conversion from BSTR to string

Hello,

Example from C++ side:
interface declaration

interface ISampleInterface:public IUnknown
{
    STDMETHOD(put_Value)(BSTR _value) = 0;
    STDMETHOD(get_Value)(BSTR * _value) = 0;
};

Class example which provide that functionality:

class CTest : public ISampleInterface
{
	// skipped IUnknown
private:
	WCHAR m_szString[1024];
public:
	CTest() {wcscpy_s(m_szString,L"");}
public:
	STDMETHODIMP put_Value(BSTR _value);
	{
		if (!_value || !wcslen(_value)) return E_INVALIDARG;
		wcscpy_s(m_szString,_value);
		return NOERROR;
	}
        STDMETHODIMP get_Value(BSTR * _value)
	{
		if (!_value) return E_INVALIDARG;
		*_value = SysAllocString(m_szString);
		return NOERROR;
	}
};

Example on C# side with specify marshaling type:
Here is type specified so .NET will do marshaling stuff in RCW.

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ISampleInterface
{
    [PreserveSig]
    int put_Value([In, MarshalAs(UnmanagedType.BStr)] string _value);

    [PreserveSig]
    int get_Value([Out, MarshalAs(UnmanagedType.BStr)] out string _value);
}

Example on C# side without marshaling:
Here you directly perform marshaling type.

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ISampleInterface
{
    [PreserveSig]
    int put_Value(IntPtr _value);

    [PreserveSig]
    int get_Value(out IntPtr _value);
}

For such declaration you should have code:

// initialized somewhere
ISampleInterface _sample;
// Put value
{
    IntPtr _ptr = Marshal.StringToBSTR("some string");
    _sample.put_Value(_ptr);
    Marshal.FreeBSTR(_ptr);
}
// Get value
{
    IntPtr _ptr;
    _sample.get_Value(out _ptr);
    string _result = Marshal.PtrToStringBSTR(_ptr);
    Marshal.FreeBSTR(_ptr);
}

Regards,
Maxim.