Why Can't I Assign a List to a List in C#?

C# In object -oriented programming, the derived class inheritance is very common. However, when processing the set (such as a list), why it is not possible to use the same base class to declare them, which may be confusing.

Consider the following code:

This code will cause compilation errors when trying to use

<code class="language-csharp">interface A { }

class B : A { }

class C : B { }

class Test
{
    static void Main(string[] args)
    {
        A a = new C(); // 正确
        List<A> listOfA = new List<C>(); // 编译错误
    }
}</code>

. Although the instance of the derived class C can be assigned to the base class variable A, the collection is not good. List<C> List<A> The reason for this limit lies in the method of the C# initialization list. When creating , C# will generate the internal data structure of the list when running, and it is expected that the storage type is an object of A. However, because C is a derivative class of A, its object has additional members and occupies more memory than A object. Therefore, assigning to

will cause the memory address to be dislocated, which may cause memory errors.

List<A> In order to solve this problem and change List<C> as List<A>, you can use the following methods:

<.> 1. Expressing conversion: List<派生类> List<基类>

traversing the list and explicitly converting each element into a base class:

<.> 2. Linq:

Use linq to convert the element as follows:

<code class="language-csharp">List<A> listOfA = new List<C>().ConvertAll(x => (A)x);</code>

These methods allow safe conversion and memory address alignment to ensure the normal operation of the list.

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