c# - MVC Controller if/else 语句

Question

我有一个与数据一起提交的 MVC 表单。提交后，它会进入预览 View 表单，用户可以在其中决定是否继续发布或再次编辑表单。

除了提交数据之外，我还提供上传图片的选项。我在这个预览表单中基本上有 3 个场景:

1. 更新表单并保留现有图片
2. 更新表单并替换(或删除)现有图片
3. 更新表单并添加图片(如果之前没有)

现在，场景 2) 和 3) 工作正常，但我遇到的是场景 1)。由于某种原因，任何现有图片(来自初始提交)都将被覆盖/删除。当我尝试调试时，我注意到 if 语句的 else 子句被简单地跳过。

我怀疑这与我的 View 或模型有关，因此我发布了我的 Controller 的相关代码。

    public ActionResult UpdateErrand(
        [Bind(Exclude                       = "Picture")]errands model)
    {
        // define variables for reuse
        var userID                          = User.Identity.GetUserId();
        DateTime nowUTC                     = DateTime.Now.ToUniversalTime();
        DateTime nowLocal                   = DateTime.Now.ToLocalTime();
        // get picture and get it to bytes
        string picture                      = Request.Form["editErrandCroppedPicture"];
        byte[] imageBytes                   = Convert.FromBase64String(picture);
        // define errand
        var errand                          = new errands
        {
            // basics
            UserID                          = userID,
            FirstName                       = UserManager.FindById(userID).FirstName,
            Email                           = UserManager.FindById(userID).Email,
            Phone                           = UserManager.FindById(userID).PhoneNumber,
            //Rating                        = 

            // form
            ID                              = model.ID,
            Category                        = model.Category,
            SubCategory                     = model.SubCategory,
            Title                           = model.Title,
            Description                     = model.Description,
            Location                        = model.Location,
            ASAP                            = model.ASAP,
            StartDateTime                   = model.StartDateTime,
            DurationInHours                 = model.DurationInHours,
            EndDateTime                     = model.EndDateTime,
            DateTimePosted                  = nowLocal,
            Currency                        = model.Currency,
            Offering                        = model.Offering,
            Price                           = model.Price,
            errandTax                       = model.errandTax,
            PaymentMethod                   = model.PaymentMethod,
            LatitudePosted                  = model.LatitudePosted,
            LongitudePosted                 = model.LongitudePosted,
            LocationPosted                  = model.LocationPosted,
            Published                       = true
        };
        // workaround to ensure picture is uploaded correctly
        if (imageBytes.Length               > 2)
        {
            errand.Picture                  = imageBytes;
        }
        else
        {
            errand.Picture                  = model.Picture;
        }
        // save errand to DB
        ERRANDOM.Entry(errand).State = EntityState.Modified;
        ERRANDOM.SaveChanges();
        // track user activity: post includes activity name, timestamp along with location, and if authenticated, user ID
        var SUCCESS                         = new UserActivities
        {
            UserID                          = userID,
            ActivityName                    = "EditErrand_Success",
            ActivityTimeStampUTC            = nowUTC,
            ActivityLatitude                = model.LatitudePosted,
            ActivityLongitude               = model.LongitudePosted,
            ActivityLocation                = model.LocationPosted
        };
        ERRANDOM.UserActivityList.Add(SUCCESS);
        ERRANDOM.SaveChanges();
        return RedirectToAction("errandPreview");
    }

Answer 1

如果将实体标记为已修改 (.State = EntityState.Modified)，则其所有映射标量属性(即非导航属性)都将成为 SQL 更新语句的一部分。但是，可以从更新中排除选定的属性。操作方法如下:

...
LatitudePosted = model.LatitudePosted,
LongitudePosted = model.LongitudePosted,
LocationPosted = model.LocationPosted,
Published = true
errand.Picture = imageBytes;

ERRANDOM.Entry(errand).State = EntityState.Modified;

if (imageBytes.Length <= 2)
{
    ERRANDOM.Entry(errand).Property(e => e.Picture).IsModified = false;
}

ERRANDOM.SaveChanges();

如您所见，您现在可以无条件分配 errand.Picture = imageBytes:稍后当不需要更新时它将被排除。在(大概)没有更新图片的最常见情况下，这还可以节省更新任务的带宽。