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Trace.WriteLineIf Method (Boolean, String, String)

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)

<ConditionalAttribute("TRACE")> _
Public Shared Sub WriteLineIf ( _
	condition As Boolean, _
	message As String, _
	category As String _


Type: System.Boolean
true to cause a message to be written; otherwise, false.
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 WriteLine 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 WriteLineIf 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 WriteLine. The second example always calls WriteLineIf, even when mySwitch.TraceError is false and no tracing output is produced. This can result in unnecessary execution of arbitrarily complex code.

First example

    Trace.WriteLine("aNumber = " + aNumber + " out of range");

Second example

Trace.WriteLineIf(mySwitch.TraceError, "aNumber = " + aNumber + " out of range");

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

If the switch is set to the TraceLevel Error or higher, 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 Verbose, the example outputs the second error message and the category on the same line as the first message. The second message is followed by a line terminator.

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

    Public Shared Sub MyErrorMethod(category As String)
        ' Write the message if the TraceSwitch level is set to Error or higher.
        Trace.WriteIf(generalSwitch.TraceError, "My error message. ")

        ' Write a second message if the TraceSwitch level is set to Verbose.
        Trace.WriteLineIf(generalSwitch.TraceVerbose, _
            "My second error message.", category)
    End Sub

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.