Question

当我尝试使用Mvc的实体框架时。我想在创建数据库时插入一些数据，但是Fword类具有树属性。当我尝试将其添加到List或List时，在Seed方法中得到了 NullPointerException 。谁能告诉我我哪里出问题了？

这是DataAccess类

public class MyContext : DbContext
{ 
    public MyContext () : base("MyContext")
    {
        Database.SetInitializer(new VeritabaniOlusturucu());
    }


    public DbSet<FWord> words { get; set; }
    public DbSet<WordType> WT { get; set; }
    public DbSet<WordFrequency> WF { get; set; }

  }

在下面的类种子方法中，我得到了错误。

public class VeritabaniOlusturucu : CreateDatabaseIfNotExists<EWLContext>
{
    protected override void Seed(MyContext  context)
    {
        WType wt = new WType();
        wt.Type = "determiner";
        FWord fw = new FWord();
        fw.Word = "try";
        fw.WT.Add(wt);//I got here error
        fw.WF.Add(new WordFrequency { Frequency = "B1" });//and here
        fw.WF.Add(new WordFrequency { Frequency = "B2" });//and here
        context.FWords.Add(fw);
        context.SaveChanges();
    }

}

这是实体类

[Serializable]
public class FWord
{
    public FWord()
    {

    }
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
    public String Word { get; set; }
    public List<WordType> WT { get; set; }
    public List<WordFrequency> WF { get; set; }
}
[Serializable]
public class WordType
{
    public WordType()
    {

    }
    [Key]
    public int Id { get; set; }
    public String Type { get; set; }

    public virtual FWord Freq { get; set; }
}
[Serializable]
public class WordFrequency
{
    public WordFrequency()
    {

    }
    [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }
    public String Frequency { get; set; }
    public virtual FWord Freq { get; set; }
 }

谢谢。

Answer 1

您的列表为空。您必须实例化它，然后才能向其中添加项目。

protected override void Seed(MyContext  context)
    {
        WType wt = new WType();
        wt.Type = "determiner";
        FWord fw = new FWord();
        fw.Word = "try";
        fw.WT = new List<WordType>();
        fw.WT.Add(wt);
        fw.WF.Add(new WordFrequency { Frequency = "B1" });//and here
        fw.WF.Add(new WordFrequency { Frequency = "B2" });//and here
        context.FWords.Add(fw);
        context.SaveChanges();
    }

Answer 2

您的列表是未初始化的属性，除非您进行设置，否则它为null。如果即使未设置也要避免这种情况，请为其设置默认值：

public List<WordType> WT { get; set; } = new List<WordType>()
public List<WordFrequency> WF { get; set; } = new List<WordFrequency>();

List.Add处的C＃Mvc实体框架Nullpointer异常

2 个答案: