Redrawing Client Area with transparency

I need to render only the Client Area of a Control (TextBox) on winforms with transparency using GDI, because GDI+ doesn't support transparency for dynamic controls. I've already searched a lot, for two days, and founded two links:

https://msdn.microsoft.com/en-us/library/windows/desktop/dd162910(v=vs.85).aspx

http://www.winprog.org/tutorial/transparency.html

I think if i join these two solutions, i can obtain what i want, but i don't know how to put them in code. Someone could help me please?

I tried use AlphaBlend, but is not working, here is my code:

protected override void WndProc(ref Message m)
    {
        switch (m.Msg)
        {
            case PranchetaApp.Prancheta.Utils.GDIWrapper.WM_PAINT:

                Bitmap memBmp = new Bitmap(this.ClientRectangle.Width, this.ClientRectangle.Height, PixelFormat.Format32bppArgb);

                IntPtr memdc = PranchetaApp.Prancheta.Utils.GDIWrapper.CreateCompatibleDC(this.CreateGraphics().GetHdc());
                PranchetaApp.Prancheta.Utils.GDIWrapper.SelectObject(memdc, memBmp.GetHbitmap());

                Graphics memDC = Graphics.FromHdc(memdc);
                Graphics clientDC = this.CreateGraphics();
                memDC.FillRectangle(new SolidBrush(Color.Blue), 0, 0, this.ClientRectangle.Width, this.ClientRectangle.Height);

                IntPtr hdc = clientDC.GetHdc();
                IntPtr hMemdc = memDC.GetHdc();
                //PranchetaApp.Prancheta.Utils.GDIWrapper.BitBlt(hdc, 0, 0, this.ClientRectangle.Width, this.ClientRectangle.Height, hMemdc, 0, 0, PranchetaApp.Prancheta.Utils.GDIWrapper.SRCCOPY);

                PranchetaApp.Prancheta.Utils.GDIWrapper.BLENDFUNCTION bfn;
                bfn.AlphaFormat = 0;
                bfn.BlendFlags = 0;
                bfn.BlendOp = 0;
                bfn.SourceConstantAlpha = 0;

                PranchetaApp.Prancheta.Utils.GDIWrapper.AlphaBlend(hdc, 0, 0, this.ClientRectangle.Width, this.ClientRectangle.Height, hMemdc, 0, 0, this.ClientRectangle.Height, this.ClientRectangle.Height, bfn);

                clientDC.ReleaseHdc(hdc);
                memDC.ReleaseHdc(hMemdc);

                break;

            default:
                base.WndProc(ref m);
                break;

        }
    }

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