How does [in, out] work with dllimport?

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What is [In,Out] exactly doing in the following example?

[DllImport("DllName.dll")]
public static extern int MethodFromDll([In, Out] char[] session_file, [In, Out] char[] phrase, [In] char[] indexbuild, [In, Out] char[] result);

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Posted 14-Mar-16 2:27am

Krati Garg

Updated 14-Mar-16 6:51am

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Richard MacCutchan 14-Mar-16 9:56am

It just means that the parameter is used to pass information to the called method, and receive inforation from it. It only has meaning to the compiler.

2 solutions

Solution 1

These are attributes telling the operating system that the parameter following the attribute is an input as well as an output parameter. In some cases, you need only one of them; and in some other cases you need both. To decide whether you need either of these attributes, you should check MSDN for the particular function you are using.

For example, consider the
[GetWindowPlacement] function, which is declared as:

C++

BOOL WINAPI GetWindowPlacement(
  _In_    HWND            hWnd,
  _Inout_ WINDOWPLACEMENT *lpwndpl
);

So it is easy to see that the lpwndpl parameter needs to be declared in your code with both of these parameters.

Posted 14-Mar-16 6:37am

Hiske Bekkering

Comments

Richard MacCutchan 14-Mar-16 12:50pm

This is nothing to do with the operating system, they are compiler directives.

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Matt T Heffron · Accepted Answer · 2016-03-14T06:51:00

These attributes are used to tell the compiler what data marshaling is required across the call, from the Managed to Unmanaged code and/or back (or across COM calls).
[In] -- The InAttribute Class[^] Indicates that data should be marshaled from the caller to the callee.
[Out] -- The OutAttribute Class[^] Indicates that data should be marshaled from callee back to caller.
and [In, Out] does what you would expect; it indicates that data should be marshaled in both directions.
See the Remarks in the above links.