实体框架-空引用异常

Question

当我尝试从具有EF db上下文的数据库中获取实体时，我遇到了NullReferenceException：

这是我的测试代码：

// it works fine
using (var sql = new SqlConnection("--here is my connection string--"))
{
    sql.Open();
    var cmd = sql.CreateCommand();
    cmd.CommandText = "select field1, field2 from my_table";
    var reader = cmd.ExecuteReader();

    reader.Read();

    // here is true value
    var val = reader.GetValue(0);
}

// failed
var ctx = new BackupDbContext();
var query = ctx.Database.SqlQuery<MyEntity>("select field1, field2 from my_table");
var res = query.ToList(); // NullReferenceException

// failed
var tt = ctx.MyEntities.ToList(); // NullReferenceException

我的DbContext和映射：

public class MyEntityMapping : EntityTypeConfiguration<MyEntity>
{
    public MyEntityMapping()
    {
        ToTable("my_table");

        HasKey(p => p.Field1);

        Property(p => p.Field1).HasColumnName("field1");
        Property(p => p.Field2).HasColumnName("field2");
    }
}    

public class BackupDbContext: DbContext
{
    static BackupDbContext()
    {
        Database.SetInitializer<BackupDbContext>(null);
    }

    public BackupDbContext():base("name=BackupDbContext")
    {
        Database.Log = sql => Debug.Write(sql);
    }
    public virtual DbSet<MyEntity> MyEntities { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema("dbo");

        modelBuilder.Configurations
                .Add(new MyEntityMapping());
    }
}

我不知道例外的原因是什么。

我有以下堆栈跟踪：

有谁知道为什么会这样，还是可以给我一个提示？

编辑：

连接：...

<connectionStrings>
<add name="BackupDbContext" connectionString="Data Source=My_datasource;Encrypt=False;Initial Catalog=my_db;Integrated Security=True;" providerName="System.Data.SqlClient" />      
</connectionStrings>

...

Answer 1

您应该在webconfig文件中定义一个ConnectionString。

 <connectionStrings>
<add name="DefaultConnection" connectionString="Data Source=Your server nane;Initial Catalog=Your database name;Integrated Security=True"
  providerName="System.Data.SqlClient" />
</connectionStrings>

然后，您必须在DbContext中使用它

  public BackupDbContext():base("DefaultConnection")
  {
    Database.Log = sql => Debug.Write(sql);
  }

现在，此语句应该起作用。

  var ctx = new BackupDbContext();
  var tt = ctx.MyEntities.ToList();