Debug.WriteIf Method (Boolean, String, String)


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Writes a category name and message to the trace listeners in the Listeners collection if a condition is true.

Namespace:   System.Diagnostics
Assembly:  System (in System.dll)

Public Shared Sub WriteIf (
	condition As Boolean,
	message As String,
	category As String


Type: System.Boolean

The conditional expression to evaluate. If the condition is true, the category name and message are written to the trace listeners in the collection.

Type: System.String

A message to write.

Type: System.String

A category name used to organize the output.

By default, the output is written to an instance of DefaultTraceListener.

The category parameter can be used to group output messages.

This method calls the TraceListener.Write method of the trace listener.

Notes to Implementers:

You can minimize the performance penalty of instrumenting your application by using If...Then statements instead of using WriteIf statements. The following two code examples send the same debugging message. However, the first example is much faster when tracing is off, because if mySwitch.TraceError evaluates to false, you do not call Write. The second example always calls WriteIf, even when mySwitch.TraceError is false and no tracing output is produced. This can result in unnecessary execution of arbitrarily complex code.

First example:

    Debug.Write("aNumber = " + aNumber + " out of range");

Second example:

Debug.WriteIf(mySwitch.TraceError, "aNumber = " + aNumber + " out of range");

The following example creates a TraceSwitch named generalSwitch. This switch is set outside of the code sample.

If the switch is set to the TraceLevelVerbose, the example outputs the first error message to the Listeners. For information on adding a listener to the Listeners collection, see the TraceListenerCollection class.

Then, if the TraceLevel is set to Error or higher, the example outputs the second error message on the same line as the first message. A line terminator follows the second message.

' Class-level declaration.
' Create a TraceSwitch.
Private Shared generalSwitch As New TraceSwitch("General", "Entire Application")

Public Shared Sub MyErrorMethod(myObject As Object, category As String)
    ' Write the message if the TraceSwitch level is set to Verbose.
    Debug.WriteIf(generalSwitch.TraceVerbose, myObject.ToString() & _
        " is not a valid object for category: ", category)

    ' Write a second message if the TraceSwitch level is set to Error or higher.
    Debug.WriteLineIf(generalSwitch.TraceError, " Please use a different category.")
End Sub 'MyErrorMethod

Universal Windows Platform
Available since 10
.NET Framework
Available since 1.1
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