Question

我有以下代码：

internal static IQueryable<UserModel> SelectUser(this IQueryable<User> query, bool getChildren) {
    log.DebugFormat("create the select statement for an individual user {0}", getChildren ? "with children" : "without children");
    var res = query.Select(u => new UserModel {
        // [...] unimportant stuff
        Groups = u.Groups.Select(g => new GroupModel {
            Name = g.Name,
            Description = g.Description,
            ID = g.ID
        }).Take(10),
        CreatedGroups = !getChildren ? null : u.CreatedGroups.Select(g => new GroupModel {
            Name = g.Name,
            Description = g.Description,
            ID = g.ID
        }).Take(10)
        // [...] more stuff like above just different types
    });
    return res;
}

此代码完美无缺。但我有点恼火，我用这个代码违反了DRY原则。最重要的是，有更多情况，我需要将Group映射到GroupModel。

所以我决定创建一个扩展方法：

public static IEnumerable<GroupModel> SelectChildGroups(this ICollection<Group> query) {
    return query.Select(g => new GroupModel {
        Name = g.Name,
        ID = g.ID
    });
}

这给了我以下例外：

'LINQ to Entities' erkennt die Methode 'System.Collections.Generic.IEnumerable`1[Coding.Lizards.Gekkota.Web.Models.GroupModel] SelectChildGroups(System.Collections.Generic.ICollection`1[Coding.Lizards.Gekkota.Web.Models.Group])' nicht, und diese Methode kann nicht in einen Speicherausdruck übersetzt werden.

英文：

'LINQ to Entities' does not recognize the method 'System.Collections.Generic.IEnumerable`1[Coding.Lizards.Gekkota.Web.Models.GroupModel] SelectChildGroups(System.Collections.Generic.ICollection`1[Coding.Lizards.Gekkota.Web.Models.Group])', and this method cannot be translated into a store expression.

我能够将错误追溯到我发布的功能。有人知道如何解决它吗？

修改

属性Groups和CreatedGroups属于ICollection<Group>类型。该代码与Linq-To-Entities一起使用。

Answer 1

尝试将Func存储在变量中，然后重复使用它。像这样：

internal static IQueryable<UserModel> SelectUser(this IQueryable<User> query, bool getChildren) {
    log.DebugFormat("create the select statement for an individual user {0}", getChildren ? "with children" : "without children");

    var res = query.Select(u => new UserModel {
        // [...] unimportant stuff
        Groups = u.Groups.AsQueryable().Select(MyLinqExpressions.fSelect).Take(10),
        CreatedGroups = !getChildren ? null : u.CreatedGroups.AsQueryable().Select(MyLinqExpressions.fSelect).Take(10)
        // [...] more stuff like above just different types
    });
    return res;
}

然后是另一个班级：

public static class MyLinqExpressions {
    public static Func<Group,GroupModel>
        fSelect = g => new GroupModel {
            Name = g.Name,
            Description = g.Description,
            ID = g.ID
        };
}

重用LINQ选择功能

修改

1 个答案: