Windows Forms: How to create overlay modal popup in C#

Creating a simple UI as shows below allows you to open a dialog box, then overlay form.

C# windows form overlay

Creating a custom panel control allows you to overlay form as shown below.

public class GlassyPanel : Panel
{
    const int WS_EX_TRANSPARENT = 0x20;

    int opacity = 70;

    public int Opacity
    {
        get
        {
            return opacity;
        }
        set
        {
            if (value < 0 || value > 100) throw new ArgumentException("Value must be between 0 and 100");
            opacity = value;
        }
    }

    protected override CreateParams CreateParams
    {
        get
        {
            var cp = base.CreateParams;
            cp.ExStyle = cp.ExStyle | WS_EX_TRANSPARENT;
            return cp;
        }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        using (var b = new SolidBrush(Color.FromArgb(opacity * 255 / 100, Color.LightGray)))
        {
            e.Graphics.FillRectangle(b, ClientRectangle);
        }
        base.OnPaint(e);
    }
}

Next, Modify your form constructor as shown below.

public frmOverlayModal()
{
    InitializeComponent();
    panel = new GlassyPanel();
    panel.Dock = DockStyle.Fill;
    this.Controls.Add(panel);
    panel.Hide();
    panel.SendToBack();
}

Adding a click event handler to the Open button allows you to show an open dialog box with overlay form.

private void btnOpen_Click(object sender, EventArgs e)
{
    panel.BringToFront();
    panel.Visible = true;

    using (OpenFileDialog openFileDialog = new OpenFileDialog())
    {
        if (openFileDialog.ShowDialog(this) == DialogResult.OK)
        {
            //handle your data
        }
    }
    panel.Hide();
    panel.SendToBack();
}