如何将可空类型隐式转换为不可空类型

Question

我有一个可为空的 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;
        }
    }
}

扩展方法隐式转换string?为string但它不适用于其他基本类型，例如int?orbool?

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?and bool?？

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

Answer 1

您可以通过为不可空引用类型提供一种扩展方法并为非托管（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;
        }
    }
}

像这样使用：

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();