ref，val和out对方法参数意味着什么？

问题描述

我正在寻找一个清晰，简洁和准确的答案。

I'm looking for a clear, concise and accurate answer.

虽然可以链接到很好的解释，但实际上是实际的答案。

Ideally as the actual answer, although links to good explanations welcome.

这也适用于VB.Net ，但关键字不同- ByRef 和 ByVal 。

This also applies to VB.Net, but the keywords are different - ByRef and ByVal.

推荐答案

默认情况下（在C＃中），将对象传递给函数实际上会将引用的副本传递给该对象。更改参数本身只会更改参数中的值，而不会更改指定的变量。

By default (in C#), passing an object to a function actually passes a copy of the reference to that object. Changing the parameter itself only changes the value in the parameter, and not the variable that was specified.

void Test1(string param)
{
    param = "new value";
}

string s1 = "initial value";
Test1(s1);
// s1 == "initial value"

使用 out 或 ref 传递对函数调用中指定的变量的引用。 out 或 ref 参数的值的任何更改都将传递回调用方。

Using out or ref passes a reference to the variable specified in the call to the function. Any changes to the value of an out or ref parameter will be passed back to the caller.

out 和 ref 的行为相同，只是略有不同： ref 参数需要在调用之前进行初始化，而 out 参数可以未初始化。通过扩展，保证 ref 参数在方法开始时被初始化，而 out 参数被视为未初始化

Both out and ref behave identically except for one slight difference: ref parameters are required to be initialised before calling, while out parameters can be uninitialised. By extension, ref parameters are guaranteed to be initialised at the start of the method, while out parameters are treated as uninitialised.

void Test2(ref string param)
{
    param = "new value";
}

void Test3(out string param)
{
    // Use of param here will not compile
    param = "another value";
}

string s2 = "initial value";
string s3;
Test2(ref s2);
// s2 == "new value"
// Test2(ref s3); // Passing ref s3 will not compile
Test3(out s2);
// s2 == "another value"
Test3(out s3);
// s3 == "another value"

编辑 ：如指出， out 和 ref 之间的差异仅由C＃编译器而不是CLR强制执行。据我所知，VB没有等效于 out 并实现了 ref （如 ByRef ），与CLR的支持相匹配。

Edit: As dp points out, the difference between out and ref is only enforced by the C# compiler, not by the CLR. As far as I know, VB has no equivalent for out and implements ref (as ByRef) only, matching the support of the CLR.

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