查找對象圖中具有相同鍵的實體，以防止「ObjectStateManager中已存在具有相同鍵的對象」錯誤

Question

我首先使用EF代碼來開發我的3層WinForm應用程序，我使用斷開的POCO s作爲我的模型實體。我的所有實體都從BaseEntity類繼承而來。

我用斷開POCO S，所以處理實體的State在客戶端側，並且在ApplyChanges()方法，附上我的實體圖形（例如，與它的Order是OrderLines和Products）到我DbContext然後SYNCH每個實體的State與它的客戶端State。

public class BaseEntity 
{ 

    int _dataBaseId = -1; 

    public virtual int DataBaseId // DataBaseId override in each entity to return it's key 
    { 
     get { return _dataBaseId; } 
    } 

    public States State { get; set; } 

    public enum States 
    { 
     Unchanged, 
     Added, 
     Modified, 
     Deleted 
    } 
} 

所以，當我要保存相關實體的圖形，我用下面的方法：

public static EntityState ConvertState(BaseEntity.States state) 
    { 
     switch (state) 
     { 
      case BaseEntity.States.Added: 
       return EntityState.Added; 
      case BaseEntity.States.Modified: 
       return EntityState.Modified; 
      case BaseEntity.States.Deleted: 
       return EntityState.Deleted; 
      default: 
       return EntityState.Unchanged; 
     } 
    } 

    public void ApplyChanges<TEntity>(TEntity root) where TEntity : BaseEntity 
    { 
     _dbContext.Set<TEntity>().Add(root); 
     foreach (var entry in _dbContext.ChangeTracker 
     .Entries<BaseEntity>()) 
     { 
      BaseEntity stateInfo = entry.Entity; 
      entry.State = ConvertState(stateInfo.State); 
     } 
    } 

但如果我的圖形中包含具有相同鍵2個或多個實體我給這個錯誤：

An object with the same key already exists in the ObjectStateManager... 

我怎麼能檢測與相同的密鑰的實體在我的圖（root），使他們獨特在我的ApplyChanges()方法？

Answer 1

我改變了我的BaseEntity到

public class BaseEntity 
{ 
    public int Id {get; set;} 
    public States State { get; set; } 
    public bool MustDelete {get; set;} 

    public enum States 
    { 
    Unchanged, 
    Added, 
    Modified, 
    Deleted 
    } 
} 

而且也改變了按照我的BaseDomainService<T>類方法：

public class BaseDomainService<T> where T : class 
{ 
    protected readonly DbContext _dbContext; 

    public BaseDomainService(IUnitOfWork uow) 
    { 
     _dbContext = (DbContext)uow; 
    } 
    ..... 


    public static EntityState ConvertState(BaseEntity.States state) 
    { 
     switch (state) 
     { 
      case BaseEntity.States.Added: 
       return EntityState.Added; 
      case BaseEntity.States.Modified: 
       return EntityState.Modified; 
      case BaseEntity.States.Deleted: 
       return EntityState.Deleted; 
      default: 
       return EntityState.Unchanged; 
     } 
    }  

    public void ApplyChanges<TEntity>(TEntity root) where TEntity : BaseEntity 
    { 
     _dbContext.Set<TEntity>().Add(root); 
     foreach (var entry in _dbContext.ChangeTracker 
     .Entries<BaseEntity>()) 
     { 
      if (FoundAnEntityWithSameKeyInDbContext<TEntity>(entry)) 
       entry.State = EntityState.Detached; 
      else 
      { 
       BaseEntity stateInfo = entry.Entity; 
       if (stateInfo.MustDelete == true) 
        entry.State = EntityState.Detached; 
       else 
        entry.State = ConvertState(stateInfo.State); 
      } 
     } 
    } 

    private bool FoundAnEntityWithSameKeyInDbContext<TEntity>(DbEntityEntry<BaseEntity> entry) where TEntity : BaseEntity 
    { 
     var tmp = _dbContext.ChangeTracker.Entries<BaseEntity>().Count(t => t.Entity.Id == entry.Entity.Id && t.Entity.Id != 0 && t.Entity.GetType() == entry.Entity.GetType()); 
     if (tmp > 1) 
      return true; 
     return false; 
    } 
} 

那麼，問題就迎刃而解了。

Answer 2

當您調用_dbContext.Set<TEntity>().Add(root);時，它會告訴上下文，圖中所有實體的狀態爲EntityState.Added。但是不允許有兩個具有相同ID和EntityState.Added的實體，並且拋出異常。

嘗試將行更改爲 _dbContext.Set<TEntity>().Attach(root);。這會將圖形放入 EntityState.Unchanged的上下文中。已經在其他狀態中的上下文中的實體將其狀態設置爲「未更改」。

現在你應該可以去修復狀態。

震撼了這個答案，因爲Attach導致相同的錯誤 - 按評論

參考文獻：

When to use DbSet<T>.Add() vs DbSet<T>.Attach()

Why does Entity Framework Reinsert Existing Objects into My Database?

