How do I blur the entire screen in a Winforms program?

Yes, every form must implement the IBlur interface and must also contain public (not private) Blur() and UnBlur() methods for this approach to work. 

Thanks for this solution.

Do I need to add the IBlur reference to every form?

When I run my program as is, an exception is generated:

Unable to cast object of type 'WindowsApplication1.AppointmentDetailForm' to type 'WindowsApplication1.IBlur'.


The exception is in this code:

private void BlurAll()
  {
  foreach (Form f in Application.OpenForms)
  {
  ((IBlur)f).Blur();
  }
  }

When I tried in this form to add the IBlur interface to the form in this fashion:

 public partial class PatientSummaryForm : Form, IBlur



I get this error when trying to compile:

Error    106    'WindowsApplication1.PatientSummaryForm' does not implement interface member 'WindowsApplication1.IBlur.Blur()'    E:\c# projects\Eclipse\Scheduler\Oslerscheduler\OslerScheduler\PatientSummaryForm.Designer.cs    3    19    OslerScheduler

Error    107    'WindowsApplication1.PatientSummaryForm' does not implement interface member 'WindowsApplication1.IBlur.UnBlur()'    E:\c# projects\Eclipse\Scheduler\Oslerscheduler\OslerScheduler\PatientSummaryForm.Designer.cs    3    19    OslerScheduler



Can you tell me what I am doing wrong here?

What I'd suggest here is that you make the Blur and UnBlur methods on each form public rather than private.

You then add an interface IBlur and make all forms implement this interface:

    public interface IBlur
    {
        void Blur();
        void UnBlur();
    }

You can then add BlurAll and UnBlurAll methods (say) to Form1 which would now look like this:

    public partial class Form1 : Form, IBlur
    {
        private PictureBox pb;

        public Form1()
        {
            InitializeComponent();
            pb = new PictureBox();
            panel1.Controls.Add(pb);
            pb.Dock = DockStyle.Fill;
        }

        public void Blur()
        {
            Bitmap bmp = Screenshot.TakeSnapshot(panel1);
            BitmapFilter.GaussianBlur(bmp, 4);

            pb.Image = bmp;
            pb.BringToFront();
        }

        public void UnBlur()
        {
            pb.Image = null;
            pb.SendToBack();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // open another form which has the Blur infrastructure already added
            Form2 f2 = new Form2();
            f2.Show();
        }

        private void button2_Click(object sender, EventArgs e)
        {          
            BlurAll();
            MessageBox.Show("It's blurred now!");
            UnBlurAll();
        }

        private void BlurAll()
        {
            foreach (Form f in Application.OpenForms)
            {
                ((IBlur)f).Blur();
            }
        }

        private void UnBlurAll()
        {
            foreach (Form f in Application.OpenForms)
            {
                ((IBlur)f).UnBlur();
            }
        }      
    }