c# - 即使签名匹配，也无法将一种类型的委托(delegate)分配给另一种

Question

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!

Answer 1

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(delegate)类型转换为另一种委托(delegate)类型，因为它们的声明是相似的。

来自语言规范:

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.

尝试其中之一:

// 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);