disable key event when the focus is on autocompletion box of textbox

Question

In my project there is a Form mainForm in which there are two textBoxes txtUserName and txtPassword and also a button btnLogin

Answer 1

You should not be handling the Enter key at all. You can remove your KeyDown handler, and instead use the AcceptButton property of the form to set the button that gets "clicked" when Enter is pressed. This is already supposed to not "click" the button when another control has already handled the Enter key.

That isn't enough for your situation, because standard Windows behaviour is for the Enter key to press the default button. Press Win+R, for example, to get the Run... dialog, start typing C:\Use, press Down to select C:\Users, press Enter, and see what happens.

In order to override that behaviour, you need to make the text box tell the form that it will be handling the Enter key itself, so that the form won't send it to the default button. This can be done by creating a derived class and overriding IsInputKey:

public class MyTextBox : TextBox
{
    protected override bool IsInputKey(Keys keyData)
    {
        return base.IsInputKey(keyData) || ((keyData & ~Keys.Shift) == Keys.Enter && IsDroppedDown);
    }
}

However, TextBox implements autocompletion using the SHAutoComplete function, which automatically creates an IAutoComplete object behind the scenes. That object cannot be accessed, and because of that, the IsDroppedDown property that I used in IsInputKey cannot be created. It would be implemented using IAutoCompleteDropDown.GetDropDownStatus, but since the object is inaccessible, you cannot (reliably) determine whether the dropdown list is showing.

You would need to either implement the auto completion without using the built-in AutoComplete* properties, or you would need to always suppress the Enter key (remove the && IsDroppedDown in the above IsInputKey).

Update: here's how to create an IAutoComplete object manually. The strings Administrator and Clerk are hardcoded. The GetDropDownStatus function is used to suppress any default button's handling of Enter when the drop down list is visible. Feedback welcome.

IAutoComplete.cs:

using System;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

[ComImport]
[Guid("00bb2762-6a77-11d0-a535-00c04fd7d062")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[CoClass(typeof(IAutoCompleteClass))]
interface IAutoComplete
{
    void Init(HandleRef hwndEdit, IEnumString punkACL, string pwszRegKeyPath, string pwszQuickComplete);
    void Enable(bool fEnable);
}

IAutoComplete2.cs:

using System;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

[Guid("EAC04BC0-3791-11d2-BB95-0060977B464C")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IAutoComplete2
{
    void Init(HandleRef hwndEdit, IEnumString punkACL, string pwszRegKeyPath, string pwszQuickComplete);
    void Enable(bool fEnable);
    void SetOptions(AutoCompleteOptions dwFlag);
    AutoCompleteOptions GetOptions();
};

AutoCompleteOptions.cs:

using System;

[Flags]
enum AutoCompleteOptions : int
{
    None = 0x00,
    AutoSuggest = 0x01,
    AutoAppend = 0x02,
    Search = 0x04,
    FilterPrefixes = 0x08,
    UseTab = 0x10,
    UpDownKeyDropsList = 0x20,
    RtlReading = 0x40,
    WordFilter = 0x80,
    NoPrefixFiltering = 0x100,
}

IAutoCompleteDropDown.cs:

using System;
using System.Runtime.InteropServices;
using System.Text;

[Guid("3CD141F4-3C6A-11d2-BCAA-00C04FD929DB")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
interface IAutoCompleteDropDown
{
    void GetDropDownStatus(out AutoCompleteDropDownFlags dwFlags, out StringBuilder wszString);
    void ResetEnumerator();
}

AutoCompleteDropDownFlags.cs:

using System;

[Flags]
enum AutoCompleteDropDownFlags : int
{
    None = 0x00,
    Visible = 0x01
}

IAutoCompleteClass.cs:

using System;
using System.Runtime.InteropServices;

[ComImport]
[Guid("00BB2763-6A77-11D0-A535-00C04FD7D062")]
class IAutoCompleteClass
{
}

EnumString.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

class EnumString : IEnumString
{
    const int E_INVALIDARG = unchecked((int)0x80070057);
    const int S_OK = 0;
    const int S_FALSE = 1;

    int current;
    string[] strings;

    public EnumString(IEnumerable strings)
    {
        this.current = 0;
        this.strings = strings.ToArray();
    }

    public void Clone(out IEnumString ppenum)
    {
        ppenum = new EnumString(strings);
    }

    public int Next(int celt, string[] rgelt, IntPtr pceltFetched)
    {
        if (celt < 0)
            return E_INVALIDARG;

        int num = 0;
        while (current < strings.Length && celt != 0)
        {
            rgelt[num] = strings[current];
            current++;
            num++;
            celt--;
        }

        if (pceltFetched != IntPtr.Zero)
            Marshal.WriteInt32(pceltFetched, num);

        if (celt != 0)
            return S_FALSE;

        return S_OK;
    }

    public void Reset()
    {
        current = 0;
    }

    public int Skip(int celt)
    {
        if (celt < 0)
            return E_INVALIDARG;

        if (strings.Length - current > celt)
        {
            current = strings.Length;
            return S_FALSE;
        }

        current += celt;
        return S_OK;
    }
}

MyTextBox.cs:

using System;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

public class MyTextBox : TextBox
{
    IAutoComplete2 autoComplete;
    IAutoCompleteDropDown autoCompleteDropDown;

    public bool IsDroppedDown
    {
        get
        {
            if (autoCompleteDropDown == null)
                return false;

            AutoCompleteDropDownFlags dwFlags;
            StringBuilder wszString;
            autoCompleteDropDown.GetDropDownStatus(out dwFlags, out wszString);
            return (dwFlags & AutoCompleteDropDownFlags.Visible) != AutoCompleteDropDownFlags.None;
        }
    }

    protected override void CreateHandle()
    {
        base.CreateHandle();

        autoComplete = (IAutoComplete2)new IAutoComplete();
        autoCompleteDropDown = (IAutoCompleteDropDown)autoComplete;
        autoComplete.SetOptions(AutoCompleteOptions.AutoSuggest);
        autoComplete.Init(new HandleRef(this, this.Handle), new EnumString(new string[] { "Administrator", "Clerk" }), null, null);
    }

    protected override void DestroyHandle()
    {
        ReleaseAutoComplete();
        base.DestroyHandle();
    }

    protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            ReleaseAutoComplete();
        }
        base.Dispose(disposing);
    }

    protected override bool IsInputKey(Keys keyData)
    {
        return base.IsInputKey(keyData) || ((keyData & ~Keys.Shift) == Keys.Enter && IsDroppedDown);
    }

    void ReleaseAutoComplete()
    {
        if (autoComplete != null)
        {
            Marshal.ReleaseComObject(autoComplete);
            autoComplete = null;
            autoCompleteDropDown = null;
        }
    }
}