Making Do with Absent Foreign Keys

Provide better support for working with disconnected entities

DbSet.Attach method

Answer 3

還有就是搜索數據庫並檢查是否與相同主鍵的記錄已經存在，我不知道這是你在找什麼，但代碼如下方式：

public static class ObjectSetExtensions 
{ 
    #region Constants 

    private const BindingFlags KeyPropertyBindingFlags = 
     BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic; 

    #endregion 

    #region Public Methods and Operators 

     public static bool RecordExists<TEntity>(
     this ObjectSet<TEntity> set, 
     TEntity entity) where TEntity : class 
    { 
     Contract.Requires(set != null); 
     Contract.Requires(entity != null); 

     var expressionParameter = Expression.Parameter(typeof(TEntity)); 
     var keyProperties = set.GetKeyProperties(); 

     var matchExpression = 
      keyProperties.Select(
       pi => 
       Expression.Equal(
        Expression.Property(expressionParameter, pi.Last()), 
        Expression.Constant(pi.Last().GetValue(entity, null)))) 
       .Aggregate<BinaryExpression, Expression>(
        null, 
        (current, predicate) => (current == null) ? predicate : 
         Expression.AndAlso(current, predicate)); 

     var existing = 
      set.SingleOrDefault(Expression.Lambda<Func<TEntity, bool>>(
      matchExpression, 
      new[] { expressionParameter })); 

     return existing != null; 
    } 

    #endregion 

    #region Methods 

    private static IEnumerable<PropertyPathCollection> GetKeyProperties<TEntity>(this ObjectSet<TEntity> objectSet) 
     where TEntity : class 
    { 
     Contract.Requires(objectSet != null); 

     var entityType = typeof(TEntity); 

     return 
      objectSet.EntitySet.ElementType.KeyMembers.Select(
       c => new PropertyPathCollection(entityType.GetProperty(c.Name, KeyPropertyBindingFlags))); 
    } 

    #endregion 
} 

public sealed class PropertyPathCollection : IEnumerable<PropertyInfo> 
{ 
    // Fields 
    #region Static Fields 

    public static readonly PropertyPathCollection Empty = new PropertyPathCollection(); 

    #endregion 

    #region Fields 

    private readonly List<PropertyInfo> components; 

    #endregion 

    // Methods 
    #region Constructors and Destructors 

    public PropertyPathCollection(IEnumerable<PropertyInfo> components) 
    { 
     this.components = new List<PropertyInfo>(); 
     this.components.AddRange(components); 
    } 

    public PropertyPathCollection(PropertyInfo component) 
    { 
     this.components = new List<PropertyInfo> { component }; 
    } 

    private PropertyPathCollection() 
    { 
     this.components = new List<PropertyInfo>(); 
    } 

    #endregion 

    #region Public Properties 

    public int Count 
    { 
     get 
     { 
      return this.components.Count; 
     } 
    } 

    #endregion 

    #region Public Indexers 

    public PropertyInfo this[int index] 
    { 
     get 
     { 
      return this.components[index]; 
     } 
    } 

    #endregion 

    #region Public Methods and Operators 

    public static bool Equals(PropertyPathCollection other) 
    { 
     if (ReferenceEquals(null, other)) 
     { 
      return false; 
     } 

     return true; 
    } 

    public static bool operator ==(PropertyPathCollection left, PropertyPathCollection right) 
    { 
     return Equals(left, right); 
    } 

    public static bool operator !=(PropertyPathCollection left, PropertyPathCollection right) 
    { 
     return !Equals(left, right); 
    } 

    public override bool Equals(object obj) 
    { 
     if (ReferenceEquals(null, obj)) 
     { 
      return false; 
     } 

     if (ReferenceEquals(this, obj)) 
     { 
      return true; 
     } 

     if (obj.GetType() != typeof(PropertyPathCollection)) 
     { 
      return false; 
     } 

     return Equals((PropertyPathCollection)obj); 
    } 

    public override int GetHashCode() 
    { 
     return this.components.Aggregate(0, (t, n) => (t + n.GetHashCode())); 
    } 

    #endregion 

    #region Explicit Interface Methods 

    IEnumerator<PropertyInfo> IEnumerable<PropertyInfo>.GetEnumerator() 
    { 
     return this.components.GetEnumerator(); 
    } 

    IEnumerator IEnumerable.GetEnumerator() 
    { 
     return this.components.GetEnumerator(); 
    } 

    #endregion 
} 

而且用法是這樣的：

var context = this.DbContext; 
var adapter = context as IObjectContextAdapter; 
var objectContext = adapter.ObjectContext; 

objectContext.CreateObjectSet<TEntity>().RecordExists(instance);