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C# ПІДРУЧНИКИ / c# / Hungry Minds - Visual C# Blueprint.pdf

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WRITE ERRORS TO THE APPLICATION LOG

hen working with exceptions, there are cases Wwhere you want to persist the error/exception

information to a durable store. You can persist errors by using the Event Log that is built into the Windows NT and 2000 operating systems. If you log error/exception information, you can analyze a reoccurring problem and understand the sequence of events that occur to cause the problem. Logging to the Event Log allows you to perform some troubleshooting without having to run the application in a debug mode.

To access the Event Log, you will have to use the System.Diagnostics namespace. With this referenced,

you can create an event log source which will give context to the entries that you write to the Event Log (source name for application and which log you want to write to – Application, Security, System, or a custom event log). With that Event Log object you will call the WriteEntry method to put entries into the event log. When writing errors to the log, you will want to classify the severity of the error. These severities will affect what icon and type classification the error is given in the event viewer.

The task below will take you through the basic steps of setting up and logging to an Event Log.

WRITE ERRORS TO THE APPLICATION LOG



class name	Á Save the file.
		‡ Create string variables
		‡ Create string variables
function.	for the type of log, the source
	of the error, and the error
	message.

° Add an if statement to check for the existence of the event log and set the

CreateEventSource property.

· Create a new EventLog variable and set the Source for the event log.

WORKING WITH ERRORS 15

You can control the severity for entries that you place in the Application Error Log. After running this sample, open the Event Viewer and note that each one has a different severity and each severity has a different icon.

Example:

string sLog = "Application";

string sSource = "MySharedPhotoAlbum";

string sErrorMsg1 = "Message for Information."; string sErrorMsg2 = "Message for Error."; string sErrorMsg3 = "Message for Warning.";

if ( !EventLog.SourceExists(sSource) ) { EventLog.CreateEventSource(sSource,sLog); }

EventLog elMain = new EventLog(); elMain.Source = sSource;

if ( elMain.Log.ToUpper() != sLog.ToUpper() ){ Console.WriteLine

("Source is not available to use!"); return;}

elMain.WriteEntry(sErrorMsg1,EventLogEntryType.Information);

elMain.WriteEntry(sErrorMsg2,EventLogEntryType.Error);

elMain.WriteEntry(sErrorMsg3,EventLogEntryType.Warning);

‚ Add an if statement to write a message if some other application is accessing

the log.

— Add the WriteEntry function to write the details of the log entry and write a message to the console about the update being successful.

± Set a debug stop.

¡ Press F5 to save, build, and run the console application.

■ A message appears about the event log being updated.

285

APPENDIX

BASIC EXAMPLES

DECLARING VARIABLES


	Visual Basic	C#	JScript
	Dim x As Integer	int x;	var x : int;
	Dim x As Integer = 10	int x = 10;	var x : int = 10;


COMMENTS

	Visual Basic	C#	JScript
	‘ comment	// comment	// comment
	x = 1 ‘ comment	/* multiline	/* multiline
	Rem comment	comment */	comment */


ASSIGNMENT STATEMENTS

	Visual Basic	C#	JScript
	nVal = 7	nVal = 7;	nVal = 7;

IF...ELSE STATEMENTS

Visual Basic

If nCnt <= nMax Then

nTotal += nCnt ‘ Same as nTotal = nTotal + nCnt nCnt += 1 ‘ Same as nCnt = nCnt + 1

Else

nTotal += nCnt nCnt -= 1

End If

C#	JScript
if (nCnt <= nMax)	if(nCnt < nMax) {
{	nTotal += nCnt;
nTotal += nCnt;	nCnt ++;
nCnt++;	}
}	else {
else {	nTotal += nCnt;
nTotal += nCnt;	nCnt --;
nCnt --;	};
}

286

C# QUICK REFERENCE A

CASE STATEMENTS

Visual Basic

Select Case n

Case 0

MsgBox ("Zero")

‘Visual Basic exits the Select at the end of a Case.

Case 1

MsgBox ("One")

Case 2

MsgBox ("Two")

Case Else

MsgBox ("Default")

End Select

	C#	JScript
	switch(n) {	switch(n) {
	case 0:	case 0 :
	MessageBox.Show("Zero");	MessageBox.Show("Zero");
	break;	break;
	case 1:	case 1 :
	MessageBox.Show("One");	MessageBox.Show("One");
	break;	break;
	case 2:	case 2 :
	MessageBox.Show("Two");	MessageBox.Show("Two");
	break;	default :
	default:	MessageBox.Show("Default");
	MessageBox.Show("?");	}
	}

FOR LOOPS

	Visual Basic	JScript
	For n = 1 To 10	for (var n = 0; n < 10; n++) {
	MsgBox("The number is " & n)	Response.Write("The number is " + n);
	Next	}
	For Each i In iArray	for (prop in obj){
	Box.Show(i)	obj[prop] = 42;
	Next i Msg	}

	C#
	for (int i = 1; i <= 10; i++)
	MessageBox.Show("The number is {0}", i);
	foreach (int i in iArray)
	{
	MessageBox.Show (i.ToString());
	}

287

APPENDIX

BASIC EXAMPLES

WHILE LOOPS


Visual Basic		C#	JScript
While n < 100 ‘ Test at start of loop		while (n < 100)	while (n < 100) {
n += 1	‘ Same as n = n + 1	n++;	n++; }
End While ‘

PARAMETER PASSING BY VALUE

Visual Basic

Public Sub ABC(ByVal y As Long) ‘ The argument Y is passed by value.

‘ If ABC changes y, the changes do not affect x. End Sub

ABC(x) ‘ Call the procedure

You can force parameters to be passed by value, regardless of how they are declared, by enclosing the parameters in extra parentheses.

ABC((x))

//The method: void ABC(int x)

{

...

}

//Calling the method: ABC(i);

JScript

ABC(i,j);

PARAMETER PASSING BY REFERENCE

Visual Basic

Public Sub ABC(ByRef y As Long) ‘ The parameter of ABC is declared by reference: ‘ If ABC changes y, the changes are made to the value of x.

End Sub

ABC(x) ‘ Call the procedure

//The method: void ABC(ref int x)

{

...

}

//Calling the method: ABC(ref i);

JScript

N/A (objects (including arrays) are passed by reference, but the object to which the variable refers to cannot be changed in the caller). Properties and methods changed in the callee are visible to the caller.

/* Reference parameters are supported for external object, but not internal JScript functions */

comPlusObject.SomeMethod(&foo);

288

C# QUICK REFERENCE A

STRUCTURED EXCEPTION HANDLING

Visual Basic	JScript
Try	try {
If x = 0 Then	if (x == 0) {
Throw New Exception("x equals zero")	throw new Error(513, "x equals zero");
Else	}
Throw New Exception("x does not equal zero")	else {
End If	throw new Error(514, "x does not equal zero");
Catch	}
MessageBox.Show("Error: " & Err.Description)	}
Finally	catch(e) {
MessageBox.Show("Executing finally block.")	Response.Write("Error number: " + e.number + "<BR>");
End Try	Response.Write("Error description: " + e.message + "<BR>");
	}
	finally {
	Response.Write("Executing finally block.");
	}

C#
// try-catch-finally
try
{

if (x == 0)

throw new System.Exception ("x equals zero"); else

throw new System.Exception ("x does not equal zero");

}

catch (System.Exception err)

{

System.Console.WriteLine(err.Message);

}

finally

{

System.Console.WriteLine("executing finally block");

}

SET AN OBJECT REFERENCE TO NOTHING

Visual Basic	C#	JScript
o = Nothing	o = null;	o = null;

289

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