如何创建 Nullable<T>从类型变量? [英] How to create a Nullable&lt;T&gt; from a Type variable?

问题描述

我正在处理表达式，我需要一个方法来接收某种类型(目前未知)的对象.像这样:

I'm working with expressions and I need a method which receives an object of some type (currently unknown). Something like this:

public static void Foobar(object Meh) { }

我需要的是让这个方法返回一个 Nullable 版本的 Meh，但是类型 T 来自 Meh.GetType().所以返回结果是 Nullable，其中 MehType 是 Meh 的类型.

What I need to is make this method return a Nullable<T> version of Meh, but the type T is from Meh.GetType(). So the return would be Nullable<MehType>, where MehType is the type of Meh.

有什么想法或建议吗?

谢谢

更新:我需要这个的原因是因为这个例外:

Update: the reason why I needed this is because of this exception:

未为类型System.Nullable`1[System.Int32]"和System.Int32"定义二元运算符 Equal.

return Expression.Equal(leftExpr, rightExpr);

其中 leftExpr 是一个 System.Nullable1[[System.Int32 并且 rightExpr 是一个 System.Int32.

where leftExpr is a System.Nullable1[[System.Int32 and rightExpr is a System.Int32.

推荐答案

如果你在编译时不知道类型，只能用 object 来表达 - 并且尽快你装箱一个可为空的值类型，你最终得到一个空引用，或者一个装箱的不可空值类型.

If you don't know the type at compile time, the only way of expressing it is as object - and as soon as you box a nullable value type, you end up with either a null reference, or a boxed non-nullable value type.

所以这些片段在结果方面完全相同:

So these snippets are exactly equivalent in terms of the results:

int? nullable = 3;
object result = nullable;

int nonNullable = 3;
object result = nonNullable;

换句话说，我认为你无法真正表达你想要做什么.

In other words, I don't think you can really express what you're trying to do.

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