Question

我收到编译错误，“无法隐式转换类型'System.Collections.ObjectModel.ObservableCollection到ProddataRecsObservable'。存在显式转换”请参阅以下代码段中的注释。

//I created a custom class called ProddataRecsObservable derived from
//ObservableCollection<Proddata> so I can do some special CRUD operations for my production
//data records.

public class ProddataRecsObservable : ObservableCollection<Proddata>
{

}        

//I have another class which maps an object to a reader and the function MappAll returns an 
//Observable collection of type <T>.

public abstract class MapperBase<T>
{
    protected abstract T Map(IDataRecord record);

    public ObservableCollection<T> Mapall(IDataReader reader)
    {
        ObservableCollection<T> collection = new ObservableCollection<T>();

        while (reader.Read())
        {
            try
            {
                collection.Add(Map(reader));
            }
            catch
            {
                throw;
            }
        }

        return collection;
    }
}

//I have another class derived from MapperBase called ProddataMapper.

public class ProddataMapper : WTS.Data.Mapper.MapperBase<Proddata>
{
    protected override Proddata Map(System.Data.IDataRecord record)
    {
        Proddata p = new Proddata();    

        p.PSTAT = (DBNull.Value == record["PSTAT"]) ? "" : record["PSTAT"].ToString();

 return p;
    }
}

//In my calling code, I attempt to set a ProddataRecsObservable variable equal to the 
//result of MapAll() from my ProddataMapper class but get the compile error. The error even 
//tells me they are the same type which is strange. How can I get around this?

//Calling Code:

ProddataMapper prodMapper = new ProddataMapper(); 
ProddataRecsObservable recs = prodMapper.Mapall(ProddataReader); //ERROR'S HERE <-

Answer 1

我没有看到它告诉你类型完全相同。 ProddataRecsObservable与ObservableCollection<Proddata>不同 - 是什么让你认为它们属于同一类型？第一种类型派生自第二种类型 - 它不会使它们成为相同的类型。

由于继承关系，ProddataRecsObservable的每个实例都是ObservableCollection<Proddata>的实例，但反之则不然。

你在做什么相当于：

class Test : object {}

object Foo()
{
    return new object();
}
...
Test t = Foo();

你希望它能起作用吗？如果是这样，你为什么期望它起作用？编译器如何知道Foo将实际返回Test的实例（实际上它不在此处 - 您的方法也不会返回您的实例衍生类）？如果您不希望希望它能够正常工作，那么为什么您希望您的示例代码能够正常工作？

Answer 2

ObservableCollection<Proddata>无法升级到ProddataRecsObservable，ProddataRecsObservable中ObservableCollection<Prodddata>还有其他逻辑{{1}}不知道。

Answer 3

在基类中创建MapAll（）抽象，然后在ProdDataMapper中提供覆盖：

public override ProddataRecsObservable Mapall(IDataReader reader)
{
   // do the downcast explicitly
    return (ProddataRecsObservable) base.MapAll(reader);
}

Answer 4

John Skeet是对的。尝试在得到错误的分配中将结果显式地转换为ProddataRecsObservable。

为什么我不能将从ObservableCollection <proddata>派生的对象设置为等于以ObservableCollection <proddata>返回的对象？</proddata> </proddata>

4 个答案: