c# – LINQ to Entities和LINQ to Objects处理强制转换的方式不

我最近加入了一个项目,其中Sort方法有条件地将lambda表达式传递给LINQ查询以定义应该对哪个属性进行排序.问题是lambda表达式是在Func< TEntity,Object>中传递的.而不是表达式< Func< TEntity,Object>>因此,排序发生在内存而不是数据库中(因为调用了带有IEnumerable的OrderBy的重载).这是SortWithDelegate中的版本(见下文).

当我使用表达式< Func< TEntity,Object>> (参见下面的SortWithExpression)然后,当在orderBy参数中传递字符串属性时,在数据库中正确完成排序.但是,当我尝试使用Expression< Func< TEntity,Object>>对整数(或日期时间)进行排序时我收到以下错误：

无法将类型“System.Int32”强制转换为“System.Object”类型. LINQ to Entities仅支持转换EDM原语或枚举类型.

为了避免这种情况,我必须将整数或日期时间字段包装在匿名类型内部,如下所示：orderByFunc = sl => new {sl.ParentUnit.Id};.我知道我需要这样做,因为Func的返回类型是Object.但是,我不明白为什么在使用LINQ to Entities提供程序而不是LINQ to Objects提供程序时需要执行此操作？

void Main()
{
    var _context = new MyContext();

    string sortProperty = "Id";
    bool sortAscending = false;


    IQueryable<Qualification> qualifications = _context.Qualifications.Include(q => q.ParentUnit);

    qualifications = SortWithExpression(sortProperty,sortAscending,qualifications);

    qualifications.Dump();

}

private static IQueryable<Qualification> SortWithDelegate(string orderBy,bool sortAscending,IQueryable<Qualification> qualificationsQuery)
{
    Func<Qualification,Object> orderByFunc;

    switch (orderBy)
    {
        case "Name":
            orderByFunc = sl => sl.Name;
            break;
        case "ParentUnit":
            orderByFunc = sl => sl.ParentUnit.Name;
            break;
        case "Id":
            orderByFunc = sl => sl.ParentUnit.Id;
            break;
        case "Created":
            orderByFunc = sl => sl.Created;
            break;
        default:
            orderByFunc = sl => sl.Name;
            break;
    }

    qualificationsQuery = sortAscending
        ? qualificationsQuery.OrderBy(orderByFunc).AsQueryable()
            : qualificationsQuery.OrderByDescending(orderByFunc).AsQueryable();

    return qualificationsQuery;
}

private static IQueryable<Qualification> SortWithExpression(string orderBy,IQueryable<Qualification> qualificationsQuery)
{
    Expression<Func<Qualification,Object>> orderByFunc;

    switch (orderBy)
    {
        case "Name":
            orderByFunc = sl => sl.Name;
            break;
        case "ParentUnit":
            orderByFunc = sl => sl.ParentUnit.Name;
            break;
        case "Id":
            orderByFunc = sl => new {sl.ParentUnit.Id};
            break;
        case "Created":
            orderByFunc = sl => new {sl.Created};
            break;
        default:
            orderByFunc = sl => sl.Name;
            break;
    }

    qualificationsQuery = sortAscending
        ? qualificationsQuery.OrderBy(orderByFunc)
            : qualificationsQuery.OrderByDescending(orderByFunc);

    return qualificationsQuery;
}

添加

我以为我会为这个问题添加自己的解决方案.为了避免装箱int和datetime,我在IQueryable< T>上创建了一个通用的扩展方法.我传入lambda表达式来指示sort字段,以及一个布尔值,指示排序顺序是否应该升序：

public static IQueryable<TSource> OrderBy<TSource,TResult>(this IQueryable<TSource> query,Expression<Func<TSource,TResult>> func,bool sortAscending)
    {
        return sortAscending ?
            query.OrderBy(func) :
                query.OrderByDescending(func);
    }

    private static IQueryable<Qualification> Sort(string orderBy,IQueryable<Qualification> qualificationsQuery)
    {
        switch (orderBy)
        {
            case "Name":
                return qualificationsQuery.OrderBy(sl => sl.Name,sortAscending);
            case "ParentUnit":
                return qualificationsQuery.OrderBy(s1 => s1.ParentUnit.Name,sortAscending);
            default:
                return qualificationsQuery.OrderBy(sl => sl.Name,sortAscending);
        }
    }