你是正确的，在语义上，ref同时提供“in”和“out”功能，而out只提供“out”功能。有一些事情需要考虑:

out requires that the method accepting the parameter MUST, at some point before returning, assign a value to the variable. You find this pattern in some of the key/value data storage classes like Dictionary<K,V>, where you have functions like TryGetValue. This function takes an out parameter that holds what the value will be if retrieved. It wouldn't make sense for the caller to pass a value into this function, so out is used to guarantee that some value will be in the variable after the call, even if it isn't "real" data (in the case of TryGetValue where the key isn't present). out and ref parameters are marshaled differently when dealing with interop code

Also, as an aside, it's important to note that while reference types and value types differ in the nature of their value, every variable in your application points to a location of memory that holds a value, even for reference types. It just happens that, with reference types, the value contained in that location of memory is another memory location. When you pass values to a function (or do any other variable assignment), the value of that variable is copied into the other variable. For value types, that means that the entire content of the type is copied. For reference types, that means that the memory location is copied. Either way, it does create a copy of the data contained in the variable. The only real relevance that this holds deals with assignment semantics; when assigning a variable or passing by value (the default), when a new assignment is made to the original (or new) variable, it does not affect the other variable. In the case of reference types, yes, changes made to the instance are available on both sides, but that's because the actual variable is just a pointer to another memory location; the content of the variable--the memory location--didn't actually change.

与ref关键字一起传递表示原始变量和函数参数实际上都指向相同的内存位置。同样，这只影响赋值语义。如果将一个新值赋给其中一个变量，那么由于其他变量指向相同的内存位置，新值将反映在另一侧。

在c#中如何使用in或out或ref ?

All keywords in C# have the same functionality but with some boundaries. in arguments cannot be modified by the called method. ref arguments may be modified. ref must be initialized before being used by caller it can be read and updated in the method. out arguments must be modified by the caller. out arguments must be initialized in the method Variables passed as in arguments must be initialized before being passed in a method call. However, the called method may not assign a value or modify the argument.

以下类型的方法不能使用in、ref和out关键字:

Async方法，通过使用Async修饰符来定义。 迭代器方法，其中包括yield return或yield break语句。

只是在OP的注释中澄清一下，对ref和out的使用是“对在方法外部声明的值类型或结构的引用”，这已经在incorrect中建立了。

考虑在StringBuilder上使用ref，它是一种引用类型:

private void Nullify(StringBuilder sb, string message)
{
    sb.Append(message);
    sb = null;
}

// -- snip --

StringBuilder sb = new StringBuilder();
string message = "Hi Guy";
Nullify(sb, message);
System.Console.WriteLine(sb.ToString());

// Output
// Hi Guy

与此相反:

private void Nullify(ref StringBuilder sb, string message)
{
    sb.Append(message);
    sb = null;
}

// -- snip --

StringBuilder sb = new StringBuilder();
string message = "Hi Guy";
Nullify(ref sb, message);
System.Console.WriteLine(sb.ToString());

// Output
// NullReferenceException

作为ref传递的参数必须在传递给方法之前初始化，而out形参在传递给方法之前不需要初始化。

使用out表示该参数未被使用，仅被设置。这有助于调用者理解您总是在初始化参数。

此外，ref和out不仅适用于值类型。它们还允许您重置引用类型从方法中引用的对象。

下面是我从这篇关于c# Out Vs Ref的codeproject文章中摘录的一些注释

只有当我们期望从一个函数或方法得到多个输出时，才应该使用它。对结构的思考也是一个不错的选择。 REF和OUT是关键字，它们指示数据如何从调用方传递到被调用方，反之亦然。 在REF中，数据以两种方式传递。从呼叫者到被呼叫者，反之亦然。 In Out数据仅以一种方式从被调用方传递给调用方。在这种情况下，如果Caller试图向被调用方发送数据，它将被忽略/拒绝。

如果你是一个视觉人，那么请看看这个yourtube视频，它演示了实际的区别https://www.youtube.com/watch?v=lYdcY5zulXA

下图更直观地显示了差异