Draw semi transparent overlay image all over the windows form having some controls

This is going to require another form that you display on top of the existing one. Its Opacity property can create the intended effect. Add a new class to your project and paste the code shown below. Call the Close() method to remove the effect again.

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

class Plexiglass : Form {
    public Plexiglass(Form tocover) {
        this.BackColor = Color.DarkGray;
        this.Opacity = 0.30;      // Tweak as desired
        this.FormBorderStyle = FormBorderStyle.None;
        this.ControlBox = false;
        this.ShowInTaskbar = false;
        this.StartPosition = FormStartPosition.Manual;
        this.AutoScaleMode = AutoScaleMode.None;
        this.Location = tocover.PointToScreen(Point.Empty);
        this.ClientSize = tocover.ClientSize;
        tocover.LocationChanged += Cover_LocationChanged;
        tocover.ClientSizeChanged += Cover_ClientSizeChanged;
        this.Show(tocover);
        tocover.Focus();
        // Disable Aero transitions, the plexiglass gets too visible
        if (Environment.OSVersion.Version.Major >= 6) {
            int value = 1;
            DwmSetWindowAttribute(tocover.Handle, DWMWA_TRANSITIONS_FORCEDISABLED, ref value, 4);
        }
    }
    private void Cover_LocationChanged(object sender, EventArgs e) {
        // Ensure the plexiglass follows the owner
        this.Location = this.Owner.PointToScreen(Point.Empty);
    }
    private void Cover_ClientSizeChanged(object sender, EventArgs e) {
        // Ensure the plexiglass keeps the owner covered
        this.ClientSize = this.Owner.ClientSize;
    }
    protected override void OnFormClosing(FormClosingEventArgs e) {
        // Restore owner
        this.Owner.LocationChanged -= Cover_LocationChanged;
        this.Owner.ClientSizeChanged -= Cover_ClientSizeChanged;
        if (!this.Owner.IsDisposed && Environment.OSVersion.Version.Major >= 6) {
            int value = 1;
            DwmSetWindowAttribute(this.Owner.Handle, DWMWA_TRANSITIONS_FORCEDISABLED, ref value, 4);
        }
        base.OnFormClosing(e);
    }
    protected override void OnActivated(EventArgs e) {
        // Always keep the owner activated instead
        this.BeginInvoke(new Action(() => this.Owner.Activate()));
    }
    private const int DWMWA_TRANSITIONS_FORCEDISABLED = 3;
    [DllImport("dwmapi.dll")]
    private static extern int DwmSetWindowAttribute(IntPtr hWnd, int attr, ref int value, int attrLen);
}