为什么不能将通用类型转换为null? [英] Why can't I cast generic-type with null?

问题描述

一个类似问题，但是而不使用T参数.

A similar question, but without casting the T parameter.

这不起作用

public T GetValueG<T>(string Query)
{
    object value = DBNull.Value;    // Read from Database which can return DBNull
    if (value is DBNull)
        return (T)null;
    else
        return (T)value;
}

错误CS0037无法将null转换为'T'，因为它是不可为空的值类型

Error CS0037 Cannot convert null to 'T' because it is a non-nullable value type

但这可行

public T GetValueG<T>(string Query)
{
    object value = DBNull.Value;    // Read from Database which can return DBNull
    if (value is DBNull)
        value = null;

    return (T)value;
}

我知道使用约束类型where T: struct将使T能够直接使用null进行强制转换.

I know that using constraint type where T: struct will make T to be able to be cast with null directly.

但是在第二种方法中，为什么它接受100％可以为null的对象?为什么我不能在没有对象或任何东西的情况下直接返回null?

But in the second approach, why does it accept an object which is 100% can be null? Why can't I return null directly without an object or something either?

推荐答案

您尝试执行的操作是错误的.整数是不可为null的类型，这意味着您不能将null作为int的值或任何其他不可为null的类型返回(如名称所示).

What you're trying to do is wrong. Integers are non-nullable types, meaning you can't return null as a value of int or any other not nullable type (as the name suggests...)

例如:

int myInt = null;-您的IDE会立即将其标志为integer(非空类型).

int myInt = null; - your IDE will right away flag it up with integer being a non-nullable type.

您想要做的是使用您要使用的实际类型的可空类型，例如int?或bool?.

What you want to do is use a nullable type of the actual type you're trying to use e.g. int? or bool?.

您应该返回null或default(T)而不是没有意义的(T)null，因为您不能将null转换为例如一个类类型，它将抛出Object Reference Not Set To An Instance....

You should be returning null or default(T) rather than (T)null that doesn't make sense, as you can't cast null to e.g. a class type, it'll throw Object Reference Not Set To An Instance....

我会将您的功能更改为以下内容:

I would change your function to the following:

public T GetValueG<T>(string Query)
{

    object value = DBNull.Value;    // Read from Database which can return DBNull
    if (value is DBNull)
        return default(T);  // `return default;` also valid since C# 7.1 

    return (T)value;
}

如果数据库中的值可以为null，则您的模型应该实现该类型的nullable，以避免以后产生异常.

If a value can be null in your database, then your model should be implementing a nullable of that type to avoid casting exceptions later down the line.

您的函数应这样调用GetValueG<int?>("<query>");.

可以找到非空值的默认值

Default values for non-nullables can be found here.

在代码中，您将执行myNullableInt.HasValue，而不是使用myInt == null检查空值.

In your code, rather than checking for null values using myInt == null you'd do myNullableInt.HasValue.

摘要

默认关键字.默认情况下将为可为null的类型返回null. 以及其他非空类型的默认值.根据OP的需要.

The default keyword. will return null for nullable types by default. and default values for other non-nullable types. as OP needs.

default(bool?)->空

default(bool)->假

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