尝试返回IQueryable< MyType>时发生转换错误 [英] Casting errors when attempting to return an IQueryable<MyType>

问题描述

我有一个查询，应该返回一个IQueryable<MyType>.代码如下:

I have a query that ought to return an IQueryable<MyType>. The code looks like this:

public IQueryable<MyType> GetFooList()
{
    var query = (from x in dbContext.TableX
                 join y in dbContext.TableY on x.our_id equals y.our_id 
                 join z in dbContext.TableZ on y.our_id equals z.our_id 
                 join a in dbContext.TableA on z.other_id equals a.other_id 
                 where !string.IsNullOrEmpty(x.status)
                 select new
                 {
                   (fields....)
                 })
                 .AsQueryable();
    IQueryable<MyType> result = (IQueryable<MyType>) query;
    return result;
}

在调用控制器操作中，我要过滤此列表以获取运行时指定的值；在各种调用操作之间，要过滤的参数将有所不同.例如:

In the calling controller actions, I want to filter this list for values specified at run time; the parameters to filter for will differ between the various calling actions. E.g.:

List<MyType> FooList = Interface.GetFooList()
    .Where( specific conditions )
    .ToList();

在行设置result上，引发异常:

On the line setting result, an exception gets raised:

无效的强制转换异常未由用户代码处理

无法转换类型的对象 'System.Data.Entity.Infrastructure.DbQuery'1 [<> f__AnonymousType9'9 [System.String，System.Nullable`1 [System.DateTime]，System.String，System.String，System.String，System.String ，System.Int32，System.Nullable'1 [System.DateTime]，System.Nullable'1 [System.DateTime]]]' 键入"System.Linq.IQueryable'1 [MyType]".

Unable to cast object of type 'System.Data.Entity.Infrastructure.DbQuery'1[<>f__AnonymousType9'9[System.String,System.Nullable`1[System.DateTime],System.String,System.String,System.String,System.String,System.Int32,System.Nullable'1[System.DateTime],System.Nullable'1[System.DateTime]]]' to type 'System.Linq.IQueryable'1[MyType]'.

所以我认为这是一个转换问题，并在调用AsQueryable()之前添加了.Cast<MyType>().这会产生另一个错误:

So I figured this was a casting problem, and added .Cast<MyType>() before the call to AsQueryable(). This generates a different error:

无法将类型匿名类型"转换为类型"MyType". LINQ至 实体仅支持强制转换EDM基本类型或枚举类型.

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

如果我不进行任何强制转换，则会在调用操作而不是Entity Frameworks访问器中引发这些错误.

And if I don't do any casting, these errors get raised in the calling actions instead of the Entity Frameworks accessor.

我尝试了所有链接的类似问题"中的建议，但无济于事-错误不断出现.我什至尝试包括.Select(obj => new MyType() {fields...} )来摆脱匿名类型.那也不起作用.

I've tried the suggestions in all the linked "Similar Questions", to no avail -- the errors keep going back and forth. I even tried including .Select(obj => new MyType() {fields...} ) to get away from the anonymous type. That didn't work either.

我感觉自己似乎遗漏了一些明显的东西.

I feel like I'm missing something subtly obvious.

编辑后添加

我更新了代码以选择一种类型:select new MyType() {fields...}.这工作正常.然后，调用方法在我过滤结果并从查询中列出的行上抛出了 NotSupportedException :

I updated the code to select a type: select new MyType() {fields...}. This worked properly. Then the calling method threw a NotSupportedException, on the line where I filter the results and make a list from the query:

实体或复杂类型'MyType'不能在 LINQ to Entities查询.

The entity or complex type 'MyType' cannot be constructed in a LINQ to Entities query.

ETA2

我将EF表属性复制到一个新类MyTypeDTO.我用MyTypeDTO替换了对MyType的所有使用.我收到此错误:

I copied the EF table properties to a new class, MyTypeDTO. I replaced all use of MyType with MyTypeDTO. I got this error:

LINQ to Entities不支持指定的类型成员'our_id'.仅支持初始化程序，实体成员和实体导航属性.

The specified type member 'our_id' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

这是DTO中的财产:

public int our_id { get; set; }

因此，我删除了获取/设置，重建并重新运行.不，我遇到了同样的错误.

So I removed the get/set, rebuilt, and reran. Nope, I got the same error.

推荐答案

问题非常难以捉摸.查询的匿名类型中的字段之一在MyType中不存在.我在创建用于此方法的视图模型时发现了这一点.

The problem was incredibly subtle. One of the fields in the query's anonymous type did not exist in MyType. I discovered this while I was creating a view model to use in this method.

就其价值而言，视图模型可以正常工作.例如:

For what it's worth, the view model works just fine. Eg:

public IQueryable<MyViewModel> GetFooList(int parameter)
{
    var query = (from x in dbContext.TableX
                 join y in dbContext.TableY on x.our_id equals y.our_id
                 join z in dbContext.TableZ on y.our_id equals z.our_id
                 join a in dbContext.TableA on z.other_id  equals a.other_id 
                 where x.their_id == parameter
                 select new MyViewModel()
                 {
                    field1 = x.field1,
                    field2 = y.field2,
                    field3 = z.field3 //<= this field did not exist in MyType
                 }
                 ).AsQueryable();

    return query;
}

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