Confused about static

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I have a C# program. I'm watching a directory for changes to an xml file, and when the file changes I want to ready the xml and update a rich textbox.
It all works when I specify the file. It reads it and populates the RTB fine. But when I add code to try to do it automatically when the files changes I get the error

Error 1 An object reference is required for the non-static field, method, or property 'xml_reader.Form1.readXML(string)'

I've read several examples on this but I still don't get it.

this fires when it should

C#

private static void OnChanged(object source, FileSystemEventArgs e)
 {
     // Specify what is done when a file is changed, created, or deleted.
     string path = e.FullPath;
     Console.WriteLine("File: " + path + " " + e.ChangeType);
    readXML(path);
 }

I want to call

C#

private void readXML(string Filename)
{
    int linenumbers = 0;
    string stringvalue;
    string stringname;
    String title = "POS           PILOT                 LAPS    ELAPSED      SEED         FAST LAP \n";
    richTextBox1.Text = title;
    XmlTextReader reader = new XmlTextReader(Filename);
    while (reader.Read())
    {
        switch (reader.NodeType)
        {
            case XmlNodeType.Element: // The node is an element.
                if (reader.Name == "Driver")
                {

                    while (reader.MoveToNextAttribute()) // Read the attributes.
                    {
                        stringvalue = reader.Value;
                        stringname = reader.Name;
                        if (stringname == "position")
                        {
                            richTextBox1.Text += (" " + stringvalue + "\t");
                        }
                        if (stringname == "name")
                        {
                            string longname = stringvalue;
                            longname = longname.PadRight(30);
                            int namelength = longname.Length;
                            Console.WriteLine(longname + namelength);
                            richTextBox1.Text += (longname);
                        }
                        if (stringname == "laps")
                        {
                            if (stringvalue == " ")
                                stringvalue = "0";
                            richTextBox1.Text += ("  " + stringvalue + "\t\t");
                        }
                        if (stringname == "seed")
                        {
                            if (stringvalue == " ")
                                stringvalue = "0.000";
                            richTextBox1.Text += (stringvalue + "\t\t");
                        }
                        if (stringname == "elapsedTime")
                        {
                            if (stringvalue == " ")
                                stringvalue = "0.000";
                            richTextBox1.Text += (" " + stringvalue + "\t\t");
                        }
                        if (stringname == "fastLap")
                        {
                            if (stringvalue == " ")
                                stringvalue = "0.000";
                            stringvalue = stringvalue.PadRight(8);
                            richTextBox1.Text += (" " + stringvalue + Environment.NewLine);
                            linenumbers++;
                        }
                    }
                }
                break;
            case XmlNodeType.Text: //Display the text in each element.
                break;
            case XmlNodeType.EndElement: //Display the end of the element.
                break;
        }
    }
    reader.Close();
    colorTextbox(linenumbers);
}

If I change the function so that it is "static void readXML(string Filename)"
then I get errors on richtextbox1.

What is the easiest way to fix it?
Thanks in advance.

What I have tried:

making the called routine static.

Posted 16-Jun-16 10:04am

Member 3652617

Updated 16-Jun-16 14:40pm

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Comments

Matt T Heffron 16-Jun-16 16:52pm

Try removing the static from the OnChanged method.

2 solutions

Solution 1

In addition to my comment above about making the OnChanged method not be static, there are other things very wrong with your code. Not the least of which is the abuse of string concatenation! You should use StringBuilder Class (System.Text)[^]:

