C＃ - 为什么我需要初始化[Out]参数

Question

I have a couple of methods imported from a native .dll, using the following syntax:

internal static class DllClass {
    [DllImport("Example.dll", EntryPoint = "ExampleFunction")]
    public static extern int ExampleFunction([Out] ExampleStruct param);
}

Now, because I specified param as [Out] , I would expect at least one of the following snippets to be valid:

ExampleStruct s;
DllCass.ExampleFunction(s);

ExampleStruct s;
DllCass.ExampleFunction([Out] s);

ExampleStruct s;
DllCass.ExampleFunction(out s);

However, none of them works. The only way I found to make it work was by initializing s.

ExampleStruct s = new ExampleStruct();
DllCass.ExampleFunction(s);

I have managed to fix that by rewriting the first snippet to the following code, but that feels kinda redundant.

internal static class DllClass {
    [DllImport("Example.dll", EntryPoint = "ExampleFunction")]
    public static extern int ExampleFunction([Out] out ExampleClass param);
}

I've read What's the difference between [Out] and out in C#? and because the accepted answer states that [Out] and out are equivalent in the context, it left me wondering why it didn't work for me and if my "solution" is appropriate.

Should I use both? Should I use only out ? Should I use only [Out] ?

Answer 1

The OutAttribute determines the runtime behavior of the parameter, but it has no impact at compile time . The out keyword is required if you want to use compile-time semantics.

Using just the out keyword will change the runtime marshaling, so the OutAttribute is optional. See this answer for more info.

Answer 2

Are you initializing it to a value inside the function? That's where the requirement comes in: out parameters must be assigned to at least once within the function which declares the parameter.