I have 2 model classes:

public class MoReg
{
    #region properties
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int RegID { get; set; }

    [Required]
    public long RegRef { get; set; }

    [Required]
    public DateTime? RegDate { get; set; }

    [Required]
    public int? RegType { get; set; }

    [Required]
    public bool RegDeported { get; set; }

    [Required]
    public string? RegNotes { get; set; }

    //relations
    public MoAtt xType { get; set; } = null!;

    public List<MoRegDet> xRegDetList { get; set; }
    #endregion
}

public class MoRegDet
{
    #region properties
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int RegDetID { get; set; }

    [Required]
    public long RegDetRef { get; set; }

    [Required]
    public int RegIDx { get; set; } // المفتاح الأجنبي

    [Required]
    public DateTime? RegDetDate { get; set; }

    [Required]
    public double? RegDetMadeen { get; set; }

    [Required]
    public double? RegDetDaen { get; set; }

    [Required]
    public int? RegDetAcc { get; set; }

    [Required]
    public int? RegDetCurr { get; set; }

    [Required]
    public double? RegDetRate { get; set; }

    [Required]
    public double? RegDetMadeen2 { get; set; }

    [Required]
    public double? RegDetDaen2 { get; set; }

    [Required]
    public string? RegDetNotes { get; set; }

    // relations
    public MoAcc xAcc { get; set; } = null!;
    public MoCurrency xCurr { get; set; } = null!;

    // navigation properties
    [ForeignKey(nameof(RegIDx))]
    public MoReg xReg { get; set; } = null!;
    #endregion
}

I have a DbContext file called AppDb with the following code:

//Regs
modelBuilder.Entity<MoReg>(entity =>
 {
     entity.ToTable("regs");
     entity.HasKey(e => e.RegID);
     entity.Property(e => e.RegID);
     entity.Property(e => e.RegRef).IsRequired();
     entity.Property(e => e.RegDate).IsRequired();
     entity.Property(e => e.RegType).IsRequired();
     entity.Property(e => e.RegDeported).IsRequired();
     entity.Property(e => e.RegNotes).IsRequired(false);

     entity.HasOne(e => e.xType)
          .WithMany()
          .HasForeignKey(e => e.RegType)
          .OnDelete(DeleteBehavior.Restrict);
 });

// Regs det
modelBuilder.Entity<MoRegDet>(entity =>
 {
     entity.ToTable("regs_det");
     entity.HasKey(e => e.RegDetID);
     entity.Property(e => e.RegDetID);
     entity.Property(e => e.RegDetRef).IsRequired();
     entity.Property(e => e.RegIDx).IsRequired();
     entity.Property(e => e.RegDetDate).IsRequired();
     entity.Property(e => e.RegDetMadeen).IsRequired();
     entity.Property(e => e.RegDetDaen).IsRequired();
     entity.Property(e => e.RegDetAcc).IsRequired();
     entity.Property(e => e.RegDetCurr).IsRequired();
     entity.Property(e => e.RegDetRate).IsRequired();
     entity.Property(e => e.RegDetMadeen2).IsRequired();
     entity.Property(e => e.RegDetDaen2).IsRequired();
     entity.Property(e => e.RegDetNotes).IsRequired(false);

     entity.HasOne(e => e.xReg)
          .WithMany(reg => reg.xRegDetList)
          .HasForeignKey(e => e.RegIDx)
          .OnDelete(DeleteBehavior.Cascade);

     entity.HasOne(e => e.xAcc)
          .WithMany()
          .HasForeignKey(e => e.RegDetAcc)
          .OnDelete(DeleteBehavior.Restrict);

     entity.HasOne(e => e.xCurr)
          .WithMany()
          .HasForeignKey(e => e.RegDetCurr)
          .OnDelete(DeleteBehavior.Restrict);
});

I created a manager file RegsManager that has the following code:

// Reg
public List<MoReg> xRegsList;
public MoReg xReg { get; set; }

// Reg Det
// public List<MoRegDet> xRegsDetList;
public MoRegDet xRegDet { get; set; }

// Others
public AttsManager _Mngr_Atts;
public AccManager _Mngr_Accs;
public CurrenciesManager _Mngr_Currs;
public RatesManager _Mngr_Rates;

#region insert
public void Insert(AppDb xDb, MoReg xNew)
{
     using (var transaction = xDb.Database.BeginTransaction())
     {
         try
         {
             xDb.Db_MoRegs.Add(xNew);
             xDb.SaveChanges();

             foreach (var detail in xNew.xRegDetList)
             {
                 detail.RegIDx = xNew.RegID;
             }

             xDb.Db_MoRegsDet.AddRange(xNew.xRegDetList);
             xDb.SaveChanges();

             transaction.Commit();
         }
         catch (Exception ex)
         {
             MessageBox.Show(ex.Message);
             transaction.Rollback();
         }
     }
}
#endregion

When I run the following code when I click on the “Save” button:

// set data
_Manager.xReg = new MoReg()
 {
     RegRef = _Manager.GetRef(),
     RegDate = TxtRegDate.Value,
     RegType = 1,
     RegDeported = RdYes.Checked ? true : false,
     RegNotes = TxtRegNotes.Text.Trim(),
     xRegDetList = (List<MoRegDet>)Dgv.DataSource

 };

AppDb xDb = new AppDb();

if (xDb.IsConnected())
{
     _Manager.Insert(xDb, _Manager.xReg);
     _Manager.xOpResult = AppEnums.Errs.NoErr;
}

The problem is when I clear the line:

xRegDetList = (List<MoRegDet>)Dgv.DataSource

and just run:

xDb.Db_MoRegs.Add(xNew);
xDb.SaveChanges();

I can save just the header but if I keep it (xRegDetList = (List<MoRegDet>)Dgv.DataSource) I get an error

Could not save changes. Please configure your entity type accordingly

I tried for more than 5 hours I didn’t get any method to make it work correctly.

I hope someone can help me.

I tried to insert just the header and it worked and I tried by AI and nothing worked.