Cannot convert type via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

问题

In C#, if I have a parameter for a function where the parameter type is of an interface, how do a pass in an object that implements the interface.

Here is an example:

The parameter for a function is as follows:

List<ICustomRequired>

The list that I already have is as follows:

List<CustomObject> exampleList

CustomObject inherits from the ICustomRequired interface

What is the correct syntax to pass the exampleList as a parameter?

This is how I thought to do the above task:

exampleList as List<ICustomRequired>

However I am getting the following error:

Cannot convert type via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

Thanks

回答1:

You cannot cast a List of one type to a List of a different type.

And if you think about it, you would be glad that you can't. Imagine the havoc you could cause if it was possible:

 interface ICustomRequired
 {
 }

 class ImplementationOne : ICustomRequired
 {
 }

 class ImplementationTwo: ICustomRequired
 {
 }

 var listOne = new List<ImplementationOne>();
 var castReference = listOne as List<ICustomRequired>();
 // Because you did a cast, the two instances would point
 // to the same in-memory object

 // Now I can do this....
 castReference.Add(new ImplementationTwo());

 // listOne was constructed as a list of ImplementationOne objects,
 // but I just managed to insert an object of a different type

Note, however, that this line of code is legal:

 exampleList as IEnumerable<ICustomRequired>;

This would be safe, because IEnumerable does not provide you with any means to add new objects.

IEnumerable<T> is actually defined as IEnumerable<out t>, which means the type parameter is Covariant.

Are you able to change the parameter of the function to IEnumerable<ICustomRequired>?

Otherwise your only option will be to create a new List.

var newList = (exampleList as IEnumerable<ICustomRequired>).ToList();

var newList = exampleList.Cast<ICustomRequired>().ToList();

回答2:

You can't do that, you have to convert list

exampleList.Cast<ICustomRequired>().ToList();

回答3:

In additional to List.Cast, C#'s generics provide a good support for Covariance and contravariance. This example causes it to work in the way I think you originally intended.

public class Program
{
    public static void Main()
    {
         Foo(new List<Fim>());
    }

    public static void Foo<T>(List<T> bar) where T : IFim
    {
        throw new NotImplementedException();
    }

    public class IFim{}
    public class Fim : IFim{}
}

来源：https://stackoverflow.com/questions/30888325/cannot-convert-type-via-a-reference-conversion-boxing-conversion-unboxing-conv

标签

interface

type-conversion