EF - 不能将运算符'=='应用于'TId'和'TId'类型的操作数 [英] EF - Cannot apply operator '==' to operands of type 'TId' and 'TId'

问题描述

  public class GenericRepository< TEntity，TId>< p>我有这个通用类，它使用实体框架6.x。其中TEntity，class，IIdentifyable< TId> 
 {
 public virtual TEntity GetById（TId id）
 {
 using（var context = new DbContext（））
 {
 var dbSet = context。设置< TEntity>（）; 
 var currentItem = dbSet.FirstOrDefault（x => x.Id == id）; 
返回currentItem; 
 
 
 
 public virtual bool Exists（TId id）
 {
 using（var context = new DbContext（））
 { 
 var dbSet = context.Set< TEntity>（）; 
 var exists = dbSet.Any（x => x.Id == id）; 
 return存在; 
} 
} 
} 
  

p>

  public interface IIdentifyable：IIdentifyable< int> 
 {
} 
 
 public interface IIdentifyable< out TId> 
 {
 TId Id {get; } 
} 
  

看起来像这样的实体：

  public class CustomerEntity：IIdentifyable< int> 
 {
 public string Name {get;组; } 
 public int Id {get; set; } 
} 
 
 public class ProductEntity：IIdentifyable< Guid> 
 {
 public string Name {get;组; } 
 public Guid Id {get; set; } 
} 
  

我的问题是它不能编译。我得到这个错误：

无法应用运算符' == '到' TId '和' TId '的操作数

我试图将它更改为 x => Equals（x.Id，id），但EF无法翻译它。任何方式吗？

我知道我可以使用 Find（）而不是 FirstOrDefault 。但我比上面提到的方法更需要这个。 有什么方法可以让EF比较 TId 和 TId ？ <$ c $目前只有 guid 和 int 。我已经看到了下面的问题，但他们没有处理有关转换为SQL的问题。

无法将运算符==应用于C＃中的泛型类型？

如何解决运算符'！='不能应用于'T'和'T'类型的操作数 解决方案

这是一个泛型的已知问题，通常使用 EqualityComparer< T> .Default 来代替 == 运算符。然而，这种方法不适用于LINQ to Entities。

解决这个问题的一种方法是使用 Expression System.Linq.Expressions 命名空间中的类>：
$ b

  public class GenericRepository< TEntity，TId>其中TEntity：class，IIdentifyable< TId> 
 {
 protected static Expression< Func< TEntity，bool>> EqualsPredicate（TId id）
 {
表达式< Func< TEntity，TId>> selector = x => x.Id; 
 Expression< Func< TId>> closure =（）=> ID; 
 return Expression.Lambda< Func< TEntity，bool>>（
 Expression.Equal（selector.Body，closure.Body），
 selector.Parameters）; 
 
 
 $ / code $ / pre 
 
 
并像这样使用它：
  dbSet.FirstOrDefault（EqualsPredicate（id））; 
  
或
  dbSet.Any（EqualsPredicate（ID））; 
  
等。
 
 
 
 更新：这是另一种适用于EF的方式。
 
 
 
将以下约束添加到 GenericRepository 类 
 
 
 其中TId：IEquatable< TId> 
  
，然后使用等于方法
  x => x.Id.Equals（ID）; 
  
 
I have this generic class, which uses Entity Framework 6.x.
public class GenericRepository<TEntity, TId> where TEntity, class, IIdentifyable<TId>
{
    public virtual TEntity GetById(TId id)
    {
        using (var context = new DbContext())
        {
            var dbSet = context.Set<TEntity>();
            var currentItem = dbSet.FirstOrDefault(x => x.Id == id);
            return currentItem;
        }
    }

    public virtual bool Exists(TId id)
    {
        using (var context = new DbContext())
        {
            var dbSet = context.Set<TEntity>();
            var exists = dbSet.Any(x => x.Id == id);
            return exists ;
        }
    }
}
And these interfaces:
public interface IIdentifyable : IIdentifyable<int>
{
}

public interface IIdentifyable<out TId>
{
    TId Id { get; }
}
And entities that looks like this:
public class CustomerEntity : IIdentifyable<int>
{
    public string Name { get; set; }
    public int Id { get;set; }
}

public class ProductEntity : IIdentifyable<Guid>
{
    public string Name { get; set; }
    public Guid Id { get;set; }
}
My issue is that it doesn't compile. I get this error:

  
Cannot apply operator '==' to operands of type 'TId' and 'TId'
I tried to change it to x => Equals(x.Id, id), but then EF cannot translate it. Any way around it?

I know that I can use Find() instead of FirstOrDefault. But I need this for more than the methods mentioned above. Is there any way I can let EF compare TId with TId? TId is currently only guid and int. I've already seen the questions below, but they don't handle the issue regarding translation to SQL.

Can't operator == be applied to generic types in C#?

How to solve Operator '!=' cannot be applied to operands of type 'T' and 'T'
解决方案
This is a known issue with generics which normally is handled by using EqualityComparer<T>.Default instead of the == operator. However this approach doesn't work with LINQ to Entities.

One way to solve it is to build the predicate dynamically using the Expression class from the System.Linq.Expressions namespace like this:
public class GenericRepository<TEntity, TId> where TEntity: class, IIdentifyable<TId>
{
    protected static Expression<Func<TEntity, bool>> EqualsPredicate(TId id)
    {
        Expression<Func<TEntity, TId>> selector = x => x.Id;
        Expression<Func<TId>> closure = () => id;
        return Expression.Lambda<Func<TEntity, bool>>(
            Expression.Equal(selector.Body, closure.Body),
            selector.Parameters);
    }
}
and use it like this:
dbSet.FirstOrDefault(EqualsPredicate(id));
or 
dbSet.Any(EqualsPredicate(id));
etc.

Update: Here is another way that works with EF.

Add the following constraint to the GenericRepository class
where TId : IEquatable<TId>
and then use Equals method
x => x.Id.Equals(id);


                        
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