Quickstart: Analyze a local image with C#

In this quickstart, you analyze a local image to extract visual features using Computer Vision.


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

Analyze Image request

With the Analyze Image method, you can extract visual features based on image content. You can upload an image or specify an image URL and choose which features to return, including:

  • A detailed list of tags related to the image content.
  • A description of image content in a complete sentence.
  • The coordinates, gender, and age of any faces contained in the image.
  • The ImageType (clip art or a line drawing).
  • The dominant color, the accent color, or whether an image is black & white.
  • The category defined in this taxonomy.
  • Does the image contain adult or sexually suggestive content?

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.
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.
            Console.WriteLine("Analyze an image:");
            Console.Write("Enter the path to the image you wish to analyze: ");
            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 the analysis of the specified image file by using
        /// the Computer Vision REST API.
        /// </summary>
        /// <param name="imageFilePath">The image file to analyze.</param>
        static async Task MakeAnalysisRequest(string imageFilePath)
                HttpClient client = new HttpClient();

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

                // Request parameters. A third optional parameter is "details".
                string requestParameters =

                // 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);

                // Get the JSON response.
                string contentString = await response.Content.ReadAsStringAsync();

                // Display the JSON response.
            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);

Analyze Image response

A successful response is returned in JSON, for example:

   "categories": [
         "name": "abstract_",
         "score": 0.00390625
         "name": "others_",
         "score": 0.0234375
         "name": "outdoor_",
         "score": 0.00390625
   "description": {
      "tags": [
      "captions": [
            "text": "a close up of an empty city street at night",
            "confidence": 0.7965622853462756
   "requestId": "dddf1ac9-7e66-4c47-bdef-222f3fe5aa23",
   "metadata": {
      "width": 3733,
      "height": 1986,
      "format": "Jpeg"
   "color": {
      "dominantColorForeground": "Black",
      "dominantColorBackground": "Black",
      "dominantColors": [
      "accentColor": "666666",
      "isBWImg": true

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.