在没有泛型的情况下将A类转换为B类 [英] Cast class A to class B without generics

问题描述

我有两个彼此没有联系的班级:

I have two classes that have no connection to one another :

public class A
{
   public String Address {get;set}
}

public class B 
{
   public String Address {get;set}
}

List<A> addressList = DB.Addresses.GetAll();

我这样做

List<B> addressListOther = addressList.Cast<B>().ToList();

输出为:

其他信息:无法将类型为"A"的对象转换为类型"B".

Additional information: Unable to cast object of type 'A' to type 'B'.

任何想法如何解决?

推荐答案

您可以使用Select()代替这种方式:

You can use Select() instead of that way:

List<B> addressListOther = addressList.Select(a => new B { Address = a.Address}).ToList();

或者您可以在类B中覆盖explicit operator:

Or you can override explicit operator in class B:

public static explicit operator B(A a)  // explicit A to B conversion operator
{
    return new B { Address = a.Address };
}

然后，

List<B> addressListOther = aList.Select(a => (B)a).ToList();

---

此异常的原因:

---

The reason of this exception:

Cast将抛出InvalidCastException，因为它试图将A转换为object，然后将其转换为B:

Cast will throw InvalidCastException, because it tries to convert A to object, then cast it to B:

A myA = ...;
object myObject = myA ;
B myB= (B)myObject; // Exception will be thrown here

此异常的原因是， 装箱值 只能拆箱到 完全相同类型 .


其他信息:

The reason of this exception is, a boxed value can only be unboxed to a variable of the exact same type.


Additional Information:

这是 Cast<TResult>(this IEnumerable source) Cast<TResult>(this IEnumerable source) 方法，如果您感兴趣的话:

Here is the implemetation of the Cast<TResult>(this IEnumerable source) method, if you interested:

public static IEnumerable<TResult> Cast<TResult>(this IEnumerable source) {
    IEnumerable<TResult> typedSource = source as IEnumerable<TResult>;
    if (typedSource != null) return typedSource;
    if (source == null) throw Error.ArgumentNull("source");
    return CastIterator<TResult>(source);
}

如您所见，它返回CastIterator:

static IEnumerable<TResult> CastIterator<TResult>(IEnumerable source) {
    foreach (object obj in source) yield return (TResult)obj;
}

看上面的代码.它将使用foreach循环遍历源，并将所有项目转换为object，然后转换为(TResult).

Look at the above code. It will iterate over source with foreach loop, and converts all items to object, then to (TResult).

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