我想过滤一个实现接口的实体列表。
模型:
public interface IEntity
{
int Id {get; set;}
}
public interface IOther
{
int Other {get; set;}
}
public class MyEntity : IEntity, IOther
{
public int Id {get; set;}
public int Other {get; set;}
}
控制器:
public abstract class GenericApiController<T> : ApiController
where T : IEntity
{
public HttpResponseMessage Get(int other)
{
var query = Repository.AsQueryable()
.Cast<IOther>()
.Where(x => x.Other == other);
return Ok(query.ToList());
}
}
但是,我收到“ LINQ to Entities仅支持转换EDM基本类型或枚举类型”异常。
一种解决方案是在GenericApiController上具有
where T : IOther
,但是可悲的是我不能这样做,因为不是每个IEntity都也实现IOther。我正在研究是否可以执行以下操作:
public abstract class GenericApiController<T> : ApiController
where T : IEntity
{
public HttpResponseMessage Get(int other)
where T : IOther
{
var query = Repository.AsQueryable()
.Where(x => x.Other == other);
return Ok(query.ToList());
}
}
请注意对
Get()
的额外限制,但这是不可能的(据我所知)。有什么建议么?
最佳答案
您可以为继承IOther
的类编写特定的控制器,但不能完全解决问题。
在下面的表达式中(Repository
是IQueryable<T>
,T
继承自IOther
),C#编译器考虑从T
到IOther
的隐式强制转换,以调用Other
属性。
var query = Repository.Where(x => x.Other == other);
因此,您可以对实体进行强制转换和LINQ相同的
NotSupportedException
。解决方案是在运行时使用Reflection构建查询。
这是为了限制编译器在表达式级别上的工作,并在运行时执行从表达式到函数的转换。
通用查询表达式为:
Expression<Func<IQueryable<T>, int, IQueryable<T>>> QueryExpression =
(repository, other) => repository.Where(x => x.Other == other);
使用调试控制台,您将看到编译器如何添加隐式转换器:
QueryExpression.ToString():(存储库,其他)=>存储库。(x =>(Convert(x).Other == other))
此表达式有两件事需要更改:
消除转换器的使用
调用
Other
声明的T
属性而不是IOther
声明的属性为此,我们使用
ExpressionVisitor
。public abstract class GenericApiControllerForIOther<T> : ApiController
where T : IOther
{
public HttpResponseMessage Get(int other)
{
var query = QueryFunction(Repository, other);
return Ok(query.ToList());
}
// the generic query expression
static Expression<Func<IQueryable<T>, int, IQueryable<T>>> QueryExpression =
(repository, other) => repository.Where(x => x.Other == other);
// the function built from the generci expression
static Func<IQueryable<T>, int, IQueryable<T>> queryFunction = null;
static Func<IQueryable<T>, int, IQueryable<T>> QueryFunction
{
get
{
if (queryFunction == null)
{
// rebuild a new lambda expression without reference to the IOther type
TypeReplacer replacer = new TypeReplacer(typeof(IOther), typeof(T));
Expression newExp = replacer.Visit(QueryExpression.Body);
Expression<Func<IQueryable<T>, int, IQueryable<T>>> newLambdaExp = Expression.Lambda<Func<IQueryable<T>, int, IQueryable<T>>>(newExp, QueryExpression.Parameters);
// newLambdaExp.ToString(): (repository, other) => repository.Where(x => (x.Other == other))
// convert the expression to a function
queryFunction = newLambdaExp.Compile();
}
return queryFunction;
}
}
class TypeReplacer : ExpressionVisitor
{
public TypeReplacer(Type oldType, Type newType)
{
OldType = oldType;
NewType = newType;
}
Type OldType;
Type NewType;
protected override Expression VisitMember(MemberExpression node)
{
// replace IOther.Property by T.Property
MemberInfo memberInfo = node.Member;
if (memberInfo.DeclaringType == OldType)
{
MemberInfo newMemberInfo = NewType.GetMember(memberInfo.Name).First();
return Expression.MakeMemberAccess(Visit(node.Expression), newMemberInfo);
}
return base.VisitMember(node);
}
protected override Expression VisitUnary(UnaryExpression node)
{
// remove the Convert operator
if (node.NodeType == ExpressionType.Convert
&& node.Type == OldType
&& node.Operand.Type == NewType)
return node.Operand;
return base.VisitUnary(node);
}
}
}