EFCore 3 - 保存项目抛出 sql 异常“无法更新标识列”

Question

i'm working on Entity Framework Core 3.0 in a .net Core 3.0 but when i trying to update this item (only this class, in a db first model) it gives me this error

Sql Exception. cannot update identity column 'IDNRR'

i just get an item from X table and then save it with context.Update command

Unit test with exception

var c = _dbContext.TblWoTaskExtServices.SingleOrDefault(x => x.Idnrr == 15273778);
_dbContext.Update(c);
_dbContext.SaveChanges(); -> execute this give me the sql exception

class definition

modelBuilder.Entity<TblWoTaskExtServices>(entity =>
{
    entity.HasKey(e => new { e.ExternalServiceCode, e.IdWotask });

    entity.ToTable("tbl_WO_TASK_EXT_SERVICES");

    entity.HasIndex(e => e.Idnrr)
        .HasName("IX_tbl_WO_TASK_EXT_SERVICES")
        .IsUnique();

    entity.Property(e => e.ExternalServiceCode)
        .HasMaxLength(255)
        .IsUnicode(false);

    entity.Property(e => e.IdWotask).HasColumnName("IdWOTask");

    entity.Property(e => e.DateIns).HasColumnType("smalldatetime");

    entity.Property(e => e.DateMod).HasColumnType("smalldatetime");

    entity.Property(e => e.ExternalServiceDescription).HasColumnType("text");

    entity.Property(e => e.Idnrr)
        .HasColumnName("IDNRR")
        .ValueGeneratedOnAdd();

    entity.Property(e => e.Notes).HasColumnType("text");

    entity.Property(e => e.TotalCost).HasColumnType("decimal(18, 2)");

    entity.Property(e => e.TotalCostFinal).HasColumnType("decimal(18, 2)");

    entity.HasOne(d => d.IdCompanyNavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCompany)
        .OnDelete(DeleteBehavior.ClientSetNull)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_COMPANIES");

    entity.HasOne(d => d.IdCostClassNavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCostClass)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_COST_CLASSES");

    entity.HasOne(d => d.IdCurrencyNavigation)
        .WithMany(p => p.TblWoTaskExtServices)
        .HasPrincipalKey(p => p.Idnrr)
        .HasForeignKey(d => d.IdCurrency)
        .HasConstraintName("FK_tbl_WO_TASK_EXT_SERVICES_tbl_Currencies");
    });
}

Answer 1

Solved without use of Update() method

as @metacube suggest

Also, you have not changed the returned entity but used DbContext.Update on it, which changed entity state to "Modified". I suggest simply changing the needed properties of the tracked entity and then call SaveChanges