Add support for tasks to ListViewGroup

• You can set the LVGF_TASK flag on mask and set pszTask to a valid string and cchTask to the length of the string

Proposed API

This would add the following APIs to ListView and ListViewGroup. I modelled these after the ColumnClick event args/handler/method

public delegate void GroupLinkClickEventHandler(object sender, GroupLinkClickEventArgs e);
public class GroupLinkClickEventArgs : EventArgs
{
    public GroupLinkClickEventArgs(int column) { }

    public int GroupLink { get; }
}
public class ListView
{
    ...
    public event GroupLinkClickEventHandler GroupLinkClick { add; remove; }
    protected void OnGroupLinkClick(GroupEventArgs e) { }
    ...
}
public class ListViewGroup
{
    ...
    public string Task { get; set; }
    ...
}

Example

var listView = new ListView();
var group = new ListViewGroup
{
    Task = "Task"
};
listView.Groups.Add(group);
listView.GroupLinkClick += (sender, e) => MessageBox.Show("Link Clicked");

E.g.

[hughbe]

In my local implementation I went with the LVN_LINKCLICK. Although this would add more structs to the internal interop, this is no harm - they’re not exposed and relatively simple.

I think making the distinction between the header and the task itself is important so I think LVN_LINKCLICK is the way to go

I agree

Example LVN_CLICKED in WndProc

case (int)User32.WindowMessage.WM_REFLECT_NOTIFY:
    User32.NMHDR* nmhdr = (User32.NMHDR*)m.LParam;
    switch (nmhdr->code)
    {
        case (int)LVN.LINKCLICK:
            NMLVLINK* link = (NMLVLINK*)m.LParam;
            string task = "Clicked";
            var group = new LVGROUPW
            {
                cbSize = (uint)sizeof(LVGROUPW),
                mask = LVGF.TASK,
                cchTask = (uint)task.Length
            };
            fixed (char* pTask = "Clicked")
            {
                group.pszTask = pTask;
                User32.SendMessageW(Handle, (User32.WindowMessage)LVM.SETGROUPINFO, (IntPtr)link->iSubItem, ref group);
                MessageBox.Show("Link Clicked");
            }
            break;

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

internal static partial class Interop
{
    internal static partial class ComCtl32
    {
        public struct NMLVLINK
        {
            public User32.NMHDR nmhdr;
            public LITEM link;
            public int iItem;
            public int iSubItem;
        }
    }
}

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

internal static partial class Interop
{
    internal static partial class ComCtl32
    {
        public unsafe struct LITEM
        {
            private const int MAX_LINKID_TEXT = 48;
            private const int L_MAX_URL_LENGTH = 2048 + 32 + 3;

            public LIF mask;
            public int iLink;
            public LIS state;
            public LIS stateMask;
            public fixed char szID[MAX_LINKID_TEXT];
            public fixed char szUrl[L_MAX_URL_LENGTH];
        }
    }
}

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;

internal static partial class Interop
{
    internal static partial class ComCtl32
    {
        [Flags]
        public enum LIF : uint
        {
            ITEMINDEX = 0x00000001,
            STATE = 0x00000002,
            ITEMID = 0x00000004,
            URL = 0x00000008,
        }
    }
}

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;

internal static partial class Interop
{
    internal static partial class ComCtl32
    {
        [Flags]
        public enum LIS : uint
        {
            FOCUSED = 0x00000001,
            ENABLED = 0x00000002,
            VISITED = 0x00000004,
            HOTTRACK = 0x00000008,
            DEFAULTCOLORS = 0x00000010,
        }
    }
}

[JeremyKuhne]

I think making the distinction between the header and the task itself is important so I think LVN_LINKCLICK is the way to go

I agree as well