Question

正在编译此查询而没有错误：

            var _entityList = context.customer

                .Join(context.applications,
                cust => cust.cust_id,
                app => app.cust_id,
                (cust, app) => new { customer = cust, application = app })

                .Join(context.advices,
                cust => cust.application.app_id,
                sa => sa.app_id,
                (cust, sa) => new { customer = cust, advice = sa })

                .GroupBy(g => new { g.customer.customer.cust_id, g.customer.customer.cust_code, g.customer.customer.cust_name })
                .Select(g => new { cust_id = g.Key.cust_id, cust_code = g.Key.cust_code, cust_name = g.Key.cust_name })
                .ToList();

在上面的查询中添加条件where子句会返回编译时类型转换错误：

            var _entityList = context.customer

                .Join(context.applications,
                cust => cust.cust_id,
                app => app.cust_id,
                (cust, app) => new { customer = cust, application = app })

                .Join(context.advices,
                cust => cust.application.app_id,
                sa => sa.app_id,
                (cust, sa) => new { customer = cust, advice = sa });

            if (custcode != null && custcode != "")
                _entityList = _entityList.Where(e => e.customer.customer.cust_code == custcode);

            _entityList = _entityList
                .GroupBy(g => new { g.customer.customer.cust_id, g.customer.customer.cust_code, g.customer.customer.cust_name })
                .Select(g => new { cust_id = g.Key.cust_id, cust_code = g.Key.cust_code, cust_name = g.Key.cust_name })
                .ToList(); // error on this line

无法隐式将System.Collections.Generic.List<AnonymousType#1>类型转换为System.Linq.IQueryable<AnonymousType#2>

我错过了什么？

Answer 1

考虑以下简化代码示例：

var _entityList = Enumerable.Range(0, 1)
    .Select(i=>new {i1 =i, i2 = i+1});
_entityList = _entityList
    //.Select(i => new { i1 = i.i1, i2 = i.i2 })    // works
    //.Select(i => i)                               // works
    .Select(i => new { i })                         // fails
    .ToList();

在您的第一个场景中，只涉及一个匿名类型，因此_entityList是List<AnonymousType#1>。在第二个场景中，您将从一个匿名类型更改返回的类型：

new { 
    customer = cust, 
    advice = sa 
}

到另一个人：

new { 
    cust_id = g.Key.cust_id, 
    cust_code = g.Key.cust_code, 
    cust_name = g.Key.cust_name 
}

因此发生转换错误。

试试这个：

var _entityList = context.customer
    .Join(context.applications,
            cust => cust.cust_id,
            app => app.cust_id,
            (cust, app) => new { customer = cust, application = app })
    .Join(context.advices,
            cust => cust.application.app_id,
            sa => sa.app_id,
            (cust, sa) => new { customer = cust, advice = sa })
    .Where(e => (custcode != null && custcode != "") 
        ? e.customer.customer.cust_code == custcode : true)
    .GroupBy(g => new { 
        g.customer.customer.cust_id, 
        g.customer.customer.cust_code, 
        g.customer.customer.cust_name })
    .Select(g => new { 
        cust_id = g.Key.cust_id, 
        cust_code = g.Key.cust_code, 
        cust_name = g.Key.cust_name })
    .ToList();

Answer 2

更改

_entityList = _entityList
            .GroupBy(g => new { g.customer.customer.cust_id, g.customer.customer.cust_code, g.customer.customer.cust_name })
            .Select(g => new { cust_id = g.Key.cust_id, cust_code = g.Key.cust_code, cust_name = g.Key.cust_name })
            .ToList(); // error on this line

到

var result = _entityList
            .GroupBy(g => new { g.customer.customer.cust_id, g.customer.customer.cust_code, g.customer.customer.cust_name })
            .Select(g => new { cust_id = g.Key.cust_id, cust_code = g.Key.cust_code, cust_name = g.Key.cust_name })
            .ToList();

Answer 3

当您应用When子句时，使用类型

(cust, sa) => new { customer = cust, advice = sa }.

但是在你改成

之后

new { cust_id = g.Key.cust_id, cust_code = g.Key.cust_code, cust_name = g.Key.cust_name }

它们是编译器的不同类型。

无法将类型'System.Collections.Generic.List <anonymoustype＃1>'隐式转换为'System.Linq.IQueryable <anonymoustype＃2>'</anonymoustype＃2> </anonymoustype＃1>

3 个答案: