How to make autocomplete on a TextBox show suggestions when empty
Solution 1:
Take into account that this is a hack. I managed to solve that problem and the lack of API functionality doing a trivial and nasty thing. I'll show you this with code:
dim source as AutoCompleteStringCollection = new AutoCompleteStringColection()
dim values() as String = new String() {" Monday", _
" Tuesday", _
" Wednesday", _
" Thursday", _
" Friday", _
" Saturday", _
" Sunday" }
TextBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend
TextBox1.AutoCompleteSource = AutoCompleteSource.CustomSource
TextBox1.AutoCompleteCustomSource = source
That is, prepend a whitespace to every string in the autocomplete list. Then, it's your knowlodge about that fact and use it for your convenient objective.
For example, you could add a whitespace in the TextBox when clicked, focused, etc. (Note that this could be done with any character. The idea is to know that every string in the autocomplete list begins with the same character)
You MUST be aware of that. In fact, consider extending TextBox form and manage the correct trimming of the inputed string.
Again, called this recommended or not is at your own decision. What this answer tends to do, is to solve the problem of wanting a TextBox drops down a suggestion list without starting typing with the restrictions of the API, also called, a workaround or ugly-hack.
Solution 2:
Solution to this is to wrap and call the AutoSuggest WinShell functions directly. Then you can enable this feature without too many headaches (and even get a few extra ones for free).
Below is a working example that allows you to show the AutoSuggest list even though nothing is typed in the box (when user presses up/down arrows)
Same code as in this Gist: https://gist.github.com/sverrirs/e6a64faaab341882c3e801792f5e87ae
public void EnableAutoSuggest(TextBox tb, string[] suggestions) {
// Try to enable a more advanced settings for AutoComplete via the WinShell interface
try {
var source = new SourceCustomList() {StringList = suggestions.ToArray()};
// For options descriptions see:
// https://docs.microsoft.com/en-us/windows/desktop/api/shldisp/ne-shldisp-_tagautocompleteoptions
var options = AUTOCOMPLETEOPTIONS.ACO_UPDOWNKEYDROPSLIST | AUTOCOMPLETEOPTIONS.ACO_USETAB |
AUTOCOMPLETEOPTIONS.ACO_AUTOAPPEND | AUTOCOMPLETEOPTIONS.ACO_AUTOSUGGEST;
AutoCompleteExt.Enable(tb.Handle, source, options);
}
catch (Exception) {
// Incase of an error, let's fall back to the default
var source = new AutoCompleteStringCollection();
source.AddRange(suggestions);
tb.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
tb.AutoCompleteSource = AutoCompleteSource.CustomSource;
tb.AutoCompleteCustomSource = source;
}
}
public void DisableAutoSuggest(TextBox tb) {
tb.AutoCompleteCustomSource = null;
AutoCompleteExt.Disable(tb.Handle);
}
And the implementation of AutoCompleteExt
using System;
using System.Runtime.InteropServices;
/// <summary>
/// From: https://www.codeproject.com/Articles/3792/C-does-Shell-Part-4
/// Note: The UCOMIEnumString interface is deprecated in .NET as of 2018!
/// </summary>
public class AutoCompleteExt {
public static Guid CLSID_AutoComplete = new Guid("{00BB2763-6A77-11D0-A535-00C04FD7D062}");
private static object GetAutoComplete()
{
return Activator.CreateInstance(Type.GetTypeFromCLSID(CLSID_AutoComplete));
}
public static void Enable(IntPtr controlHandle, SourceCustomList items, AUTOCOMPLETEOPTIONS options) {
if (controlHandle == IntPtr.Zero)
return;
IAutoComplete2 iac = null;
try {
iac = (IAutoComplete2) GetAutoComplete();
int ret = iac.Init(controlHandle, items, "", "");
ret = iac.SetOptions(options);
ret = iac.Enable(true);
}
finally {
if (iac != null)
Marshal.ReleaseComObject(iac);
}
}
public static void Disable(IntPtr controlHandle) {
if (controlHandle == IntPtr.Zero)
return;
IAutoComplete2 iac = null;
try {
iac = (IAutoComplete2) GetAutoComplete();
iac.Enable(false);
}
finally {
if (iac != null)
Marshal.ReleaseComObject(iac);
}
}
}
/// <summary>
/// From https://www.pinvoke.net/default.aspx/Interfaces.IAutoComplete2
/// </summary>
[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("EAC04BC0-3791-11D2-BB95-0060977B464C")]
public interface IAutoComplete2 {
[PreserveSig]
int Init(
// Handle to the window for the system edit control that is to
// have autocompletion enabled.
IntPtr hwndEdit,
// Pointer to the IUnknown interface of the string list object that
// is responsible for generating candidates for the completed
// string. The object must expose an IEnumString interface.
[MarshalAs(UnmanagedType.IUnknown)] object punkACL,
// Pointer to an optional null-terminated Unicode string that gives
// the registry path, including the value name, where the format
// string is stored as a REG_SZ value. The autocomplete object
// first looks for the path under HKEY_CURRENT_USER . If it fails,
// it then tries HKEY_LOCAL_MACHINE . For a discussion of the
// format string, see the definition of pwszQuickComplete.
