Pass a function pointer from C++ to be called by C# - Arguments of functions include a wide char string (LPCWSTR)

Question

I am writing a C# library to be used by native C++ application. I am using C++/CLI as the Interoperability mechanisim.

I require to pass a callback function from C++ to C# (using C++/CLI as the intermediate layer). C# library needs to call the C++ function with a zero terminated string of wide characters; i.e. the prototype of the callback function is

Func(LPCWSTR pszString);

There are other parameters but they are immaterial for this discussion.

I searched net and found Marshal.GetDelegateForFunctionPointer Method wich I can use. The problem with this is that it converts System.String from C# to char* and not wchar_t* which I am looking for.

Also, what is the best method of achieving this- code example including the C++/CLI portion, if possible. C++/CLI dll is dependent on C# dll. Method needs to be called synchronously.

Cory Nelson · Accepted Answer · 2011-07-22 03:56:23Z

9

GetDelegateForFunctionPointer will work, but you need to add a [MarshalAs(UnmanagedType.LPWStr)] attribute to the parameter in your delegate declaration in order for String to get converted into wchar_t*:

delegate void MyDelegate([MarshalAs(UnmanagedType.LPWStr)] string foo)

IntPtr func = ...;
MyDelegate del = (MyDelegate)Marshal.GetDelegateForFunctionPointer(func,
                                 typeof(MyDelegate));

To pass a modifiable string, give a StringBuilder. You need to explicitly reserve space for the unmanaged function to work with:

delegate void MyDelegate([MarshalAs(UnmanagedType.LPWStr)] StringBuilder foo)

StringBuilder sb = new StringBuilder(64); // reserve 64 characters.

del(sb);

edited Jul 22, 2011 at 3:56

answered Jun 8, 2011 at 16:52

Cory Nelson

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7 Comments

David Heffernan Over a year ago

+1 Out of curiosity, is there no way to declare a delegate to have Charset=Charset.Unicode like you do with DLLImport?

Anil Padia Over a year ago

Hi Cory. Many thanks. Your answer has actually solved my problem. How can I achieve the following second requirement: delegate void MyDelegate([MarshalAs(UnmanagedType.LPWStr)] string foo, out string val1, out int val2); In other words, C# calls the callback, giving a string parameter, and in C++, based on that string parameters, val1 and val2 need to be passed back to C#

Ben Voigt Over a year ago

@Anil: For LPSTR and LPWSTR output parameters in p/invoke, use StringBuilder.

Anil Padia Over a year ago

Hi Ben, can you please provide an example as Cory did. I am new to C# and C++/CLI world.

user541686 Over a year ago

@David: Yes there actually is, see my answer. :-)

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user541686 · Accepted Answer · 2011-07-22 03:59:50Z

2

See the little-known UnmanagedFunctionPointer attribute, which is like DllImport for delegates, if you'd like to use CharSet or whatnot.

answered Jul 22, 2011 at 3:59

user541686

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