无法将一种类型转换为另一种类型错误 [英] cannot convert one type to another type error

问题描述

我有一个接口和一个实现该接口的类，如下所示:

I have a interface and a class which implements that interface as below:

public interface Grid_Result
{
}
public partial class Contract_GetSubsetList_Result : Grid_Result
{
}

然后有一个我调用的方法以返回IEnumerable的Grid_Result项:

I then have a method which I call to return an IEnumerable of Grid_Result items:

public IEnumerable<Grid_Result> GetModelForGrid(string fitlerBy, int filterType, int startIndex, int pageSize){...}

所有这些都可以正常工作，但是当我尝试使用以下方法调用我的方法时:

All this works fine, but when I try to call my method with:

List<Contract_GetSubsetList_Result> results = testcontroller.GetModelForGrid(regNumFilter, filterBy,
            startIndex, pageSize).ToList();

我得到一个错误:

Cannot implicitly convert type 'System.Collections.Generic.List<Test.Grid_Result>' to 'System.Collections.Generic.List<Test.Contract_GetSubsetList_Result>'

我不确定为什么，因为Contract_GetSubsetList_Result确实实现了Grid_Result接口.

I'm not sure why, as Contract_GetSubsetList_Result does implement the Grid_Result interface.

推荐答案

因为 IEnumerable< T> 是协变的，而 List< T> 不是，这意味着您可以t将 List< T> 强制转换为 List< {T的超类}>

Because IEnumerable<T> is covariant but List<T> is not, meaning you can't cast a List<T> to a List<{superclass of T}>

您可以使用 Cast< T> 在创建列表之前投射:

You can use Cast<T> to cast before the list is created:

List<Contract_GetSubsetList_Result> results = 
    testcontroller.GetModelForGrid(regNumFilter, filterBy, startIndex, pageSize)
                  .Cast<Contract_GetSubsetList_Result> // if you KNOW all objects will be of type Contract_GetSubsetList_Result
                  .ToList();

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