串行端口数据和C＃中的表单刷新

Question

I want to draw rectangle. Its width and height get from serial port. And I want to refresh page for repainting when i get data. my code is here;

public partial class Form1 : Form
{
    string RxString;
    static int x = 1;
    SerialPort serialPort1 = new SerialPort();
    public Form1()
    {
        InitializeComponent();


    }

    private void Form1_Load(object sender, EventArgs e)
    {

        serialPort1.PortName = "COM3";
        serialPort1.BaudRate = 9600;

        serialPort1.Open();
        RxString = serialPort1.ReadLine();

        x = Convert.ToInt32(RxString);
        Graphics g = this.CreateGraphics();
        Pen pen = new Pen(Color.Black, 2);

        g.DrawRectangle(pen, 100, 100, x, x);

   }

But there is a error at line of (x = Convert.ToInt32(RxString);)

A first chance exception of type 'System.FormatException' occurred in mscorlib.dll

What should I do? after that how can i refresh the form page for repaint image?

Answer 1

Since your code example isn't complete, and you don't say what the string value that fails is, I can't answer the question about your exception. However, I can show you how to deal with the painting correctly.

You need to maintain data in your class that keeps track of what you want to draw. Then, you override the OnPaint() method (because here, you are trying to the object of the declaring class itself…in other scenarios, you might need to handle some other object's Paint event) where you draw, based on the data you got.

Any time that data changes, you call Invalidate() to signal the need to redraw.

For example:

public partial class Form1 : Form
{
    string RxString;
    int x = 1;
    SerialPort serialPort1 = new SerialPort();

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {

        serialPort1.PortName = "COM3";
        serialPort1.BaudRate = 9600;

        serialPort1.Open();

        // The next three lines of code, you'll do wherever you want
        // to receive data and update the displayed graphics. Naturally,
        // the call to serialPort1.ReadLine() might vary, depending on
        // how you are managing the I/O on the serial port.
        RxString = serialPort1.ReadLine();

        x = Convert.ToInt32(RxString);
        Invalidate();
   }

   protected override void OnPaint(PaintEventArgs e)
   {
        e.Graphics.DrawRectangle(Pens.Black, 100, 100, x, x);
   }
}

I made some other changes to your code to improve it:

1. The x member should not be static, as the rest of the code is also not static, and so you in theory could have multiple instances of this form, all trying to use the same x at the same time.
2. Use the stock Pens.Black object instead of creating one yourself.

Also note that your original code failed to dispose the Graphics instance it created, which was a serious bug. It becomes moot in the above, because you don't actually need to create a Graphics instance. You just draw to the one passed to you (and don't dispose it…it's the caller's object, not yours).