Quickstart: Generate a thumbnail with C#

In this quickstart, you generate a thumbnail from an image using Computer Vision.


To use Computer Vision, you need a subscription key; see Obtaining Subscription Keys.

Get Thumbnail request

With the Get Thumbnail method, you can generate a thumbnail of an image. You specify the height and width, which can differ from the aspect ratio of the input image. Computer Vision uses smart cropping to intelligently identify the region of interest and generate cropping coordinates based on that region.

To run the sample, do the following steps:

  1. Create a new Visual C# Console App in Visual Studio.
  2. Install the Newtonsoft.Json NuGet package.
    1. On the menu, click Tools, select NuGet Package Manager, then Manage NuGet Packages for Solution.
    2. Click the Browse tab, and in the Search box type "Newtonsoft.Json".
    3. Select Newtonsoft.Json when it displays, then click the checkbox next to your project name, and Install.
  3. Replace Program.cs with the following code.
  4. Replace <Subscription Key> with your valid subscription key.
  5. Change the uriBase value to the location where you obtained your subscription keys, if necessary.
  6. Run the program.
  7. At the prompt, enter the path to a local image.

The thumbnail is saved to the same folder as the local image, using the original name with the suffix "_thumb".

using Newtonsoft.Json.Linq;
using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

namespace CSHttpClientSample
    static class Program
        // Replace <Subscription Key> with your valid subscription key.
        const string subscriptionKey = "<Subscription Key>";

        // You must use the same region in your REST call as you used to
        // get your subscription keys. For example, if you got your
        // subscription keys from westus, replace "westcentralus" in the URL
        // below with "westus".
        // Free trial subscription keys are generated in the westcentralus region.
        // If you use a free trial subscription key, you shouldn't need to change
        // this region.
        const string uriBase =

        static void Main()
            // Get the path and filename to process from the user.
                "Enter the path to the image you wish to use to create a thumbnail image: ");
            string imageFilePath = Console.ReadLine();

            if (File.Exists(imageFilePath))
                // Make the REST API call.
                Console.WriteLine("\nWait a moment for the results to appear.\n");
                Console.WriteLine("\nInvalid file path");
            Console.WriteLine("\nPress Enter to exit...");

        /// <summary>
        /// Gets a thumbnail image from the specified image file by using
        /// the Computer Vision REST API.
        /// </summary>
        /// <param name="imageFilePath">The image file to use to create the thumbnail image.</param>
        static async Task MakeThumbNailRequest(string imageFilePath)
                HttpClient client = new HttpClient();

                // Request headers.
                    "Ocp-Apim-Subscription-Key", subscriptionKey);

                // Request parameters.
                string requestParameters = "width=200&height=150&smartCropping=true";

                // Assemble the URI for the REST API Call.
                string uri = uriBase + "?" + requestParameters;

                HttpResponseMessage response;

                // Request body.
                // Posts a locally stored JPEG image.
                byte[] byteData = GetImageAsByteArray(imageFilePath);

                using (ByteArrayContent content = new ByteArrayContent(byteData))
                    // This example uses content type "application/octet-stream".
                    // The other content types you can use are "application/json"
                    // and "multipart/form-data".
                    content.Headers.ContentType =
                        new MediaTypeHeaderValue("application/octet-stream");

                    // Make the REST API call.
                    response = await client.PostAsync(uri, content);

                if (response.IsSuccessStatusCode)
                    // Display the response data.
                    Console.WriteLine("\nResponse:\n{0}", response);

                    // Get the image data.
                    byte[] thumbnailImageData =
                        await response.Content.ReadAsByteArrayAsync();

                    // Save the thumbnail to the same folder as the original image,
                    // using the original name with the suffix "_thumb".
                    // Note: This will overwrite an existing file of the same name.
                    string thumbnailFilePath =
                        imageFilePath.Insert(imageFilePath.Length - 4, "_thumb");
                    File.WriteAllBytes(thumbnailFilePath, thumbnailImageData);
                    Console.WriteLine("\nThumbnail written to: {0}", thumbnailFilePath);
                    // Display the JSON error data.
                    string errorString = await response.Content.ReadAsStringAsync();
            catch (Exception e)
                Console.WriteLine("\n" + e.Message);

        /// <summary>
        /// Returns the contents of the specified file as a byte array.
        /// </summary>
        /// <param name="imageFilePath">The image file to read.</param>
        /// <returns>The byte array of the image data.</returns>
        static byte[] GetImageAsByteArray(string imageFilePath)
            using (FileStream fileStream =
                new FileStream(imageFilePath, FileMode.Open, FileAccess.Read))
                BinaryReader binaryReader = new BinaryReader(fileStream);
                return binaryReader.ReadBytes((int)fileStream.Length);

Get Thumbnail response

A successful response contains the thumbnail image binary. If the request fails, the response contains an error code and a message to help determine what went wrong.


StatusCode: 200, ReasonPhrase: 'OK', Version: 1.1, Content: System.Net.Http.StreamContent, Headers:
  Pragma: no-cache
  apim-request-id: 131eb5b4-5807-466d-9656-4c1ef0a64c9b
  Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
  x-content-type-options: nosniff
  Cache-Control: no-cache
  Date: Tue, 06 Jun 2017 20:54:07 GMT
  X-AspNet-Version: 4.0.30319
  X-Powered-By: ASP.NET
  Content-Length: 5800
  Content-Type: image/jpeg
  Expires: -1

Next steps

Explore a basic Windows application that uses Computer Vision to perform optical character recognition (OCR); create smart-cropped thumbnails; plus detect, categorize, tag, and describe visual features, including faces, in an image.