Problem creating panels at runtime

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Can anybody help??

I have a small application which creates a number of panels at runtime, each panel needs a label and some lines inside it. The number of panels is depicted simply from an integer entered in a text box, and I'm using a FOR loop to create the controls with a nested WHILE loop which draws the lines.

The code I have written so far creates the panels and labels, but I can't get the lines to draw. I've used a breakpoint to try and narrow down the problem and it seems as though the panels and labels aren't actually being drawn until the method has finished executing.

My code:

C#

    public void CreatePanels()
    {
        int PanelPosX = 50;
        int PanelPosY = 500;
        int LabelPosX = 10;
        int LabelPosY = 10;

        for (int i = 0; i < (Convert.ToInt32(textBox2.Text)); i++)
        {
            // Create a new panel

            Panel pnlOverview = new Panel();
            pnlOverview.Name = "InspectorPanel" + i.ToString();
            pnlOverview.Text = "Inspector Panel " + i.ToString();
            pnlOverview.Location = new Point(PanelPosX, PanelPosY);
            pnlOverview.Size = new Size(974, 136);
            pnlOverview.BorderStyle = System.Windows.Forms.BorderStyle.FixedSingle;
            Controls.Add(pnlOverview);
            PanelPosY += 170;

            // Add a label inside the panel

            Label lblInspectorName = new Label();
            lblInspectorName.Name = "InspectorName" + i.ToString();
            lblInspectorName.Text = " Inspector " + i.ToString();
            lblInspectorName.Width = 100;
            lblInspectorName.Height = 13;
            lblInspectorName.Location = new Point(LabelPosX, LabelPosY);
            lblInspectorName.Size = new Size(974, 136);
            pnlOverview.Controls.Add(lblInspectorName);

            // Draw some lines inside the panel

            int x = 10;
            int y = 0;

            Pen OVTable = new Pen(Color.Black, 0);

            while (y < 5)
            {
                Graphics mp = pnlOverview.CreateGraphics();
                mp.DrawLine(OVTable, x, 40, x, 100);
                y++;
                x += 8;
            }
        }
        return;
    }
}

Any help would be much appreciated

IW

[edit]Code block added - OriginalGriff[/edit]

Posted 10-Feb-13 0:25am

Wilsoni

Updated 10-Feb-13 0:38am

OriginalGriff

v2

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OriginalGriff · Accepted Answer · 2013-02-10T00:47:00

It's a bit more fundamental than that, I'm afraid.

When you create a Graphics context and draw something, the drawing is not preserved - it is transitory and will survive until the next time Windows decides the control needs to be re-drawn. Which in your case will be immediately after your method ends...:laugh:
At which point the lines you have drawn will be erased to prepare the drawing surface fro the controls to drawn themselves.

So, there are two things you must do:
1) First, if you create a Graphics object you are responsible for Disposing of it - failing to do that (as in your code above) will cause cause problems in teh long run, because Graphics contexts are scarce resources and you will run out of them a long time before the Garbage Collector gets round to deleting them for you. At this point you program will crash. If you ever create a scarce resource, enclose it in a using block:

C#

using (Graphics mp = pnlOverview.CreateGraphics())
    {
    mp.DrawLine(OVTable, x, 40, x, 100);
    y++;
    x += 8;
    }

2) Don't do it that way - you need to handle the Paint event for the panel and draw the lines in there. Just add the event handler when you construct the panel, and move the while loop into that, replacing the mp with e.Graphics and you should be fine.

Sergey Alexandrovich Kryukov · Accepted Answer · 2013-02-10T02:51:00

The whole idea to create the instance of System.Drawing.Graphics is wrong. You need to do all rendering (some exclusions apply) in the handler of the Paint event or in the overridden virtual method OnPaint. Please see my past answers where I explain it all:
Drawing Lines between mdi child forms[^],
capture the drawing on a panel[^],
What kind of playful method is Paint? (DataGridViewImageCell.Paint(...))[^],
How to speed up my vb.net application?[^].

[EDIT]

Wilsoni wrote:
The biggest problem was the size of the label I was trying to create.

As to the problem of the size, correct layout might help. Please see my past answers:
Zom Out malfunctions when Screen resolution changes[^],
how to dock button so that it can adjust with the form[^].

—SA

Problem creating panels at runtime

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