C#

private void readXML(string Filename)
{
  int linenumbers = 0;
  string stringvalue;
  const string title = "POS           PILOT                 LAPS    ELAPSED      SEED         FAST LAP \n";
  StringBuilder builder = new StringBuilder(title, 1024); // 1k is a fair starting "guess" ;-)
  using (XmlTextReader reader = new XmlTextReader(Filename))
  {
    // The using statement ensures that reader is Closed when done.
    while (reader.Read())
    {
      switch (reader.NodeType)
      {
      case XmlNodeType.Element: // The node is an element.
        if (reader.Name == "Driver")
        {
          while (reader.MoveToNextAttribute()) // Read the attributes.
          {
            stringvalue = reader.Value;
            switch (reader.Name)
            {
            case "position":
              builder.Append(" ").Append(stringvalue).Append("\t");
              break;
            case "name":
              string longname = stringvalue.PadRight(30);
              Console.WriteLine("{0} {1}", longname, longname.Length);
              builder.Append(longname);
              break;
            case "laps":
              if (string.IsNullOrWhiteSpace(stringvalue))
                stringvalue = "0";
              builder.Append("  ").Append(stringvalue).Append("\t\t");
              break;
            case "seed":
              if (string.IsNullOrWhiteSpace(stringvalue))
                stringvalue = "0.000";
              builder.Append(stringvalue).Append("\t\t");
              break;
            case "elapsedTime":
              if (string.IsNullOrWhiteSpace(stringvalue))
                stringvalue = "0.000";
              builder.Append(" ").Append(stringvalue).Append("\t\t");
              break;
            case "fastLap":
              if (string.IsNullOrWhiteSpace(stringvalue))
                stringvalue = "0.000";
              stringvalue = stringvalue.PadRight(8);
              builder.Append(" ").AppendLine(stringvalue);
              linenumbers++;
              break;
            default:
              break;
            }
          }
        }
        break;
      case XmlNodeType.Text: //Display the text in each element.
        break;
      case XmlNodeType.EndElement: //Display the end of the element.
        break;
      }
    }
  }
  // Edit: Matt Heffron: 6/17/2016
  // Dealing with the Cross-thread access error:
  // Was:
  richTextBox1.Text = builder.ToString();
  // Change to:
  Action updateText = () => richTextBox1.Text = builder.ToString();
  if (richTextBox1.InvokeRequired)
    richTextBox1.Invoke(updateText);
  else
    updateText();
  // End of Edit
  colorTextbox(linenumbers);
}

In addition, tabular data like this would be better presented using a DataGrid control instead of a RichTextBox.

Posted 16-Jun-16 11:26am

Matt T Heffron

Updated 17-Jun-16 8:50am

v4

Comments

Sergey Alexandrovich Kryukov 16-Jun-16 20:43pm

I voted 4 this time. You have some very good points, but static vs instance methods are not explained.
Yes, I know, it's extremely difficult to explain the fundamentals in brief, especially to people who try to do something advanced without understanding of the basics. This is the main problem of most of out inquirers, isn't it.

I tried to explain it in Solution 2, crediting your comment to the question.

—SA

Member 3652617 17-Jun-16 11:09am

Thanks Matt, I will definitely look at the data grid.I made the string changes you posted. When I remove static from the Onchange method, I then get an error on the line
richTextBox1.Text = builder.ToString();
I'm assuming I would get the same error if it were a data grid. The error is
An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll

Additional information: Cross-thread operation not valid: Control 'richTextBox1' accessed from a thread other than the thread it was created on.

Its probably what Sergey tried to explain below but it went over my head. I'm obviously not a programmer. Thanks for explaining it and I'll read it a few more times, but can you help me with this error as well please. Thanks Again.

Matt T Heffron 17-Jun-16 12:41pm

The Cross-thread error is because you are executing this in a different thread from the UI thread. The properties/methods of UI controls must be access from the UI thread. There a property and a method on controls to deal with this.
See my update to the Solution.
If the colorTextbox method (or any method) accesses any UI controls from a thread other than the UI thread then this same error will occur there. You'll need to look for and fix all of these!

Member 3652617 17-Jun-16 12:59pm

Thanks, I couldn't get it to run with the line
Action updateText = () => {richTextBox1.Text = builder.ToString();}
so I took out the brakets and made it
Action updateText = () => richTextBox1.Text = builder.ToString();
not sure if that broke it or not, but now the error is
on line while(reader.Read())
and its An unhandled exception of type 'System.IO.IOException' occurred in System.Xml.dll

Additional information: The process cannot access the file 'C:\Users\ldavis\Dropbox\RC\DRC\xml\driverData.xml' because it is being used by another process.

Matt T Heffron 17-Jun-16 14:49pm

Actually all you needed was a ; after the } (oops!)
The IOException means that some other process has that file open, probably for writing. Since you are calling this because of a FileWatcher Changed event, you need to wait for a little time after the "last" Changed event to determine if the file is done changing and is "readable". How long depends on how the file is being written.

