c# - 是否可以使用 IComparable 比较 Entity Framework 中的实体？

Question

我有一个实现 IComparable 的 POCO 类。

public interface IEntity : IComparable
{
    long Id { get; set; }

    Func<IEntity, bool> CompareFunction { get; }
}

public abstract class BaseEntity : IEntity
{
    public virtual long Id { get; set; }

    public Func<IEntity, bool> CompareFunction
    {
        get
        {
            Func<IEntity, bool> compare = EvaluateEquivalency;
            return compare;
        }
    }

    public static int Compare(BaseEntity left, BaseEntity right)
    {
        if (object.ReferenceEquals(left, right))
        {
            return 0;
        }

        if (object.ReferenceEquals(left, null))
        {
            return -1;
        }

        return left.CompareTo(right);
    }

    public static bool operator ==(BaseEntity left, BaseEntity right)
    {
        if (object.ReferenceEquals(left, null))
        {
            return object.ReferenceEquals(right, null);
        }

        return left.Equals(right);
    }

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

    public static bool operator <(BaseEntity left, BaseEntity right)
    {
        return Compare(left, right) < 0;
    }

    public static bool operator >(BaseEntity left, BaseEntity right)
    {
        return Compare(left, right) > 0;
    }

    public override bool Equals(object obj)
    {
        IEntity other;

        if (!(obj is IEntity)) return false;

        other = (IEntity)obj;

        if (object.ReferenceEquals(other, null))
        {
            return false;
        }

        return this.CompareTo(other) == 0;
    }

    public override int GetHashCode()
    {
        return base.GetHashCode();
    }

    public virtual int CompareTo(object obj)
    {
        if (obj == null) throw new ArgumentNullException("obj");
        if (!(obj is IEntity)) throw new ArgumentException("obj is not an IEntity");

        if (this.Id == ((IEntity)obj).Id) return 0;

        return -1;
    }

    private bool EvaluateEquivalency(IEntity toCompare)
    {
        return Equals(toCompare);
    }
}

DbContext 中有用于我的 POCO 类的 DbSet。

但是，当我执行 BaseRepository.Exists() 时，我得到了一个 System.NotSupportedException。

public class BaseRepository<TEntity> : IRepository<TEntity> where TEntity : class, IEntity
{
    ...

    private TEntity Exists(TEntity entity)
    {
        return Context.DbSet<TEntity>.FirstOrDefult(i => i.CompareTo(entity) == 0);
    }

    private TEntity ExistsV2(TEntity entity)
    {
        return Context.DbSet<TEntity>.FirstOrDefult(i => i.CompareFunction(entity) == 0);
    }

    ...
}

异常堆栈跟踪看起来像...

System.NotSupportedException was unhandled by user code
  Message=Unable to create a constant value of type '{My POCO Class}'. Only primitive types or enumeration types are supported in this context.
  Source=System.Data.Entity
  StackTrace:
       at System.Data.Objects.ELinq.ExpressionConverter.ConstantTranslator.TypedTranslate(ExpressionConverter parent, ConstantExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.EqualsTranslator.TypedTranslate(ExpressionConverter parent, BinaryExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.FirstPredicateTranslatorBase.Translate(ExpressionConverter parent, MethodCallExpression call)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.Convert()
       at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
       at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
       at System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__1[TResult](IEnumerable`1 sequence)
       at System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
       at System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
       at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)
       at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)

当我执行 BaseRepository.ExistsV2() 时，我得到一个略有不同的 System.NotSupportedException。

System.NotSupportedException was unhandled by user code
  HResult=-2146233067
  Message=The LINQ expression node type 'Invoke' is not supported in LINQ to Entities.
  Source=System.Data.Entity
  StackTrace:
       at System.Data.Objects.ELinq.ExpressionConverter.NotSupportedTranslator.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.FirstPredicateTranslatorBase.Translate(ExpressionConverter parent, MethodCallExpression call)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.Convert()
       at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
       at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
       at System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__1[TResult](IEnumerable`1 sequence)
       at System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
       at System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
       at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)
       at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source, Expression`1 predicate)

我读过 Entity Framework 不支持 IComparable？无论如何，有没有人知道该功能是否会在 EF6 中可用？

Answer 1

不，你不能这样做。

EF 要求所有操作都能够 100% 在服务器端执行。支持 IComparable 或 IEquatable 需要 EF 能够将任意 IL 转换为 SQL，而它目前还不能。

否则，它必须将整个未过滤的结果集传递给客户端。您可以使用 AsEnumerable() 实现这一点:

Context.DbSet<TEntity>.AsEnumerable().FirstOrDefault(i => i.CompareTo(entity) == 0);

虽然这当然会运行得很慢，所以如果表很大，我不会推荐它。