Entity Framework Core 多对多关系错误：对象引用未设置为对象的实例

Question

我是 C#/EF Core 的新手，不明白为什么我会收到空​​引用异常：

System.NullReferenceException
HResult=0x80004003
Message=对象引用未设置为对象的实例。
来源=WMS

堆栈跟踪：
在 WMS.Pages.Artikels.Artikel_Verkauf_erfassen.ArtikelVerkaufErfassen.d__33.MoveNext() 在 D:\Project Arbeit\Code\WMS\Pages\Artikels\Artikel Verkauf erfassen\ArtikelVerkaufErfassen.razor.cs:line 44

我有一个多对多的关系并试图获取分支的文章：

void Add()
{
    var availableQuantity = BranchItems.Find(bi => bi.ArticleId == articleSale.ArticleId)
    .BrancheArticles.Find(ba => ba.ArticleId == articleSale.ArticleId).Quantity;
}

类结构如下：

public class ArticleModel
{
    [Key]
    public int ArticleId { get; set; }

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

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

    [Required]
    [MaxLength(40)]
    [MinLength(10)]
    public string ArticleDescription { get; set; }

    [Range(50, 10000)]
    [DataType(DataType.Currency)]
    [Column(TypeName = "decimal(18, 2)")]
    public decimal Preis { get; set; }

    [Range(1,100, ErrorMessage = "sollte von 1 bis 100 Stück sein")]
    public int Menge { get; set; }

    public List<BranchArticle> BrancheArticles { get; set; } = new List<BranchArticle>();
}

public class BranchArticle
{
    public int BranchId { get; set; }
   
    public int ArticleId{ get; set; }

    public BranchModel Branch { get; set; } = new BranchModel();

    public ArticleModel Article { get; set; } = new ArticleModel();

    public int Quantity { get; set; }
}

public class BranchModel
{
    [Key]
    public int Id { get; set; }
    public Guid BranchId { get; set; }
    public string BranchCode { get; set; }
    public string BranchName { get; set; }
    public string Street { get; set; }
    public string HouseNr { get; set; }
    public string ZipCode { get; set; }
    public string Location { get; set; }
    public string ContactPerson { get; set; }
    public string TelefoneNr { get; set; }
    public string Email { get; set; }

    public List<BranchArticle> BrancheArticles { get; set; } = new List<BranchArticle>();  
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<BranchArticle>()
        .HasKey(ab => new { ab.BranchId, ab.ArticleId });

    modelBuilder.Entity<BranchArticle>()
        .HasOne<ArticleModel>(ab => ab.Article)
        .WithMany(am => am.BrancheArticles)
        .HasForeignKey(ab => ab.ArticleId);

    modelBuilder.Entity<BranchArticle>()
        .HasOne<BranchModel>(ab => ab.Branch)
        .WithMany(bm => bm.BrancheArticles)
        .HasForeignKey(ab => ab.BranchId);

    modelBuilder.Entity<BranchArticle>(entity => { entity.ToTable(name: "BranchArticles"); });

    base.OnModelCreating(modelBuilder);
}

Answer 1

可能其中一个 Find 函数失败了（返回 null）。

我建议你像这样分解你的代码。

var branchItem = BranchItems.Find(bi => bi.ArticleId == articleSale.ArticleId);
if(branchItem == null) {
    // throw an error or print an error message, etc...
} else {
    var article = branchItem.BrancheArticles.Find(ba => ba.ArticleId == articleSale.ArticleId);
    if (article == null) {
        // throw an error or print an error message, etc...
    } else { 
        var quantity = article.Quantity;
    }
}

您可能还考虑使用 Where() + SingleOrDefault() 而不是 Find()。