[MarshalAs(UnmanagedType.LPWStr)] string pwszRegKeyPath,
// Pointer to an optional string that specifies the format to be
// used if the user enters some text and presses CTRL+ENTER. Set
// this parameter to NULL to disable quick completion. Otherwise,
// the autocomplete object treats pwszQuickComplete as a sprintf
// format string, and the text in the edit box as its associated
// argument, to produce a new string. For example, set
// pwszQuickComplete to "http://www. %s.com/". When a user enters
// "MyURL" into the edit box and presses CTRL+ENTER, the text in
// the edit box is updated to "http://www.MyURL.com/".
[MarshalAs(UnmanagedType.LPWStr)] string pwszQuickComplete
);
// Enables or disables autocompletion.
[PreserveSig]
int Enable(bool value);
// Sets the current autocomplete options.
[PreserveSig]
int SetOptions(AUTOCOMPLETEOPTIONS dwFlag);
// Retrieves the current autocomplete options.
[PreserveSig]
int GetOptions(out AUTOCOMPLETEOPTIONS pdwFlag);
}
/// <summary>
/// Specifies values used by IAutoComplete2::GetOptions and
/// "IAutoComplete2.SetOptions" for options surrounding autocomplete.
/// </summary>
/// <remarks>
/// [AUTOCOMPLETEOPTIONS Enumerated Type ()]
/// http://msdn.microsoft.com/en-us/library/bb762479.aspx
/// </remarks>
[Flags]
public enum AUTOCOMPLETEOPTIONS {
/// <summary>Do not autocomplete.</summary>
ACO_NONE = 0x0000,
/// <summary>Enable the autosuggest drop-down list.</summary>
ACO_AUTOSUGGEST = 0x0001,
/// <summary>Enable autoappend.</summary>
ACO_AUTOAPPEND = 0x0002,
/// <summary>Add a search item to the list of
/// completed strings. When the user selects
/// this item, it launches a search engine.</summary>
ACO_SEARCH = 0x0004,
/// <summary>Do not match common prefixes, such as
/// "www." or "http://".</summary>
ACO_FILTERPREFIXES = 0x0008,
/// <summary>Use the TAB key to select an
/// item from the drop-down list.</summary>
ACO_USETAB = 0x0010,
/// <summary>Use the UP ARROW and DOWN ARROW keys to
/// display the autosuggest drop-down list.</summary>
ACO_UPDOWNKEYDROPSLIST = 0x0020,
/// <summary>Normal windows display text left-to-right
/// (LTR). Windows can be mirrored to display languages
/// such as Hebrew or Arabic that read right-to-left (RTL).
/// Typically, control text is displayed in the same
/// direction as the text in its parent window. If
/// ACO_RTLREADING is set, the text reads in the opposite
/// direction from the text in the parent window.</summary>
ACO_RTLREADING = 0x0040,
/// <summary>[Windows Vista and later]. If set, the
/// autocompleted suggestion is treated as a phrase
/// for search purposes. The suggestion, Microsoft
/// Office, would be treated as "Microsoft Office"
/// (where both Microsoft AND Office must appear in
/// the search results).</summary>
ACO_WORD_FILTER = 0x0080,
/// <summary>[Windows Vista and later]. Disable prefix
/// filtering when displaying the autosuggest dropdown.
/// Always display all suggestions.</summary>
ACO_NOPREFIXFILTERING = 0x0100
}
/// <summary>
/// Implements the https://docs.microsoft.com/en-us/windows/desktop/api/objidl/nn-objidl-ienumstring
/// interface for autoccomplete
/// </summary>
public class SourceCustomList : UCOMIEnumString {
public string[] StringList;
private int currentPosition = 0;
public int Next(
int celt, // Number of elements being requested.
string[] rgelt, // Array of size celt (or larger) of the
// elements of interest. The type of this
// parameter depends on the item being
// enumerated.
out int pceltFetched) // Pointer to the number of elements actually
// supplied in rgelt. The Caller can pass
// in NULL if celt is 1.
{
pceltFetched = 0;
while ((currentPosition <= StringList.Length-1) && (pceltFetched<celt))
{
rgelt[pceltFetched] = StringList[currentPosition];
pceltFetched++;
currentPosition++;
}
if (pceltFetched == celt)
return 0; // S_OK;
else
return 1; // S_FALSE;
}
/// <summary>
/// This method skips the next specified number of elements in the enumeration sequence.
/// </summary>
/// <param name="celt"></param>
/// <returns></returns>
public int Skip(
int celt) // Number of elements to be skipped.
{
currentPosition += celt;
if (currentPosition <= StringList.Length-1)
return 0;
else
return 1;
}
// This method resets the enumeration sequence to the beginning.
public Int32 Reset()
{
currentPosition = 0;
return 0;
}
// This method creates another enumerator that contains the same enumeration
// state as the current one. Using this function, a client can record a
// particular point in the enumeration sequence and return to that point at a
// later time. The new enumerator supports the same interface as the original one.
public void Clone(
out UCOMIEnumString ppenum) // Address of the IEnumString pointer
// variable that receives the interface
// pointer to the enumeration object. If
// the method is unsuccessful, the value
// of this output variable is undefined.
{
SourceCustomList clone = new SourceCustomList {
currentPosition = currentPosition,
StringList = (String[]) StringList.Clone()
};
ppenum = clone;
}
}