Understanding URL Encoding and NSUrl in MonoTouch
As a developer, working with URLs can be a complex task, especially when dealing with different platforms like iOS and MonoTouch. In this article, we’ll delve into the world of URL encoding and explore why NSUrl from an NSString string is returning null even though it’s a valid URL in a browser.
What are URL Encodings?
URL encodings, also known as percent-encodings, are used to represent special characters in URLs. These characters, such as spaces, punctuation marks, and non-alphanumeric characters, cannot be directly included in a URL without proper encoding.
In the past, only ASCII characters (up to U+007F) could be used in URLs without any encoding. However, with the advent of non-ASCII characters and special symbols, it became necessary to introduce percent-encoding schemes like URI (Uniform Resource Identifier).
How Does Percent-Encoding Work?
Percent-encoding involves replacing special characters with a % symbol followed by their hexadecimal representation. For example:
- Space:
%20 - Comma:
%2C - Ampersand:
%26
When encoding an NSString, we need to convert these special characters into their corresponding hexadecimal representations using the stringByAddingPercentEscapesUsingEncoding: method.
Creating a Valid NSUrl from an NSString
In our example, the string contains the following encoded URL:
http://google.com/search?query
The %20 represents a space. To create a valid NSUrl, we need to convert this encoded URL into its original form using the stringByAddingPercentEscapesUsingEncoding: method.
NSString *urlString = @"<a>http://google.com/search</a>?\query";
NSString *encodedUrlString = [urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
Now we can use the stringByAddingPercentEscapesUsingEncoding: method to create a valid NSUrl from our encoded string.
NSURL *foo = [NSURL URLWithString:encodedUrlString];
Why Does NSUrl Return Null?
When you call NSUrl.FromString:, it attempts to parse the input string as a URL. However, if the string is not properly encoded or contains invalid characters, it will return null.
In our case, the original string:
@\"http://google.com/search?query\"
contains double quotes (") and non-ASCII characters like \uFFFD (the unknown character in the URL). When we try to create an NSUrl from this string using NSUrl.FromString:, it returns null.
To fix this issue, we need to ensure that our input string is properly encoded before passing it to NSUrl.FromString:.
NSString *urlString = @"<a>http://google.com/search</a>?\query";
NSURL *foo = [NSURL URLWithString:[urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
By using the stringByAddingPercentEscapesUsingEncoding: method, we can convert our input string into a valid encoded URL that can be parsed by NSUrl.FromString:.
Conclusion
In this article, we explored the challenges of working with URLs in MonoTouch and how to create a valid NSUrl from an NSString. We discussed the importance of percent-encoding special characters and how to use the stringByAddingPercentEscapesUsingEncoding: method to convert our input string into a properly encoded URL.
By following these steps, you can ensure that your URLs are correctly formatted and that your app behaves as expected on different platforms.
Last modified on 2025-02-20