C++/CLI double pointer typecasting to ref IntPtr for C# access

Question

I am writing a wrapper around C DLL in C++/CLI so that it can be accessed in C# through a managed assembly. Using direct P/Invoke to access C functions via C# is not possible because C DLL raises exceptions which cannot be correctly caught using P/Invoke (exception message is lost when moving across C/C# boundary). So the idea is to create a managed CLI DLL which internally calls C DLL and wraps the exception to Exception class of CLI.

So the C DLL function has this declaration.

void InitDB(void **handle);

The C# app would required following declaration

void InitDB_cs(ref IntPtr handle);

To accomplish this I created C++/CLI function with following declaration

void InitDB_clr(IntPtr %handle);

However, I am unable to typecast ref InPtr into C function. I tried using following code but seems can't get the typecasting right.

void InitDB_clr(IntPtr %handle)
{
pin_ptr<IntPtr> ptr = handle.ToPointer();
InitDB(&ptr);
}

Error message for above code

error C2440: 'initializing' : cannot convert from 'void *' to 'cli::pin_ptr<Type>'
1>        with
1>        [
1>            Type=System::IntPtr
1>        ]
1>        Conversion from 'void*' to pointer to non-'void' requires an explicit cast
1>error C2664: 'InitDB' : cannot convert parameter 1 from 'cli::pin_ptr<Type> *' to 'void **'
1>        with
1>        [
1>            Type=System::IntPtr
1>        ]
1>        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

Sorry typing error. Its InitDB (C DLL function). Thanks for bringing to my notice. — K Singh
– K Singh, Commented Jul 24, 2011 at 23:32
No prob. I don't know CLI, but what's the error? Does it help if you cast &ptr to void**? — Kerrek SB
– Kerrek SB, Commented Jul 24, 2011 at 23:33
I tried that but it gives me typecasting error similar to above. — K Singh
– K Singh, Commented Jul 24, 2011 at 23:41
Why do you declare function InitDB_clr() with the parameter handle when you never use it? — svick
– svick, Commented Jul 24, 2011 at 23:47
I know this is not your question, but are you sure the C++/CLI wrapper will be able to handle the exceptions? No DLL should let exceptions escape. — Rudy Velthuis
– Rudy Velthuis, Commented Jul 25, 2011 at 0:28

Ben Voigt · Accepted Answer · 2011-07-25 01:13:25Z

1

That's an out parameter, not really ref, correct?

Try

void InitDB_clr(IntPtr% handle)
{
    void* ptr /* = handle.ToPointer() */;
    InitDB(&ptr);
    handle = IntPtr(ptr);
}

Uncomment the initializer if it needs to be in/out.

edited Jul 25, 2011 at 1:13

answered Jul 25, 2011 at 0:13

Ben Voigt

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4 Comments

K Singh Over a year ago

With the code above i get the following error when I compile in C++/CLI project: : error C2440: '=' : cannot convert from 'System::IntPtr ^' to 'System::IntPtr'

ildjarn Over a year ago

@KSingh : Don't use gcnew. By the way, this was already answered on the MSDN forums where you asked the same question.

K Singh Over a year ago

@ildjarn: Thanks.. I cross posted on both stackoverflow and MSDN forums. And appreciate your help, it works now :)

Ben Voigt Over a year ago

Ugh, yeah, I was thinking of C# where it's required to use new with value types. Fixed.

foxy · Accepted Answer · 2011-07-24 23:58:42Z

0

You could use C# in an unsafe context and use

unsafe void InitDB_cs(IntPtr** handle);

answered Jul 24, 2011 at 23:58

foxy

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1 Comment

K Singh Over a year ago

I dont want to have pointers/unsafe code in C#. That is one of the reason for having C++/CLI managed which will have all the functions for C# and internally interacts with C DLL.

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