将对象添加到 DBSet 时出现 InvalidCastException

Question

I have two classes

[Table("Kunden")]
public class Kunde
{

    public Guid Id { get; set; }
 

    [Required, ForeignKey("Id")]
    public virtual List<Standort> OrtStandort { get; set; }

    
    public Kunde()
    {
        Id = Guid.NewGuid();
        OrtStandort = new List<Standort>();
    }
}

[Table("Standorte")]
public class Standort
{
    public Guid Id { get; set; }

    public Standort()
    {
        Id = Guid.NewGuid();
        //KontaktAnsprechpartner = new List<Ansprechpartner>();
    }
}

public class HausarbeitContext : DbContext
{
    public DbSet<Kunde> Kunden { get; set; }
    
    public DbSet<Standort> Standort { get; set; }
    
    // ...
}
         
         
         

As soon, as the debugger hits:

 haDB/*HausarbeitContext*/.Kunden/*DbSet<Kunde>*/.Add(kunde);

I get:

System.InvalidCastException: 'Das Objekt des Typs "System.Collections.Generic.List`1[ClassDesigner.Part_Hausarbeit.Data.Standort]" kann nicht in Typ "ClassDesigner.Part_Hausarbeit.Data.Standort" umgewandelt werden.' (can not be converted)

See the Objekt: enter image description here

Answer 1

You might need to include the full definitions for the 2 entities. At a glance, one thing that stands out is this: // Knude

[Required, ForeignKey("Id")]
public virtual List<Standort> OrtStandort { get; set; }

The ForeignKey attribute is typically applied on the "One" side of a Many-to-One. Additionally, while I don't believe it is a requirement, collection references are typically declared as ICollection<T> rather than List<T> . EF will be looking to establish proxies for virtual members so it's advisable to leave these references as a lower level interface type.

Given a Kunde & Standort parent-children relationship, the FK references would more normally be:

public class Kunde
{
     [Key]
     public Guid Id { get; set; }

     public virtual ICollection<Standort> OrtStandort { get; set; } = new List<Standort>();
}

public class Standort
{
    [Key]
    public Guid Id { get; set; }
    
    public Guid KundeId { get; set; }
    [ForeignKey("KundeId")]
    public virtual Kunde Kunde { get; set; }
}

The FK for the Kunde-Standort relationship is managed in the Many side for the singular ID column pointing back to the parent.

Or, to exclude the KundeId property inside of Standort entity, leveraging a shadow property (EF Core) or mapping it without defining a field using Map(x => x.MapKey("KundeId")) in EF 6.

On a side note, when using Guids for Key columns, I would strongly recommend leveraging a Database generated approach (Marking the columns as Identity ) rather than set in code, and then setting these up to use sequential GUID generation. For example, with SQL Server, using NEWSEQUENTIALID() instead of NEWID() :

[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public Guid Id { get; set; }

If you do need to set them in code then I would look at options to generate a GUID in an index-friendly manner for your selected database and implement a Guid generator that can arrange the various elements that make up a GUID so that the database indexing can sort through them efficiently. The default randomized .Net GUID will lead to considerable Index table fragmentation. In systems that could see more than 10's of thousands of rows this can lead to considerable memory/disk usage issues and performance issues.