Cross-domain images in SharePoint provider-hosted add-ins

Use images across domains in provider-hosted add-ins.

Applies to: SharePoint 2013 | SharePoint Add-ins | SharePoint Online

You might want to display images from a SharePoint site in your provider-hosted add-ins. Because provider-hosted add-ins run on a remote web, the domains for your provider-hosted add-in and your SharePoint site are different. For example, your add-in might run on Microsoft Azure, and you're trying to show an image from Office 365. Because your provider-hosted add-in crosses domains to access the image, SharePoint requires user authorization before the provider-hosted add-in shows the image.

The Core.CrossDomainImages code sample shows how a provider-hosted add-in can use a SharePoint access token and a REST service to retrieve images from SharePoint. The REST service returns a Base64-encoded string, which is used to show the image in the browser. Use the solution in this sample to show images stored in SharePoint in provider-hosted add-ins by using either server-side or client-side code.

Note

Because the sample uses a REST endpoint, you can use either server-side or client-side code to retrieve your image.

Before you begin

To get started, download the Core.CrossDomainImages sample add-in from the Office 365 Developer patterns and practices project on GitHub.

Using the Core.CrossDomainImages code sample

When you run this sample, Default.aspx tries to load the following:

  • Image 1, by using the absolute URL.

  • Image 2, by using a server-side call to a REST endpoint that returns a Base64-encoded string.

  • Image 3, by using a client-side call to a REST endpoint that returns a Base64-encoded string.

Note

Image 1 does not render because the add-in crosses domains to get to the image in SharePoint. Notice that the URL of the provider-hosted add-in runs on localhost. Open the shortcut menu (right-click) for Image 1, and then choose Properties. Notice that the Address (URL) is trying to retrieve the image from the add-in web, not localhost. Because the provider-hosted add-in crosses domains to reach the add-in web, authentication is required in order to access the image. Verify that accessing Image 1's URL directly, as opposed to the cross-domain call in the provider-hosted add-in, resolves without an error by copying and pasting Address (URL) into a new browser window. Notice that Image 1 displays without an error. Compare the source of Image 1 to the source of Image 2. Notice that the Address (URL) points to a Base64-encoded string.

When Default.aspx loads, Page_Load runs and does the following:

  1. Sets Image1.ImageUrl to the absolute path of the image.

  2. Instantiates ImgService. ImgService is a REST endpoint that runs in the same domain as the provider-hosted add-in.

  3. Sets Image2.ImageUrl to the Base64-encoded string that ImgService.GetImage returns. Access token, site, folder, and file name are passed as parameters to ImgService.GetImage.

Note

The code in this article is provided as-is, without warranty of any kind, either express or implied, including any implied warranties of fitness for a particular purpose, merchantability, or non-infringement.

 protected void Page_Load(object sender, EventArgs e)
        {
            var spContext = SharePointContextProvider.Current.GetSharePointContext(Context);
            using (var clientContext = spContext.CreateUserClientContextForSPAppWeb())
            {
                // Set the access token in a hidden field for client-side code to use.
                hdnAccessToken.Value = spContext.UserAccessTokenForSPAppWeb;

                // Set the URLs to the images.
                Image1.ImageUrl = spContext.SPAppWebUrl + "AppImages/O365.png";
                Services.ImgService svc = new Services.ImgService();
                Image2.ImageUrl = svc.GetImage(spContext.UserAccessTokenForSPAppWeb, spContext.SPAppWebUrl.ToString(), "AppImages", "O365.png");
            }
        }

GetImage does the following:

  1. Uses url to store the GetFolderByServerRelativeUrl REST endpoint URI, which will be used to retrieve the image from SharePoint. You can learn more at Files and folders REST API reference.

  2. Instantiates an HttpWebRequest object by using url .

  3. Adds an Authorization header to the HttpWebRequest object that contains the access token.

After the GET call is made to the endpoint URI, the returned stream is a Base64-encoded string. The returned string is set to the src attribute of the image.

 public string GetImage(string accessToken, string site, string folder, string file)
        {
            // Create the HttpWebRequest to call the REST endpoint.
            string url = String.Format("{0}_api/web/GetFolderByServerRelativeUrl('{1}')/Files('{2}')/$value", site, folder, file);
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
            request.Headers.Add("Authorization", "Bearer" + " " + accessToken);
            using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
            {
                using (var sourceStream = response.GetResponseStream())
                {
                    using (var newStream = new MemoryStream())
                    {
                        sourceSteam.CopyTo(newStream);
                        byte[] bytes = newStream.ToArray();
                        return "data:image/png;base64, " + Convert.ToBase64String(bytes);
                    }
                }
            }
        }

After Page_Load finishes, Default.aspx runs the client-side code in Default.aspx, which loads Image 3. The client-side code calls GetImage by using jQuery Ajax. When GetImage returns the Base64-encoded string successfully, the src attribute on Image3 is set to the returned string.

  ...

                  $.ajax({
                url: '../Services/ImgService.svc/GetImage?accessToken=' + $('#hdnAccessToken').val() + '&site=' + encodeURIComponent(appWebUrl + '/') + '&folder=AppImages&file=O365.png',
                dataType: 'json',
                success: function (data) {
                    $('#Image3').attr('src', data.d);
                },
                error: function (err) {
                    alert('error occurred');
                }
            });

           ...

See also