.Net Core MVC：尝试上传图像时控制器中出现 NullReferenceException答案

【问题标题】：.Net Core MVC: NullReferenceException in Controller when attempting to upload image.Net Core MVC：尝试上传图像时控制器中出现 NullReferenceException
【发布时间】：2021-01-27 13:17:05
【问题描述】：

我正在尝试添加一项功能，允许用户上传他们宠物的照片。我是 MVC 新手，正在使用本教程作为指导：https://www.codaffection.com/asp-net-core-article/asp-net-core-mvc-image-upload-and-retrieve/

查看：

@model PetPhotoModel;

@{
    ViewData["Title"] = "Upload A photo of your pet";
}

<div class="row">
    <div class="col-md-4">
        <form asp-action="UploadPhoto">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="PetName" class="control-label"></label>
                <input asp-for="PetName" class="form-control" />
                <span asp-validation-for="PetName" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Title" class="control-label"></label>
                <input asp-for="Title" class="form-control" />
                <span asp-validation-for="Title" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Description" class="control-label"></label>
                <input asp-for="Description" class="form-control" />
                <span asp-validation-for="Description" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="ImageFile" class="control-label"></label>
                <input asp-for="ImageFile" accept="image/*" />
                <span asp-validation-for="ImageFile" class="text-danger"></span>
            </div>

            <div class="form-group">
                <input type="submit" value="UploadPhoto" class="btn btn-primary" />
            </div>
        </form>
    </div>
</div>

<div>
    <a asp-action="Index">Back to List</a>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

控制器（部分）：

...
  public IActionResult UploadPhoto()
        {
            return View();
        }

        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> UploadPhoto([Bind("Id,PetName,Title,Description,ImageFile")] PetPhotoModel petPhotoModel)
        {
            if (ModelState.IsValid)
            {
                //Save image to wwwroot/image
                string wwwRootPath = _hostEnvironment.WebRootPath;
                string fileName = Path.GetFileNameWithoutExtension(petPhotoModel.ImageFile.FileName);
                string extension = Path.GetExtension(petPhotoModel.ImageFile.FileName);
                petPhotoModel.ImageName = fileName = fileName + DateTime.Now.ToString("yymmssfff") + extension;
                string path = Path.Combine(wwwRootPath + "/Image/", fileName);
                using (var fileStream = new FileStream(path, FileMode.Create))
                {
                    await petPhotoModel.ImageFile.CopyToAsync(fileStream);
                }
                //GetLoggedInUser() gets the current user by id from the db context
                GetLoggedInUser().Uploads.Add(petPhotoModel);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(petPhotoModel);
        }
...

UserModel 包含PetPhotoModel 的集合，以便在数据库中创建一对多关系：

public virtual ICollection<PetPhotoModel> Uploads { get; set; }

宠物照片模型：

 public class PetPhotoModel
    {
        private DateTime _dateUploaded = DateTime.Now;

        public int Id { get; set; }

        [Required]
        public string PetName { get; set; }

        [Required]
        public string Title { get; set; }

        public string Description { get; set; }

        [Column(TypeName = "nvarchar(100)")]
        [DisplayName("Image Name")]
        public string ImageName { get; set; }

        [NotMapped]
        [DisplayName("Upload File")]
        public IFormFile ImageFile { get; set; }

        public DateTime DateUploaded
        {
            get { return _dateUploaded; }
            set { _dateUploaded = value; }
        }
...

问题是在按下提交按钮后，尽管图像已上传到视图中，但控制器根本没有从视图中获取任何有关图像的数据：

[

【问题讨论】：

我在您的表单中没有看到任何表明它是 multipart 的内容，这在传递文件时是必需的。你也可以看看这个相关的帖子：(stackoverflow.com/questions/46437044/…)。您的表单中也没有与模型属性 ImageName 相关的任何内容

标签： c# asp.net-mvc asp.net-core .net-core asp.net-core-mvc

【解决方案1】：

当表单包含任何类型的文件元素时，您必须使用“multipart/form-data”编码。由于默认编码为“application/x-www-form-urlencoded”，请将您视图中的表单标签更改为：

 <form asp-action="UploadPhoto" enctype="multipart/form-data" method="post">

【讨论】：