即使签名匹配，也无法将一种类型的委托分配给另一种类型

发布于 2024-10-08 01:21:40 字数 340 浏览 4 评论 0原文

我病态的好奇心让我想知道为什么以下失败：

// declared somewhere
public delegate int BinaryOperation(int a, int b);

// ... in a method body
Func<int, int, int> addThem = (x, y) => x + y;

BinaryOperation b1 = addThem; // doesn't compile, and casting doesn't compile
BinaryOperation b2 = (x, y) => x + y; // compiles!

原文

My morbid curiosity has me wondering why the following fails:

// declared somewhere
public delegate int BinaryOperation(int a, int b);

// ... in a method body
Func<int, int, int> addThem = (x, y) => x + y;

BinaryOperation b1 = addThem; // doesn't compile, and casting doesn't compile
BinaryOperation b2 = (x, y) => x + y; // compiles!

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浅唱ヾ落雨殇 2024-10-15 01:21:40

C# 对“结构”类型的支持非常有限。特别是，您不能简单地从一种委托类型转换为另一种委托类型，因为它们的声明是相似的。

从语言规范来看：

C# 中的委托类型是名称
等价的，不是结构上的
相等的。具体来说，两个
不同的委托类型具有
相同的参数列表和返回类型
被视为不同的代表
类型。

尝试以下之一：

// C# 2, 3, 4 (C# 1 doesn't come into it because of generics)
BinaryOperation b1 = new BinaryOperation(addThem);

// C# 3, 4
BinaryOperation b1 = (x, y) => addThem(x, y);
var b1 = new BinaryOperation(addThem);

C# has very limited support for "structural" typing. In particular, you can't cast from one delegate-type to another simply because their declarations are similar.

From the language specification:

Delegate types in C# are name
equivalent, not structurally
equivalent. Specifically, two
different delegate types that have the
same parameter lists and return type
are considered different delegate
types.

Try one of:

// C# 2, 3, 4 (C# 1 doesn't come into it because of generics)
BinaryOperation b1 = new BinaryOperation(addThem);

// C# 3, 4
BinaryOperation b1 = (x, y) => addThem(x, y);
var b1 = new BinaryOperation(addThem);

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若沐 2024-10-15 01:21:40

这是一个类似的问题：为什么不能编译？

// declared somewhere
struct Foo {
    public int x;
    public int y;
}

struct Bar {
    public int x;
    public int y;
}

// ... in a method body
Foo item = new Foo { x = 1, y = 2 };

Bar b1 = item; // doesn't compile, and casting doesn't compile
Bar b2 = new Bar { x = 1, y = 2 }; // compiles!

在这种情况下，演员阵容不工作似乎更自然一些，但实际上是同样的原因。

Here's a similar question: why doesn't this compile?

// declared somewhere
struct Foo {
    public int x;
    public int y;
}

struct Bar {
    public int x;
    public int y;
}

// ... in a method body
Foo item = new Foo { x = 1, y = 2 };

Bar b1 = item; // doesn't compile, and casting doesn't compile
Bar b2 = new Bar { x = 1, y = 2 }; // compiles!

In this case it seems a little more natural for the cast to not work, but it's really the same reason.

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