运营商'??'不能应用于'T'和'T'类型的操作数 [英] Operator '??' cannot be applied to operands of type 'T' and 'T'

问题描述

我有以下的泛型方法，但VS给了我一个编译错误。 （运算符'??'不能应用于'T'和'T'类型的操作数）

  public static T Method< T（T模型）其中T：new（）
 {
 var m = model？新的T（）; 
} 
  

有人知道为什么吗？

编辑：是否有可能原因是T可以是一个结构在我的情况，并且一个结构是一个非空类型？

解决方案

您应该添加 class 约束：

$ b

  public static T Method< T>（T model）其中T：class，new（）
 {
 var m = model ??新T（）; 
 
返回m; 
} 
  

您应该返回 m class 约束是因为T可以是一个值类型，如 int ，并且自 ?? 操作符（null-coalescing）检查类型可以为空，所以我们必须添加 class 约束来排除值类型。

编辑：Alvin Wong's答案也涵盖了可空类型的情况;这实际上是结构，但可以是操作数？运营商。请注意，如果没有Alvin的重载版本，对于可为空的类型， Method 将返回 null 。

I have the following generic method, but VS gives me a compile error on that. (Operator '??' cannot be applied to operands of type 'T' and 'T')

public static T Method<T>(T model) where T : new()
{
    var m = model ?? new T();
}

Does some one have any idea why?

Edit: Is it possible the reason is that T can be a struct in my case, and a struct is an non-nullable type?

解决方案

You should add class constraint:

public static T Method<T>(T model) where T : class, new()
{
    var m = model ?? new T();

    return m;
}

And you should return m too!

Note: As @KristofDegrave mentioned in his comment, the reason that we have to add class constraint is because T can be a value type, like int and since ?? operator (null-coalescing) check on types that can be null, so we have to add class constraint to exclude value types.

Edit: Alvin Wong's answer covered the case for nullable types too; which are structs actually, but can be operands of ?? operator. Just be aware that Method would return null without Alvin's overloaded version, for nullable types.

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