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

C# generic difference: assign a list & lt; assignment class & gt; assign a value to list & lt; base class & gt;

When using a generic list in C#, try to assign the list of derived type to the list of the type of derived type. This problem is caused by conflict between the type of safety and compilation of type safety during runtime.

Question and code example

Consider the following code:

In this example, due to the runtime check, it is effective to assign the giraffe array to the animal array. However, the distribution of List to List

will cause errors during compilation.

<code class="language-csharp">class Animal { }
class Giraffe : Animal { }

static void Main(string[] args)
{
    // 数组赋值有效，但是...
    Animal[] animals = new Giraffe[10]; 

    // 隐式转换失败
    List<Animal> animalsList = new List<Giraffe>(); //编译错误

    // 显式转换也失败
    List<Animal> animalsList2 = (List<Animal>) new List<Giraffe>(); //编译错误
}</code>

Different and security issues

This problem is related to the concept of variance in the generic type. Coordination allows the use of derived types in the base type position, and inverter allows the use of base types in the derived type position. In the above example, the List

assigning a value to List will be considered a coordinated change.

However, this assignment can cause security issues. If the new lion object is added to AnimalSList, it can be assigned to AnimalSList2 without any compilation check, even if it violates the type of animal list type safety.

solution in c#

Use the array for running during runtime:

The array supports the reference type variance during runtime, which allows assigning the derived type array to the base type array.

C# 4 The security square difference: C# 4 introduces the support for the security variance of interfaces and entrustment. Func is a coordinated change in T, and the Action

is inverted in T.

C# 2 Method: In C# 2, you can use List

.Convertall to create a new list with the required type.

Conclusion C#variance problem highlights the importance of maintaining type safety during the generic assignment. Although the number of array or runtime check can be used to achieve unsafe squares, the security variance with interfaces and entrustment can ensure the correct type of the type during compilation. For the early version of C#, you can use list

.convertall as an alternative solution.

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