Question

我有这堂课：

public class Product {
    [NotMapped]
    public bool Recommended {get;set;}

    public int ID {get;set;}
    //Other stuff
}

然后我从数据库中获取它们并分配推荐变量。我现在已经将它们全部设置为真，而我测试并且有效。

var Products = Find(SearchParams, DataContext).ToList();
foreach(var product in Products) {
    product.Recommended = true;
    if (!product.Recommended) throw new Exception();
}

好的，它有效，我们通过了。

然后我去做一组结果，看起来像这样：

public class QuoteProduct {
    public Product Product {get;set;}
    public Quote Quote {get;set;}
    //etc
}

分配产品：

var Results = new List<QuoteProduct>();
foreach(var product in Products) {
    var entry = new QuoteProduct(){
        Product = product;
        Quote = getQuote(this);
    }
    if (!entry.Product.Recommended) throw new Exception();
    Results.add(entry);
}

并且总是抛出异常！为什么？即使，就在我之前：

    entry.Product.Recommended = true;
    if (!entry.Product.Reccomended) throw new Exception();

Answer 1

懒惰的评估问题

Find(SearchParams, DataContext);可能会返回IEnumerable<>，因此每次迭代它时，都会生成新的Product类。这使得product.Recommended = true;无用。

这里：

var Products = Find(SearchParams, DataContext);
foreach(var product in Products)    // <--- here it's iterated and new classes instantiated
{
    product.Recommended = true;
    if (!product.Recommended) throw new Exception();
}

，此处：

var Results = new List<QuoteProduct>();
foreach(var product in Products)   // <-- and here are new classes instantiated
// So your previous `product.Recommended = true;` is gone!
{
    var entry = new QuoteProduct(){
        Product = product;
        Quote = getQuote(this);
    }
    if (!entry.Product.Recommended) throw new Exception();
    Results.add(entry);
}

您可以使用以下方法修复此问题：

// The key is to persist the data with (for example) ToArray() or ToList()
var Products = Find(SearchParams, DataContext).ToArray();

坚持不懈的缺点是，所有行都是＆＃39;被同时加载到内存中。 （这是懒惰评估有用的原因）

从数据库获取后无法分配给对象属性

1 个答案: