投实体到实现的接口为实体框架泛型方法使用LINQ [英] Cast Entity to Implemented Interface in a Generic Method Using LINQ for Entity Framework

问题描述

我有一个通用的方法来查询EF型TEntity的对象。我想补充一个条件为WHERE子句如果TEntity实现一个特定的接口。我的方法是：

I have a generic method to query objects of type TEntity in EF. I Want to add a condition as a where clause if TEntity implements a specific interface. The method I have is:

public TEntity GetByUserID(Guid userID)
{
    var query = this.DbSet;
    if (typeof (TEntity).IsImplementationOf<IDeletableEntity>())
    {
        query = query
            .Where((x => !((IDeletableEntity)x).IsDeleted);
    }
    return query
        .FirstOrDefault(x => x.UserID == userID);
}

IsImplementationOf&下;>（）是方法顾名思义刚刚返回真/假

IsImplementationOf<>() is method which just returns true/false as the name implies.

。当我运行这为它们实现IDeletableEntity实体地址，我得到一个错误：

When I run this for the entity Address which implement IDeletableEntity, I get an error:

无法转换的类型地址键入 IDeletableEntity。LINQ到实体仅支持铸造EDM原始或枚举类型。

Unable to cast the type 'Address' to type 'IDeletableEntity'. LINQ to Entities only supports casting EDM primitive or enumeration types.

任何想法我怎么能去解决此限制LINQ的？

Any ideas how I can go around this limitation of LINQ?

推荐答案

这是一个可行的解决方案：

This is a working solution:

public TEntity GetByUserID(Guid userID, params Include<TEntity>[] includes)
{
    var query = this.DbSet;
    query = Where<IDeletableEntity>(query, x => !x.IsDeleted);
    return query
        .FirstOrDefault(x => x.UserID == userID);
}

public static IQueryable<TEntity> Where<TPredicateWellKnownType>(IQueryable<TEntity> query, Expression<Func<TPredicateWellKnownType, bool>> predicate)
{
    if (typeof(TEntity).IsImplementationOf<TPredicateWellKnownType>())
    {
        query = ((IQueryable<TPredicateWellKnownType>)query)
            .Where(predicate)
            .Cast<TEntity>();
    }
    return query;
}

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