My Development Notes

COM Interop

• Simply add reference to COM dll in add references dialog
• If need to use a type in a COM library, use "tlbimp comassembly.dll" generates a .NET assembly with the type.
• Win32 functions return false if an error occured, then use Marshal.GetLastWin32Error() to get the error code

Or

[DllImport("user32.dll"[, EntryPoint])]
private static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);

MessageBox(new IntPtr(0), "Hello World", "My popup", 0);

//Callback required
public delegate bool CallBack(int hwnd, int lParam); 

[DllImport("user32"]
public static extern int EnumWindows(CallBack x, int y); 

EnumWindows(new CallBack(Report), 0); 

public static bool Report(int hwnd, int lParam) {
    ...
    return true; 
}

DllImport Parameters

Exposing .NET types to COM

• All methods, properties, fields etc must be public.
• Put [ComVisible(false)] above members to be hidden from COM
• Default constructor with no parameters
• No abstract types
• Avoid static methods
• Include ApplicationException.HRESULT error codes in custom exception classes (.NET exceptions i.e IOExecption automatically map to appropriate HRESULT i.e COR_E_IO HRESULT)
• "tlbexp NETAssembly.dll" to generate a COM assembly.
• Or register the assembly for COM either through VS.NET (tick Register For COM Interop) in Output section, or use regasm NETAssembly.dll

Using non-default marshaling:

[return: MarshalAs(UnmanagedType.LPWStr)]
public string MyMethod(MarshalAs([UnmanagedType.LPWStr)] string myString) {}