如何处理与 HttpClient 的并发冲突？

Question

I can't figure out how to edit the row after seeing the changes in DB.

• I have an API-project and an MVC-project. I use CRUD in my API and call them with my MVC with HttpClient
• I have a public byte[] RowVersion { get; set; } public byte[] RowVersion { get; set; } public byte[] RowVersion { get; set; } property with the attribute [Timestamp] .
• I have a clientFactory where I do CreateClient() to perform PutAsync("api.example.com/{id}") action.
• The HttpResponseMessage variable on my putasync action returns StatusCode(409) because my API successfully detected a concurrency conflict.
• I managed to display error messages before updating the concurrency; showing the newly updated rows in the database(newsDb) with help of a new client , clientFactory.CreateClient() , and comparing them with the inputs(news).
• Then I set the news.RowVersion = newsDb.RowVersion and re-display View(news).

And after clicking Save again, nothing happens - no redirects, no changes - the concurrency errors are still there:

[HttpPost("edit/{id}")]
[ValidateAntiForgeryToken]
public async Task<IActionResult> EditNewsArticle(int id, [Bind("NewsId,Author,Title,Content,CreatedDate,HashTags,RowVersion")] News news)
{
    if (id != news.NewsId)
    {
        return NotFound();
    }

    if (ModelState.IsValid)
    {
        news.UpdatedDate = DateTime.Now;
        string json = JsonConvert.SerializeObject(news);
        HttpResponseMessage putTask = await clientFactory.CreateClient().PutAsync($"https://localhost:44331/api/News/{id}", new StringContent(json, Encoding.UTF8, "application/json"));

        if (putTask.IsSuccessStatusCode)
        {
            return RedirectToAction(nameof(Index));
        }
        else if (putTask.StatusCode == HttpStatusCode.Conflict)
        {
            string jsonDb = await clientFactory.CreateClient().GetStringAsync($"https://localhost:44331/api/News/{id}");
            News newsDb = JsonConvert.DeserializeObject<News>(jsonDb);

            if (newsDb is null)
            {
                ModelState.AddModelError(string.Empty, $"Unfortunately, the news item you edited has already been deleted by another user.");
            }

            if (newsDb.Title != news.Title)
            {
                ModelState.AddModelError("Title", $"Title in database: {newsDb.Title}");
            }
            if (newsDb.Author != news.Author)
            {
                ModelState.AddModelError("Author", $"Author in database: {newsDb.Author}");
            }
            if (newsDb.Content != news.Content)
            {
                ModelState.AddModelError("Content", $"Content in database: {newsDb.Content}");
            }
            if (newsDb.HashTags != news.HashTags)
            {
                ModelState.AddModelError("HashTags", $"HashTags in database: {newsDb.HashTags}");
            }

            ModelState.AddModelError(string.Empty,
            "Editing was canceled as the selected news item was changed by someone else in the meantime." +
            "The values ​​of the change are now shown below, which are derived from the database" + 
            "If you still want to edit the user, click Save again.");

            news.RowVersion = newsDb.RowVersion;
        }
        else
        {
            ModelState.AddModelError(string.Empty, "Unknown error. Contact a support.");
            return View(news);
        }
    }

    return View(news);
}

API Put:

[HttpPut("{id}")]
public async Task<IActionResult> PutNews(int id, [FromBody] News news)
{
    if (id != news.NewsId)
    {
        return BadRequest();
    }

    context.Entry(news).State = EntityState.Modified;

    try
    {
        await context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException)
    {
        if (!NewsExists(id))
        {
            return NotFound();
        }
        else
        {
            return StatusCode(409);
        }
    }

    return CreatedAtAction("GetNews", new { id = news.NewsId }, news);
}

Answer 1

I found my issue. I needed to call ModelState.Clear(); after de-serializing the 'jsonDb', and also remove RowVersion from Bind in the attribute.