Auto-complete edit and combo, without Platform SDK

Introduction

Here's a quick article that shows how you can add the handy auto-completion effect that you see in the Win2K file-open dialog for your own edits and combo boxes - without having to download the entire Platform SDK (9 hours on a 56K modem!). If you already have the Platform SDK, this article is probably useless to you.

Demo

I'm not providing any demo project here, the code is small enough that you can just copy and paste (one function!).

Some constants

These would be defined in the Platform SDK, but since we're not using that, I'll include these here. You'll have to copy these to your app.

// See the MSDN for SHAutoComplete for a full 
// description of what these options do.
#define SHACF_DEFAULT                   0x00000000
#define SHACF_FILESYSTEM                0x00000001
#define SHACF_URLALL                    (SHACF_URLHISTORY | SHACF_URLMRU)
#define SHACF_URLHISTORY                0x00000002
#define SHACF_URLMRU                    0x00000004
#define SHACF_USETAB                    0x00000008
#define SHACF_FILESYS_ONLY              0x00000010

#define SHACF_AUTOSUGGEST_FORCE_ON      0x10000000
#define SHACF_AUTOSUGGEST_FORCE_OFF     0x20000000
#define SHACF_AUTOAPPEND_FORCE_ON       0x40000000
#define SHACF_AUTOAPPEND_FORCE_OFF      0x80000000

The simple wrapper function

typedef HRESULT (WINAPI *SHAUTOCOMPLETEFN) 
    (HWND hTarget, DWORD dwFlags);
static HINSTANCE hSHLWAPIDLL = NULL;

bool SetupForAutoComplete(HWND hTarget, DWORD dwFlags)
{
    if (hTarget == NULL)
    {
        return false;
    }

    bool ok = false;

    if (hSHLWAPIDLL == NULL)
    {
        hSHLWAPIDLL= LoadLibrary("SHLWAPI.DLL");
        if (hSHLWAPIDLL== NULL)
        {
            return false;
        }
    }

    SHAUTOCOMPLETEFN pSHAC = 
        (SHAUTOCOMPLETEFN)GetProcAddress(hSHLWAPIDLL, 
        "SHAutoComplete");

    if (pSHAC != NULL)
    {
        ok = SUCCEEDED(pSHAC(hTarget, dwFlags));
    }

    return ok;
}

Using it

First, make sure you've called ::CoInitialize(NULL); in your CWinApp's InitInstance. Without this, the call to SHAutoComplete will fail. You also need to call CoUninitialize(); in your CWinApp's ExitInstance. This initializes COM for your app.

I'd also recommend that you call FreeLibrary(hSHLWAPIDLL) in your CWinApp::ExitInstance(), just to be nice to the OS.

Now the good stuff

If you have an an edit control in your dialog class that's hooked up to a member variable, all you have to do is:

BOOL CMyDialog::OnInitDialog()
{
...
    // enable auto complete for m_myEdit
    SetupForAutoComplete(m_myEdit.m_hWnd, 
        SHACF_FILESYSTEM);
...
}

And that's it. Now, when you start typing a path, the edit control will drop down a list box with paths that match what you've typed so far - just like the file open dialog.

If you want to do this to a combo box, it's slightly more complicated:

BOOL CMyDialog::OnInitDialog()
{
...
    // the edit control of a CComboBox is always at dlg ID 1001
    CEdit * pMyEdit = (CEdit *)m_myCombo.GetDlgItem(1001);
    if (pMyEdit)
    {
        // enable auto complete for m_myCombo
        SetupForAutoComplete(pMyEdit->m_hWnd, SHACF_FILESYSTEM);
    }
...
}

And that's that.

Concerns

The heart of this whole thing is SHAutoComplete. This is a function that lives in Shlwapi.DLL and is only available on Win2K or on machines with IE 5.0 or higher installed. But, you can safely call my SetupForAutoComplete function on machines that don't meet those requirements because the function checks for the DLL and then for the particular function. If neither are there, the function leaves gracefully.

More info

SHAutoComplete can do more than file system auto-complete. It can also do URL auto-complete (like the navigation bar in IE), and it can do it all with various other fun options. See the MSDN for SHAutoComplete for a full description of what these options are.

And that's that. Enjoy responsibly. Oh yeah, thanks Michael Dunn, for your invaluable input :)