WinFormsのTextBoxでfocusを失う時にIMEへの入力文字を確定させる

TextBox2.cs

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

namespace Sayuri.Windows.Forms {
    class TextBox2 : TextBox {
        [DllImport("Imm32.dll")]
        static extern IntPtr ImmGetContext(IntPtr hWnd);
        [DllImport("Imm32.dll")]
        static extern bool ImmGetOpenStatus(IntPtr hIMC);
        [DllImport("Imm32.dll")]
        static extern bool ImmNotifyIME(IntPtr hIMC, int dwAction, int dwIndex, int dwValue);
        [DllImport("Imm32.dll")]
        static extern bool ImmReleaseContext(IntPtr hWnd, IntPtr hIMC);
        const int NI_SELECTCANDIDATESTR = 0x0015;
        const int CPS_COMPLETE = 0x0001;

        protected override void OnLostFocus(EventArgs e) {
            var context = ImmGetContext(Handle);
            if (context != IntPtr.Zero) {
                if (ImmGetOpenStatus(context))
                    ImmNotifyIME(context, NI_SELECTCANDIDATESTR, CPS_COMPLETE, 0);
                ImmReleaseContext(Handle, context);
            }
            base.OnLostFocus(e);
        }
    }
}

TextBox2.fs

namespace Sayuri.Windows.Forms
open System.Runtime.InteropServices
open System.Windows.Forms

type TextBox2 () =
    inherit TextBox ()
    
    [<DllImport "Imm32.dll">]
    static extern nativeint ImmGetContext(nativeint hWnd)
    [<DllImport "Imm32.dll">]
    static extern bool ImmGetOpenStatus(nativeint hIMC)
    [<DllImport "Imm32.dll">]
    static extern bool ImmNotifyIME(nativeint hIMC, uint32 dwAction, uint32 dwIndex, uint32 dwValue)
    [<DllImport "Imm32.dll">]
    static extern bool ImmReleaseContext(nativeint hWnd, nativeint hIMC)
    [<Literal>]
    static let NI_SELECTCANDIDATESTR = 0x0015u
    [<Literal>]
    static let CPS_COMPLETE = 0x0001u

    override this.OnLostFocus e =
        let context = ImmGetContext this.Handle
        if context <> 0n then
            if ImmGetOpenStatus context then
                ImmNotifyIME(context, NI_SELECTCANDIDATESTR, CPS_COMPLETE, 0u) |> ignore
            ImmReleaseContext(this.Handle, context) |> ignore
        base.OnLostFocus e