C# program calls vc++ function which has BSTR as input parameter

Question

From C# program, I am calling VC++ function which has BSTR as input parameter. I am not getting any return value. My c# application crashes.

C# : SampleCS,

var value = object.GetValue(10, "TEST");

VC++ : SampleCPP,

GetValue(long index, BSTR key, double * Value) { CString str = key; .. .. }

Any idea if I am missing anything or doing anything wrong?

Note : If I call the GetValue function of VC++ from VB6 then it works properly.

it works from VB6 because VB6 used BSTR which is different than the .NET strings. See http://msdn.microsoft.com/en-us/library/windows/desktop/ms221069(v=vs.85).aspx — Ňɏssa Pøngjǣrdenlarp, Jun 27 '14 at 15:03
Yes, you are right. But how should I pass string value from C#, Do I need to format it before passing? — magg, Jun 27 '14 at 15:21
since the c++ thing appears to have been written specifically for VB6, why not change **it** to now work with .NET strings? see http://stackoverflow.com/q/2380594/1070452 — Ňɏssa Pøngjǣrdenlarp, Jun 27 '14 at 15:27

score 1 · Answer 1 · answered Jun 27 '14 at 16:30

The Marshal class has methods for working with BSTRs from managed code. Try something like this:

// Note that the BSTR parameter in C++ becomes
// an IntPtr in this PInvoke declaration.
[DllImport("your.dll")]
private void GetValue(long index, IntPtr key, ref double value);

To use this method:

double result = 0;
var stringValue = "Foo";
var bstrValue = Marshal.StringToBSTR(stringValue);
try
{
    GetValue(0, bstrValue, ref result);
}
finally
{
    Marshal.FreeBSTR(bstrValue);
}

`private void GetValue(long index, [MarshalAs(UnmanagedType.BStr)]string key, ref double value);` should also work. — Chris, Jun 27 '14 at 16:37

C# program calls vc++ function which has BSTR as input parameter

1 Answers1