What's "#" means ?

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VB

#If bTrace Then
Trace.WriteLine("Begun db query at " & now())
#End If

In this code What's "#" means ? and why we enclosed the block by "#"?

Posted 13-Nov-10 4:12am

Layth Rafea

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4 solutions

Solution 2

if bTrace is equal true this line

Trace.WriteLine("Begun db query at " & now())

will compile if not will not compile

Posted 13-Nov-10 4:30am

alrosan

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OriginalGriff · Accepted Answer · 2010-11-13T04:29:00

"#" introduces a compiler instruction, in this case "#If" which if true, allows the compiler to process the code up to "#End If". If false, that code is ignored.

In your case, if bTrace is declared and true, then the "Trace.WriteLine" instruction is included in your code.

See here[^] or here[^]for more info.

Keith Barrow · Accepted Answer · 2010-11-13T04:36:00

#if is know as a preprocessor directive. Your code means: if b.Trace is true, Compile the Trace line compiled, othewise don't. For example if your code had been:

if bTrace Then
  Trace.WriteLine("Begun db query at " & now())
End If

Then the above code will always be compiled, and the if condition evaluated. This causes a loss of performance.

Often #if is used to add code you only want to execute under a debug build, without a performance hit when live code is compiled. I avoid preprocessor directives, the performance gain isn't normally enough to justify the WTF factor and messy code, but I do use them on occaision.

See
#if C# Rerence MSDN

Abhinav S · Accepted Answer · 2010-11-13T04:33:00

Solution 3

Any preprocesser directive starts with this character.
See here[^].

Posted 13-Nov-10 4:33am

Abhinav S

What's "#" means ?

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