This way when the WebView loads the image, it would go through your local socket/server that you are listening into.
I have implemented the method above (with replacing links), and my results were mostly successful. I was replacing anything that started with http:// and sometimes this would replace the content on the page. You could look for href tags but you have to remember that there could be ajax requests with absolute urls that you would have to replace. And ajax requests would not have href tags. This is just one example, and there might be others you would have to account for.
Now after going through all of this, I would not recommend this method. If you want to display a simple page and you know the user won't navigate to external websites and what not, then this might work.
I was having problems with POST requests and random headers with various websites (that needed to be removed in requests and responses). There are also too many things to account for with absolute links/images/ajax requests/sql queries/etc. Microsoft needs add in this functionality in their WebView APIs. Till then, there isn't a good way of making WebView go through proxy.