Unmanaged code, sending text to another program

Pleased to see that you've got that to work, John :)

Sorry, I didn't know how you'd defined the P/Invoke signature for SendMessage() but I should have warned you that the fourth parameter needed to be a string.

The reason why that works is because, when you use WM_SETTEXT, the underlying code is expecting the fourth parameter to be a pointer to the null terminated string that you want to send.

Strings are, of course, already pointers in C#  (i.e. references to where the string is stored on the heap) and the P/Invoke marshaller takes care of converting the .NET length-prefixed unicode string to a zero terminated ANSI string automatically.

To send data to a TextBox in another application, I'd also try WM_SETTEXT as you can send a simple string rather than a struct:

const int WM_SETTEXT = 0x0C;
SendMessage(handle, WM_SETTEXT, 0, textToSend);

Try SendMessage WM_COPYDATA as you were going to

SendMessage(hControlDestination, WM_COPYDATA, wParam, lParam)

wParam is the handle to the window passing the control or your apps window

lParam is the COPYDATASTRUCT.

COPYDATASTRUCT contains

typedef struct tagCOPYDATASTRUCT {
    ULONG_PTR dwData;
    DWORD cbData;
    PVOID lpData;
} COPYDATASTRUCT, *PCOPYDATASTRUCT;

If you are marshaling this in C#, then use

C# Definition:

struct COPYDATASTRUCT {
    public int dwData;
    public int cbData;
    public IntPtr lpData;
}

for a string, set dwData to 0,   cbData is the length of the string + 1,  lpData is an unmanaged byte array of the characters (you'll need to marshal this from a byte array if you are doing this in C#).

If you are doing this in C++ or an unmanaged environment like Delphi, then its a bit easier.

Best,