Expression generated based on interface

I'm geting the exception <strong>Unable to cast the type 'MySomeTypeThatImplementsISomeInterfaceAndIsPassedAs[T]ToTheClass' to type 'ISomeInterface'. LINQ to Entities only supports casting Entity Data Model primitive types.</strong>

my repository looks like

public interface IRepository<T> { IQueryable<T> Get(System.Linq.Expressions.Expression<System.Func<T, bool>> Query); }

Also, I have the service class

public abstract class FinanceServiceBase<TAccount, TParcel, TPayment> where TAccount : IAccount where TParcel : IParcel where TPayment : IPayment { //... IRepository<TPayment> ParcelRepository {get; private set;} public bool MakePayment(TPayment payment) { //... ParcelRepository.Get(p => p.ParcelId == 2); // here my exception is thrown // **p.ParcelId is in IParcel** //... } } //...

With this class I can control many things about finances without rewrite code for other programs. I've did the class with 3 generic parameters because I couldn't use IRepository because my repository can't be <out T>

<strong>ParcelId is Int32</strong> TParcel is typeof(ParcelToReceive) that is an entity who implement IParcel, and was generated with codeonly

The problem occurs when I call Get and the resultant lambda looks like

(**(ISomeInterface)**$p).SomeInterfaceMember ==

instead of


so, entity framework try do the cast and throws the exception. What I want to know is: is there anyway to tell linq that the lambda field p.ParcelId is from <strong>TParcel</strong> and not from <strong>IParcel</strong>.

Already tried (with no luck):

p => ((TParcel)p).ParcelId



It seems that setting the generic constraints from where TAccount : IAccount to something like where TAccount : class, IAccount tells entity framework that the expression contains a <strong>class</strong> and it will not make the explicit cast that it would do for primitive EDM and enum types.


I’m afraid you can’t do this because fundamentally you are accessing the property that is declared in the interface. LINQ-to-Entities doesn’t seem to support that; you need to call the property in the real entity type.

One way you could solve this is by passing the parcelId property as an expression tree in a parameter, and then construct a lambda expression dynamically at runtime using the property in that parameter:

public bool MakePayment(TPayment payment, Expression<Func<TParcel, int>> parcelIdExpr) { // You can use any expression involving parcelId here Expression<Func<int, bool>> expr = parcelId => parcelId == 2; // This is the parameter of the new lambda we’re creating var parameter = Expression.Parameter(typeof(TParcel)); // This constructs the lambda expression “p => expr(p.ParcelId)”, // where “expr” is the lambda expression declared above var lambda = Expression.Lambda(Expression.Invoke(expr, Expression.Invoke(parcelIdExpr, parameter)), parameter); ParcelRepository.Get((Expression<Func<TParcel, bool>>) lambda); } [...] myFinanceService.MakePayment(myPayment, p => p.ParcelId);

If you don’t want to have to pass this extra parameter every time you call MakePayment, then you could in theory retrieve the property by name with a string literal; however, this is unsafe because it doesn’t ensure that it’s the right property which implements the interface. Also, this is a very roundabout way of doing it, so no guarantees:

public bool MakePayment(TPayment payment) { Expression<Func<int, bool>> expr = parcelId => parcelId == 2; var parameter = Expression.Parameter(typeof(TParcel)); // This is the expression “p.ParcelId”, where “p” is the parameter var propertyExpression = Expression.Property(parameter, "ParcelId"); var lambda = Expression.Lambda(Expression.Invoke(expr, propertyExpression), parameter); ParcelRepository.Get((Expression<Func<TParcel, bool>>) lambda); }

You can factor this out into a generic utility method:

public static class Utils { public static Expression<Func<TParameter, TResult>> CombineLambdas<TParameter, T, TResult>( Expression<Func<TParameter, T>> lambda1, Expression<Func<T, TResult>> lambda2 ) { var parameter = Expression.Parameter(typeof(TParameter)); var lambda = Expression.Lambda(Expression.Invoke(lambda2, Expression.Invoke(lambda1, parameter)), parameter); return (Expression<Func<TParameter, TResult>>) lambda; } } public bool MakePayment(TPayment payment, Expression<Func<TParcel, int>> parcelIdExpr) { ParcelRepository.Get(Utils.CombineLambdas( parcelIdExpr, parcelId => parcelId == 2)); }

人吐槽 人点赞



用户名: 密码:
验证码: 匿名发表


查看评论:Expression generated based on interface