Example showing how to upload item images to the Square Connect API in C#

uploader.cs

using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using System.IO;
using System;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Json;

static class ImageUploader
{
    const string DOMAIN = "https://connect.squareup.com";
    const string ITEM_ID = "<your-item-id>";
    const string BEARER_TOKEN = "<your-personal-access-token>";
    const string IMAGE_FILE = "cat.jpg";

    [DataContract]
    private struct ItemImage
    {
        [DataMember]
        public string id;
        [DataMember]
        public string url;
    }

    // Uploads the image for a given item.
    // Returns a URL.
    static async public Task<string> UploadImage(
        string accessToken, string itemID, string imageFile)
    {
        using (var client = new HttpClient())
        {
            // Configure the "Authorization: Bearer ..." HTTP header
            client.DefaultRequestHeaders.Authorization =
                new AuthenticationHeaderValue("Bearer", accessToken);

            // Read the image data and add it as an HTTP multipart form data item
            var imageContent = new ByteArrayContent(File.ReadAllBytes(imageFile));
            imageContent.Headers.ContentType = MediaTypeHeaderValue.Parse("image/jpeg");

            var requestContent = new MultipartFormDataContent();
            requestContent.Add(imageContent, "image_data", Path.GetFileName(imageFile));

            // POST the image to the server
            var url = DOMAIN + "/v1/me/items/" + itemID + "/image";
            var response = await client.PostAsync(url, requestContent);

            response.EnsureSuccessStatusCode();

            // (Optional) Parse the URL out of the response using DataContractSerializer.
            // There are nicer ways to parse JSON in C#, but this way uses only built-in
            // libraries.
            var jsonStream = await response.Content.ReadAsStreamAsync();
            var itemImage = (ItemImage)
                (new DataContractJsonSerializer(typeof(ItemImage)))
                    .ReadObject(jsonStream);

            return itemImage.url;
        }
    }

    static void Main()
    {
        // Here I'm just wrapping this async code in a Task so I can run it
        // inside Main()
        Task.Run(async() =>
            {
                var url = await UploadImage(
                    BEARER_TOKEN,
                    ITEM_ID,
                    IMAGE_FILE
                );

                Console.WriteLine("Uploaded image URL: " + url);
            }
        ).Wait();
    }
}