C# Web API LINQ 实体查询 - 将 AnonymousType 转换为类对象

Question

我有以下代码:

public List<Product> ListAll()
{
    List<Product> products = new List<Product>();

    var db_products = db.Products
        .Where(p => p.Enabled == true)
        .OrderBy(p => p.Name)
        .Select(p => new
        {
            ProductId = p.ProductId,
            Name = p.Name,
            ...
        })
        .ToList();

    foreach (var dbP in db_products)
    {
        Product p = new Product();
        p.ProductId = dbP.ProductId;
        p.Name = dbP.Name;
        ...
        products.Add(p);
    }

    return products;
}

它如我所愿地工作，因为它成功地返回了一个产品对象列表。不过，有没有没有 foreach 循环的方法，所以我可以立即转换它？

我试过:

public List<Product> ListAll()
{
    List<Product> products = db.Products
        .Where(p => p.Visible == true)
        .OrderBy(p => p.Name)
        .Select(p => new
        {
            ProductId = p.ProductId,
            Name = p.Name,
            ...
        })
        .AsEnumerable()
        .Cast<Product>()
        .ToList();

    return products;
}

和

public List<Product> ListAll()
{
    List<Product> products = db.Products
        .Where(p => p.Visible == true)
        .OrderBy(p => p.Name)
        .Select(p => new Product
        {
            ProductId = p.ProductId,
            Name = p.Name,
            ...
        })
        .ToList();

    return products;
}

但两者都不起作用。对于第二个，我收到以下错误:

at System.Data.Objects.ELinq.ExpressionConverter.CheckInitializerType(Type type)
   at System.Data.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate(ExpressionConverter parent, MemberInitExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
   at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Objects.ELinq.ExpressionConverter.Convert()
   at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Data.Entity.Internal.Linq.InternalQuery`1.GetEnumerator()
   at System.Data.Entity.Infrastructure.DbQuery`1.System.Collections.Generic.IEnumerable<TResult>.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at SeashellBrawlCorvee...ListAll() in c:\Users\...\Documents\Visual Studio 2012\Projects\seashell-brawl-corvee\seashell-brawl-corvee\...\ProductController.cs:line 149
   at SeashellBrawlCorvee...ProductsRepository..ctor() in c:\Users\...\Documents\Visual Studio 2012\Projects\seashell-brawl-corvee\seashell-brawl-corvee\...\ProductsRepository.cs:line 21
   at SeashellBrawlCorvee...ProductsController..cctor() in c:\Users\...\Documents\Visual Studio 2012\Projects\seashell-brawl-corvee\seashell-brawl-corvee\...\ProductsController.cs:line 16

如果有人知道解决方案，我将不胜感激，否则我将坚持使用 foreach 循环。

预先感谢您的回复。

Answer 1

当你从一个ORM调用一个方法时，它可以返回一个代理，所以，当web api需要反序列化它时，你会遇到很多问题。

一种方法是创建一个 ViewModel .例如，使用您需要的信息创建一个类:

public class ProductViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }
    // properties
}

当您查询数据时，尝试返回 List<ProductViewModel> , 例如:

return db.Products
    .Where(p => p.Visible == true)
    .OrderBy(p => p.Name)
    .Select(p => new ProductViewModel() { Id = p.ProductId, Name = p.Name })
    .ToList();

使用 ViewModel暴露于 View或 Request是一个很好的实践。