如何将可空类型隐式转换为不可空类型

Question

我有一个可以为空的 c# 10 .net 6 项目，其扩展方法为 ThrowIfNull

using System;
using System.Runtime.CompilerServices;

#nullable enable
public static class NullExtensions
{
    public static T ThrowIfNull<T>(
        this T? argument, 
        string? message = default, 
        [CallerArgumentExpression("argument")] string? paramName = default
    )
    {
        if (argument is null)
        {
            throw new ArgumentNullException(paramName, message);
        }
        else
        {
            return argument;
        }
    }
}

扩展方法将string?隐式转换为string，但它不适用于int?或bool?等其他原始类型

public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
        
        string? foo = "foo";
        string nonNullableFoo = foo.ThrowIfNull();  // success from "string?" to "string"
        Console.WriteLine(nonNullableFoo);
        
        bool? baz = true;
        bool nonNullableBaz = baz.ThrowIfNull();    // success from "string?" to "string"
        Console.WriteLine(nonNullableFoo);
        
        int? bar = 2;
        int nonNullableBar = bar.ThrowIfNull(); // error: Cannot implicitly convert type 'int?' to 'int'
        Console.WriteLine(nonNullableBar);
    }
}

如何使扩展隐式转换int? 和bool?？

这是完整的 dotnet fiddle https://dotnetfiddle.net/LiQ8NL

Answer 1

使用空合并运算符??

要将可空值分配给非空变量，请考虑以下代码：

int? value = 28;
int result = value ?? -1;
Console.WriteLine($"The result is {result}");

• 输出：The result is 28

int? value = null;
int result = value ?? -1;
Console.WriteLine($"The result is {result}");

• 输出：The result is -1

---

编辑代码

重新排列代码如下。因此bool?可以隐式使用该类型：

int? bar = 2;
int nonNullableBar = bar ?? -1; 
Console.WriteLine(nonNullableBar);

bool? baz = true;
bool nonNullableBaz = false;
        
if (baz == true){ 
    nonNullableBaz = true;
} 
else if(baz == false){ 
    nonNullableBaz = false;
} 
else {
    /* Something */
}

---

参考文献

• Nullable Value Types
• Boolean Logical Operators

Answer 2

您可以通过为不可为空的引用类型提供一种扩展方法和为非托管（例如 int、bool、...）类型提供另一种扩展方法来实现您的目标。请注意，非托管类型需要强制转换。

public static class NullExtensions
{
    public static T ThrowIfNull<T>(
        this T? argument,
        string? message = default,
        [CallerArgumentExpression("argument")] string? paramName = default
    ) where T : notnull
    {
        if (argument is null)
        {
            throw new ArgumentNullException(paramName, message);
        }
        else
        {
            return argument;
        }
    }

    public static T ThrowIfNull<T>(
        this T? argument,
        string? message = default,
        [CallerArgumentExpression("argument")] string? paramName = default
    ) where T : unmanaged
    {
        if (argument is null)
        {
            throw new ArgumentNullException(paramName, message);
        }
        else
        {
            return (T)argument;
        }
    }
}

这样使用：

int? foo = 42;
int bar = foo.ThrowIfNull();
Console.WriteLine(bar);

string? baz = "Hello";
string quus = baz.ThrowIfNull();
Console.WriteLine(quus);

// Comment out either this
baz = null;
quus = baz.ThrowIfNull();
// Or this
foo = null;
bar = foo.ThrowIfNull();

Answer 3

在 MS dotnet docs https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters 中找到一种可能的选项

约束 "where T : struct" 类型参数必须是不可为空的值类型

同样根据这个答案https://stackoverflow.com/a/8745492/2877168 不可能使string? 和所有其他值类型工作。所以唯一的办法就是定义两个扩展方法。这是适用于string? 和int?、bool? 等的更新版本

public static class NullExtensions
{
    public static T ThrowIfNull<T>(this T? argument, string? message = default, [CallerArgumentExpression("argument")] string? paramName = default)
        where T : struct
    {
        if (argument is null)
            throw new ArgumentNullException(paramName, message);
        return (T)argument;
    }

    public static string ThrowIfNull(this string? argument, string? message = default, [CallerArgumentExpression("argument")] string? paramName = default)
    {
        if (argument is null)
            throw new ArgumentNullException(paramName, message);
        return argument;
    }
}

此版本的工作版本位于 https://dotnetfiddle.net/uBX1w6