不能隐式转换类型'System.Collections.Generic.List< AnonymousType＃1>'到'System.Linq.IQueryable< AnonymousType＃2>' [英] Cannot implicitly convert type 'System.Collections.Generic.List<AnonymousType#1>' to 'System.Linq.IQueryable<AnonymousType#2>'

问题描述

正在编译此查询，而不会出现错误：

This query is being compiled without errors:

            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();

在上述查询中添加条件where子句返回编译时类型转换错误：

While adding a conditional where clause to the above query returns compile time type conversion error:

            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

不能隐式转换类型 System。 Collections.Generic.List< AnonymousType＃1> to System.Linq.IQueryable< AnonymousType＃2>

Cannot implicitly convert type System.Collections.Generic.List<AnonymousType#1> to System.Linq.IQueryable<AnonymousType#2>

我缺少什么？

推荐答案

请考虑以下简化代码示例：

Consider the following simplified code sample:

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();

在您的第一种情况下，只涉及一种匿名类型，因此 _entityList 是列表< AnonymousType＃1> 。在第二种情况下，您可以从一个匿名类型更改返回的类型：

In your first scenario, there is only one anonymous type involved, hence _entityList is a List<AnonymousType#1>. In your 2nd scenario, you change the returned type from one anonymous type:

new { 
    customer = cust, 
    advice = sa 
}

到另一个：

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。

尝试这样：

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(); 

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