ASP.NET Core添加Listproduct路由后Create操作报NullReferenceException

原本直接访问https://localhost:7171/Category/Add时，Create操作可正常运行，但添加Listproduct Action方法后，所有Create操作均抛出NullReferenceException异常，无法访问。Listproduct Action可正常访问，删除该Action后Create操作恢复正常。

Listproduct Action代码 #

[HttpGet]
[Route("{catname}/{name}")]
public IActionResult Listproduct(string name, string catname)
{
    List<Product> products;
    
    Subcategory subcat = db.Subcategories
                           .Where(x => x.Slug == name && x.Catname == catname)
                           .FirstOrDefault();
    int subid = subcat.SubcatId;

    products = db.Products.ToArray()
                 .Where(x => x.SubcatId == subid).ToList();

    return View(products);
}

Create Action代码 #

[HttpGet]
public IActionResult Create()
{
    return View();
}
[HttpPost]
public IActionResult Create(CategoryVM model)
{
    if(! ModelState.IsValid)
    {
        return View(model);
    }

    if (db.Categories.Any(x => x.Catname == model.Catname))
    {
        ModelState.AddModelError("", $"{model.Catname} already exists");
        return View(model);
    }


    if (ModelState.IsValid)
    {
        string uniqueFileName = UploadedFile(model);

        var dto = new Category
        {

            Catname = model.Catname,
            Slug = model.Catname.ToLower(),
            Description = model.Description,
            CatImage = uniqueFileName

        };
        db.Categories.Add(dto);

        db.SaveChanges();
    }

    
    TempData["SM"] = $"{model.Catname} category added to list";
    
    return RedirectToAction("Create");
}

错误截图 #

核心原因 #

路由冲突导致访问Create操作时，请求被错误路由到Listproduct方法。ASP.NET Core中属性路由优先级高于约定路由，Listproduct的[Route("{catname}/{name}")]模板会匹配/Category/xxx这类路径（当访问/Category/Create时，catname被赋值为"Create"，name为null），此时Listproduct方法中查询不到对应的Subcategory，subcat为null，执行subcat.SubcatId时抛出NullReferenceException。

解决步骤 #

1. 修正Listproduct的路由模板，添加明确前缀避免冲突：

   [HttpGet]
   [Route("products/{catname}/{name}")] // 修改为带前缀的路由
   public IActionResult Listproduct(string name, string catname)
   {
       // 原有代码...
   }
   

   这样Listproduct的访问路径变为/Category/products/{catname}/{name}，不会和Create的路由混淆。

2. 为Create Action添加明确属性路由，确保请求正确匹配：

   [HttpGet("Create")] // 若实际用的是"Add"则改为[HttpGet("Add")]
   public IActionResult Create()
   {
       return View();
   }
   [HttpPost("Create")]
   public IActionResult Create(CategoryVM model)
   {
       // 原有代码...
   }
   

3. 增强Listproduct的空值处理，即使路由误匹配也不会崩溃：

   Subcategory subcat = db.Subcategories
                          .Where(x => x.Slug == name && x.Catname == catname)
                          .FirstOrDefault();
   if (subcat == null)
   {
       return NotFound(); // 返回404或自定义错误视图
   }
   int subid = subcat.SubcatId;
   

内容的提问来源于stack exchange，提问作者Rezo