I have a C++ struct
struct UnmanagedStruct { char* s; }; and a C# struct
struct ManagedStruct { string s; } How can I marshal the UnmanagedStruct? Do I need to use a StringBuilder?
the C++ library exposes UnmanagedStruct getStruct();
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1 Answer
Edit & Correction: For return values of p/invoke calls, the "normal" method doesn't work. I was so used to the normal, ref, and out behaviors related to method parameters that I assumed return values would work in a similar fashion. Here is the link for a solution to the return value problem:
PInvoke error when marshalling struct with a string in it
You only need to use a StringBuilder if you are passing the struct to the C++ method as a byref parameter and the string is a buffer that the method will alter. For a return value, you just need to specify the type of string, which in this case is:
struct ManagedStruct { [MarshalAs(UnmanagedType.Lpstr)] string s; } Rememer to add a property to expose the string, since s is private here (which is OK, fields should be private).
6 Comments
Pavel Minaev
There's nothing wrong with making fields public in structs that are used solely for P/Invoke purposes. All accessors would be trivial anyway, and this would not ever change in the future, so why write more code?
DevDevDev
I tried that but I keep getting Method's type signature is not PInvoke compatible
DevDevDev
Reading the Microsoft help it says "ANSI strings must be marshaled using an IntPtr and passed as an array of bytes."
Pavel Minaev
@SteveM, what help? Link, please. Most likely it's for some corner case, not applicable here.
DevDevDev
@Pavel here is the link msdn.microsoft.com/en-us/library/ms172514.aspx Also I import the DLL like this [DllImport("SomeDLL.dll", CharSet = CharSet.Ansi)] static extern ManagedStruct function( string input ); And inside the DLL the function is extern "C" UnmanagedStruct __declspec(dllexport) function( char* input ); Thanks again for your help.
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