To post an image file to a REST API using HttpClient in C#, you can follow these steps. Assume you have an image file called "example.jpg" that you want to upload:
using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;

class Program
{
    static async Task Main()
    {
        // Replace with the URL of your REST API endpoint
        string apiUrl = "https://api-endpoint.com/upload";

        // Replace with the path to your image file
        string imagePath = "path/image/example.jpg";

        // Create an HttpClient instance
        using (HttpClient httpClient = new HttpClient())
        {
            // Read the image file as bytes
            byte[] imageData = File.ReadAllBytes(imagePath);

            // Create a ByteArrayContent to hold the image data
            ByteArrayContent content = new ByteArrayContent(imageData);

            // Set the content type to "multipart/form-data"
            content.Headers.ContentType = MediaTypeHeaderValue.Parse("multipart/form-data");

            // Create a FormDataContent with the image content
            using (MultipartFormDataContent formData = new MultipartFormDataContent())
            {
                // Add the image content to the form data
                formData.Add(content, "image", "example.jpg");

                try
                {
                    // Post the form data to the API endpoint
                    HttpResponseMessage response = await httpClient.PostAsync(apiUrl, formData);

                    // Check if the request was successful
                    if (response.IsSuccessStatusCode)
                    {
                        Console.WriteLine("Image uploaded successfully.");
                    }
                    else
                    {
                        Console.WriteLine($"Error: {response.StatusCode} - {response.ReasonPhrase}");
                    }
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Exception: {ex.Message}");
                }
            }
        }
    }
}
Make sure to replace the apiUrl variable with the actual URL of your REST API endpoint and the imagePath variable with the correct path to your image file. 
The example assumes that the API expects a file named "image" in a multipart form-data request. 

Additionally, this example uses the MultipartFormDataContent class from System.Net.Http to create a multipart/form-data request and the HttpClient class to send the request to the API. 
Make sure to handle exceptions appropriately based on your application's requirements.

How to upload a user's profile picture to a REST API endpoint using HttpClient in ASP.NET Core MVC.


Let's take an real time example,to upload a user profile picture to a REST API endpoint using HttpClient in ASP.NET Core MVC, you can follow these steps: 
1.Create a Model: Define a model that represents the structure of your API request. In this case, you might create a model with a property for the profile picture.
// UserProfilePictureModel.cs
public class UserProfilePictureModel
{
    public IFormFile ProfilePicture { get; set; }
}
Update Your Controller: In your MVC controller, create an action to handle the file upload.
public class UserController : Controller
{
    private readonly HttpClient _httpClient;

    public UserController(IHttpClientFactory httpClientFactory)
    {
        _httpClient = httpClientFactory.CreateClient();
        // Configure your HttpClient if necessary (base URL, headers, etc.)
        _httpClient.BaseAddress = new Uri("https://your-api-endpoint.com");
    }

    [HttpGet]
    public IActionResult UploadProfilePicture()
    {
        return View();
    }

    [HttpPost]
    public async Task<IActionResult> UploadProfilePicture(UserProfilePictureModel model)
    {
        try
        {
            if (model.ProfilePicture != null && model.ProfilePicture.Length > 0)
            {
                // Create a MultipartFormDataContent to hold the image data
                using (var content = new MultipartFormDataContent())
                {
                    // Add the image content to the form data
                    content.Add(new StreamContent(model.ProfilePicture.OpenReadStream()), "ProfilePicture", model.ProfilePicture.FileName);

                    // Send the request to the API endpoint
                    var response = await _httpClient.PostAsync("/api/user/uploadprofilepicture", content);

                    // Check if the request was successful
                    if (response.IsSuccessStatusCode)
                    {
                        // Handle success
                        return RedirectToAction("Success");
                    }
                    else
                    {
                        // Handle API error
                        ViewData["Error"] = $"Error: {response.StatusCode} - {response.ReasonPhrase}";
                    }
                }
            }
            else
            {
                // Handle if no file is selected
                ViewData["Error"] = "No profile picture selected.";
            }
        }
        catch (Exception ex)
        {
            // Handle other exceptions
            ViewData["Error"] = $"Exception: {ex.Message}";
        }

        return View(model);
    }

    public IActionResult Success()
    {
        return View();
    }
}
2.Create a View: Create a view to display the form for uploading the profile picture.
<!-- UploadProfilePicture.cshtml -->
@model UserProfilePictureModel

<form asp-action="UploadProfilePicture" asp-controller="User" method="post" enctype="multipart/form-data">
    <div class="form-group">
        <label asp-for="ProfilePicture"></label>
        <input type="file" asp-for="ProfilePicture" class="form-control" />
        <span asp-validation-for="ProfilePicture" class="text-danger"></span>
    </div>
    <button type="submit" class="btn btn-primary">Upload Profile Picture</button>
</form>
3.Configure HttpClient in Startup.cs: Make sure to configure HttpClient in the Startup.cs file.
public void ConfigureServices(IServiceCollection services)
{
    // Other service configurations

    services.AddHttpClient();
}
Handle Validation and Display Errors: Add validation attributes to your model properties and display error messages in your view as needed. 
This is a basic example, and you might need to handle more specific requirements based on your project. Ensure proper security practices such as validating user input and handling exceptions gracefully.