Easiest way to pass modifyable System.String as modifyable LPTSTR ?

Question

BOOL PathFindOnPath( LPTSTR pszFile, LPCTSTR *ppszOtherDirs );

I am calling this API from managed C++. My pszFile is in a System.String.

What is the easiest way to pass this as LPTSTR? (Considering its an inout parameter)

I have tried pin_ptr and interior_ptr but neither seems to be accepted by the compiler.

Answer 1

You need to marshal across a (pre-allocated) StringBuilder instead of a String reference. For details, see this MSDN article on Marshaling.

Answer 2

Strings are immutable, you cannot pass it directly, even if you pin it. More seriously, you will have to deal with the possibility that the function returns a larger string. The function is unsafe by design since you cannot prevent it from returning a path string that's too large. Not much you can do about that I suppose, but you will have to use a buffer that's at least large enough for common path strings. This code will get the job done:

#include <vcclr.h>
...
    String^ file = "blah.txt";
    wchar_t path[_MAX_PATH];
    {
        pin_ptr<const wchar_t> wch = PtrToStringChars(file);
        wcscpy_s(path, _MAX_PATH, wch);
    }
    BOOL ok = PathFindOnPath(path, something);

The curly braces look odd perhaps, it ensures that the managed string doesn't stay pinned for too long.