Member 3652617 17-Jun-16 15:45pm

That makes sense. I put in an evil thread.sleep and that fixed it. Now I "think" I just need 1 more piece of syntax help.
in setupdatagrid I used
if (this.InvokeRequired)
{
this.Invoke(new Action(() => setupdatagrid()));
}
and it works, but I can't figure out the syntxx for
colorTextbox(int lines) to do the same thing.
See below and thanks for all the help

<pre lang="c#">using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml;
using System.IO;
using System.Security.Permissions;

namespace xml_reader
{

public partial class Form1 : Form
{
string filename;
public Form1()
{
InitializeComponent();
setupdatagrid();

}

private void Form1_Load(object sender, EventArgs e)
{
richTextBox1.Multiline = true;
// richTextBox1.AcceptsTab = true;
richTextBox1.SelectionColor = Color.Red;

}

private void btn_read_Click(object sender, EventArgs e)
{
OpenFileDialog openFileDialog1 = new OpenFileDialog();
// Set filter options and filter index.
openFileDialog1.Filter = "XML Files (*.xml)|*.xml";
openFileDialog1.FilterIndex = 1;
if (openFileDialog1.ShowDialog() == DialogResult.OK)
{
filename = openFileDialog1.FileName;
readXML(openFileDialog1.FileName);

FileSystemWatcher watcher = new FileSystemWatcher();
string directoryName = Path.GetDirectoryName(openFileDialog1.FileName);
watcher.Path = directoryName;
watcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite | NotifyFilters.FileName | NotifyFilters.DirectoryName;
// Only watch xml files.
watcher.Filter = "*.xml";

// Add event handlers.
watcher.Changed += new FileSystemEventHandler(OnChanged);
watcher.Created += new FileSystemEventHandler(OnChanged);

// Begin watching.
// timer1.Enabled = true;
// or
watcher.EnableRaisingEvents = true;

}
}

private void OnChanged(object source, FileSystemEventArgs e)
{
// Specify what is done when a file is changed, created, or deleted.
string path = e.FullPath;
Console.WriteLine("File: " + path + " " + e.ChangeType);
System.Threading.Thread.Sleep(1000);
readXML(path);
}

private void readXML(string Filename)
{
int linenumbers = 0;
string stringvalue;
string Pos = "POS";
string Pilot = "PILOT";
string laps = "LAPS";
string elapsed = "ELAPSED";
string seed = "SEED";
string fast = "FAST LAP";
string[] row;
setupdatagrid();

const string title = "POS PILOT LAPS ELAPSED SEED FAST LAP \n";
StringBuilder builder = new StringBuilder(title, 1024); // 1k is a fair starting "guess" ;-)
using (XmlTextReader reader = new XmlTextReader(Filename))
{
// The using statement ensures that reader is Closed when done.
while (reader.Read())
{
switch (reader.NodeType)
{
case XmlNodeType.Element: // The node is an element.
if (reader.Name == "Driver")
{
while (reader.MoveToNextAttribute()) // Read the attributes.
{
stringvalue = reader.Value;
switch (reader.Name)

Member 3652617 17-Jun-16 15:47pm

Oops, guess I can't post that much code.
so working function is

private void setupdatagrid()
{

if (this.InvokeRequired)
{
this.Invoke(new Action(() => setupdatagrid()));
}
else
{
dataGridView1.Rows.Clear();
dataGridView1.ColumnCount = 6;
.......

and the one I cant figure out is
private void colorTextbox(int lines)
{
if (this.InvokeRequired)
{
this.Invoke(new Action(() => colorTextbox()));// what is the correct Syntax?
}
else
{

int lineNumberToSelect = 0;
...........

Matt T Heffron 17-Jun-16 16:55pm

We'd both be better off if you reposted this as a new Question with a good title and description and the code formatted with the <pre ...> tags.
It also makes it easier to find for future folks searching for solving a similar issue.

Hrishikesh Ranjit Shivacharan 21-Jun-16 3:23am

When you accessing the non static method in static method this is wrong in programming language.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2016-06-16T14:40:00

A method should be static if you simply don't need "this", which is the reference to the instance of some class. This "this" is an implicit method which is passed to all instance methods. (This way, "instance method" is antonym to "static method".) You only can call an instance method on some instance (instanceName.MethodName(/* … */)) and you can only call static method on the type (TypeName.MethodName(/* … */)), so that instance becomes the reference to the instance passed to an instance method.

You can access all type members through "this", and, therefore, from a static method you can only access static member. Using static methods, in practice, is a must when you have methods unrelated to other members, such as some mathematical calculations only operating with parameters.

For some more detail, please see my past answers:
Type casting in c#,
C# windows base this key word related and its uses in the application,
What makes static methods accessible?.

Using a static method as some event handler is possible, but it is extremely unlikely to turn out useful. That explains the comment to the question by Matt T Heffron.

—SA