I'm a new C# student and I've been working on my project, however, I can't seem to get some of my textboxes to act properly. I need to ensure that some of my textboxes receive only numerical data. Currently, my medication, surgery, labs, and rehab textboxes return their messageBox message when I fill in the textbox with a numerical value which is opposite of what they are supposed to do. 

```
private void calculateButton_Click(object sender, EventArgs e)
        {
            int days;
            double medication;
            double surgery = 0;
            double labs = 0;
            double rehab = 0;

            if (int.TryParse(numDaysInHosp.Text, out days))
                if (days < 0)
                {
                    MessageBox.Show("Days In Hospital must be greater than zero.");
                }

            if (double.TryParse(medsCharges.Text, out medication))
                if (medsCharges.Text != "")
                {
                    MessageBox.Show("Charges for Medication cannot be blank.");
                    medsCharges.Focus();
                }

            if (double.TryParse(surgCharges.Text, out surgery))
               if (surgCharges.Text != "")
               {
                  MessageBox.Show("Surgical Charges cannot be blank.");
                  surgCharges.Focus();
               }

            if (double.TryParse(labFees.Text, out labs))
               if (labFees.Text != "")
               {
                    MessageBox.Show("Lab Fees cannot be blank.");
                    labFees.Focus();
               }

            if (double.TryParse(rehabCharges.Text, out rehab))
               if (rehabCharges.Text != "")
               {
                    MessageBox.Show("Rehabilitation Charges cannot be blank.");
                    rehabCharges.Focus();
               }

            { 
                double dayChrgs = CalcDayChrgs(days);
                double miscChrgs = CalcMiscChrgs(medication, surgery, labs, rehab);
                double totlCost = CalcTotlChrgs(dayChrgs, miscChrgs);

                stayChrgsLabel.Text = dayChrgs.ToString("c");
                miscChrgsLabel.Text = miscChrgs.ToString("c");
                totlCostLabel.Text = totlCost.ToString("c");
                memberLevelLabel.Text = memberLevelLabel.ToString();

            }
        }

```

We are not allowed to use the keypress method to do this. Additionally, I've tried using the following lines as well with negative results. 

```
(string.IsNullOrEmpty(TextBox.Text))
if(TextBox.Text == "") 
if(TextBox.Text.Length == 0)
String.IsNullOrWhitespace()

```

What am I not seeing in my code?