Question

I’m building an application to retrieve an image from internet. Even though it works fine, it is slow (on wrong given URL) when using try-catch statements in the application.

(1) Is this the best way to verify URL and handle wrong input – or should I use Regex (or some other method) instead?

(2) Why does the application try to find images locally if I don’t specify http:// in the textBox?

private void btnGetImage_Click(object sender, EventArgs e)
{
    String url = tbxImageURL.Text;
    byte[] imageData = new byte[1];

    using (WebClient client = new WebClient())
    {
        try
        {
            imageData = client.DownloadData(url);
            using (MemoryStream ms = new MemoryStream(imageData))
            {
                try
                {
                    Image image = Image.FromStream(ms);
                    pbxUrlImage.Image = image;
                }
                catch (ArgumentException)
                {
                    MessageBox.Show("Specified image URL had no match", 
                        "Image Not Found", MessageBoxButtons.OK, 
                        MessageBoxIcon.Error);
                }
            }
        }
        catch (ArgumentException)
        {
            MessageBox.Show("Image URL can not be an empty string", 
                "Empty Field", MessageBoxButtons.OK, 
                MessageBoxIcon.Information);
        }
        catch (WebException)
        {
            MessageBox.Show("Image URL is invalid.\nStart with http:// " +
                "and end with\na proper image extension", "Not a valid URL",
                MessageBoxButtons.OK, MessageBoxIcon.Information);
        }
    } // end of outer using statement
} // end of btnGetImage_Click

EDIT:
I tried the suggested solution by Panagiotis Kanavos (thank you for your effort!), but it only gets caught in the if-else statement if the user enters http:// and nothing more. Changing to UriKind.Absolute catches empty strings as well! Getting closer 🙂
The code as of now:

private void btnGetImage_Click(object sender, EventArgs e)
{
    String url = tbxImageURL.Text;
    byte[] imageData = new byte[1];
    Uri myUri;

    // changed to UriKind.Absolute to catch empty string
    if (Uri.TryCreate(url, UriKind.Absolute, out myUri))
    {
        using (WebClient client = new WebClient())
        {
            try
            {
                imageData = client.DownloadData(myUri);
                using (MemoryStream ms = new MemoryStream(imageData))
                {
                    imageData = client.DownloadData(myUri);
                    Image image = Image.FromStream(ms);
                    pbxUrlImage.Image = image;
                }
            }
            catch (ArgumentException)
            {
                MessageBox.Show("Specified image URL had no match",
                    "Image Not Found", MessageBoxButtons.OK, 
                    MessageBoxIcon.Error);
            }
            catch (WebException)
            {
                MessageBox.Show("Image URL is invalid.\nStart with http:// " +
                    "and end with\na proper image extension", 
                    "Not a valid URL",
                    MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }
    }
    else
    {
        MessageBox.Show("The Image Uri is invalid.\nStart with http:// " +
            "and end with\na proper image extension", "Uri was not created",
            MessageBoxButtons.OK, MessageBoxIcon.Information);
    }

I must be doing something wrong here. 🙁

Answer 1

Use Uri.TryCreate to create a new Uri object only if your url string is a valid URL. If the string is not a valid URL, TryCreate returns false.

string myString = "http://someUrl";
Uri myUri;
if (Uri.TryCreate(myString, UriKind.RelativeOrAbsolute, out myUri))
{
    //use the uri here
}

UPDATE

TryCreate or the Uri constructor will happily accept strings that may appear invalid, eg “Host: http://www.stackoverflow.com“,”Host:%20www.stackoverflow.com” or “chrome:about”. In fact, these are perfectly valid URIs that specify a custom scheme instead of “http”.

The documentation of the Uri.Scheme property provides more examples like “gopher:” (anyone remember this?), “news”, “mailto”, “uuid”.

An application can register itself as a custom protocol handler as described in MSDN or other SO questions, eg How do I register a custom URL protocol in Windows?

TryCreate doesn’t provide a way to restrict itself to specific schemes. The code needs to check the Uri.Scheme property to ensure it contains an acceptable value

UPDATE 2

Passing a weird string like "></script><script>alert(9)</script> will return true and construct a relative Uri object. Calling Uri.IsWellFormedOriginalString will return false though. So you probably need to call IsWellFormedOriginalString if you want to ensure that relative Uris are well formed.

On the other hand, calling TryCreate with UriKind.Absolute will return false in this case.

Interestingly, Uri.IsWellFormedUriString calls TryCreate internally and then returns the value of IsWellFormedOriginalString if a relative Uri was created.