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:
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);
这篇关于EF - 不能将运算符'=='应用于'TId'和'TId'类型的操作数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！