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There are cases where we can have different singletons, all of which implement an interface IFoo (for example) and a method that has accepts an IFoo parameter. Due to separation of concerns, the method may have no idea about the existence of the concrete singletons. For example:
static class PrettyPrinter
{
public static string PrettyPrint
(string textToBePrettyPrinted,
IPrettyPrinterRuleProvider rulesToUse)
{
...
return prettyPrintedString;
}
}
...
string text = "...";
string result1 = PrettyPrinter.PrettyPrint (text, KAndRTypeFormattingRules.Instance);
string result2 = PrettyPrinter.PrettyPrint (text, MicrosoftFormattingRules.Instance);/ravi
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Well sure, but would you design a class to do that with and make it a Singleton? Why? I see no reason to do so.
Look at, perhaps, System.StringComparer and its fields.
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PIEBALDconsult wrote: make it a Singleton? Why?
For performance reasons. I would prefer to not have to new up a concrete IPrettyPrinterRuleProvider each time I call PrettyPrinter.PrettyPrint().
/ravi
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And you don't; nor does it need to be a Singleton. It can be a static field of the class, for instance.
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Yes, but that would cause unneccesary coupling.
I'm not suggesting singletons are absolutely necessary (heck even while, for, switch and continue statements aren't!). My point is the singleton pattern (especially when combined with Lazy<T>) allows one to write more performant and more manageable code.
/ravi
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Ravi Bhavnani wrote: that would cause unneccesary coupling.
I don't think so.
Ravi Bhavnani wrote: one to write more performant and more manageable code.
Bullpuckey.
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Ravi Bhavnani wrote: For performance reasons. I would prefer to not have to new up
a concrete IPrettyPrinterRuleProvider
There is design, implementation and performance. Distinct but interrelated idioms.
You would NEVER impose a design limit based solely on creating an instance. You MIGHT impose a design limit based on the work that creating an instance requires. That however is very, very rare. (I am specifically referring to creating the instance.)
And I suspect for your example it does not apply.
In terms of measuring (actual values) the performance of an application if you find that a specific bit of code imposes a performance bottleneck on the application (not a just a hunk of code) then you modify the implementation to correct that. If you must modify the design to correct a measured performance problem then you have a bug in the design which requires a rework of the design and implementation.
The factors the lead to a redesign where the ONLY problem is a performance problem is probably very rare. Typically if someone cannot anticipate a performance problem at the design level then the design is incomplete or the designer does not have enough experience and thust the entire design is questionable.
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The biggest problem with singletons lies in the fact that it reduces parallelism. As you only have a single instance present, any threaded operations must be serialized in and out of the singleton, reducing the efficiency of the threading. They can also make it harder to unit test code because it can introduce global state.
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i see, this really helps, in my case I've global state (the list of users), and i've to synchronize the access, so this wont help... i think i'll just rewrite this to use normal instances and a single shared user list, apparently, is the best solution at the moment, i'm working on this with a plenty of beginners and the only thing that will be sent to the appraiser, thanks 
I'm brazilian and english (well, human languages in general) aren't my best skill, so, sorry by my english. (if you want we can speak in C# or VB.Net =p)
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Pete O'Hanlon wrote: The biggest problem with singletons lies in the fact that it reduces
parallelism. As you only have a single instance present, any threaded operations
must be serialized in and out of the singleton, reducing the efficiency of the
threading. They can also make it harder to unit test code because it can
introduce global state.
I don't agree.
The biggest problem with singletons is incorrect usage and from that overusage.
For your points...
For the first problem sometime the very nature of the need dictates serialization anyways. And in other cases there are ways that can allow multiple access.
As for the second problem there are trivial solutions for most problems of that nature, for example just implementing a Reset() method.
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jschell wrote: For the first problem sometime the very nature of the need dictates
serialization anyways.
In that case, you shouldn't be parallelising the code at this point. This concern was raised specifically for the case where you have a parallel architecture that has to be throttled because of a singleton.
jschell wrote: As for the second problem there are trivial solutions for most problems of that
nature, for example just implementing a Reset() method.
So, we're talking about introducing a method just to get through a test; not for any other purpose.
I think you misunderstood my point. I wasn't arguing that you should never use a singleton; I was just showing what the weaknesses are. If you are comfortable that you understand the pattern sufficiently well, and that you know exactly how your code is going to be used, then you can make an informed decision. If you don't know what the weaknesses are, then you can end up causing all sorts of problems for yourself.
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Pete O'Hanlon wrote: In that case, you shouldn't be parallelising the code at this point. This
concern was raised specifically for the case where you have a parallel
architecture that has to be throttled because of a singleton.
Not sure what you are referring to.
I took your response to suggest that you were addressing the Singleton Pattern, and not one specific instance of that.
Pete O'Hanlon wrote: So, we're talking about introducing a method just to get through a test; not for
any other purpose.
Yep.
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They artificially restrict your ability to change your mind later, and don't really provide any benefit. You can always just instantiate one copy of an object if you only need one!
If you have a need for a single static instance of some stuff, have a static reference to it.
static class GlobalData {
public static ContactListManager ContactListManager { get; private set; }
static GlobalData(){
ContactListManager = new ContactListManager("datastore.xml");
}
}
That way, if you discover that you want to manage disparate contacts across several data sources and therefore it shouldn't be a singleton any more, it's much easier to change. It means that if you want a temporary separate instance for some reason (e.g. importing external data), you can do so.
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Hi,
I have an 'in.xml' file which I would like to 'collapse' into an 'out.xml' file in c#. (Not necessarily asking for the code but the approach to do this ie. xslt or xmldocument add attributes) Many thanks in advance.
The files are as follows:
in.xml
<mySample xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" >
<Phase1>
<startDate>2013-01-01</startDate>
<endDate>2014-12-31</endDate>
<capital>100000.00</capital>
<fundings>
<funding>
<registrar>Genentech Pty Ltd</registrar>
<assets>100000.00</assets>
<miscellany>150000.00</miscellany>
</funding>
<funding>
<registrar>Acme Pty Ltd</registrar>
<assets>25000.00</assets>
<miscellany>25000.00</miscellany>
</funding>
</fundings>
</Phase1>
<Phase2>
<capital>150000.00</capital>
<fundings>
<funding>
<registrar>Mont Blanc Associates</registrar>
<assets>50000.00</assets>
<miscellany>25000.00</miscellany>
</funding>
<funding>
<registrar>Acme Pty Ltd</registrar>
<assets>10000.00</assets>
<miscellany>10000.00</miscellany>
</funding>
<funding>
<registrar>Triple A Commercial</registrar>
<assets>10000.00</assets>
<miscellany>10000.00</miscellany>
</funding>
</fundings>
</Phase2>
<integration>
<value>6</value>
</integration>
<years>
<value>1</value>
<value>2</value>
<value>6</value>
<value>3</value>
<value>7</value>
<value>4</value>
<value>8</value>
<value>5</value>
</years>
<bandWidths>
<value>1</value>
<value>2</value>
</bandWidths>
<regions>
<region>
<name>Trevae, Srt, 3241</name>
<state>3</state>
<zodex>3241</zodex>
</region>
<region>
<name>Silua, Rev, 2940</name>
<state>2</state>
<zodex>2940</zodex>
</region>
</regions>
<sponsors>
<sponsor>
<name>Genentech Pty Ltd</name>
<spcode>7</spcode>
</sponsor>
<sponsor>
<name>Acme Pty Ltd.</name>
<spcode>15</spcode>
</sponsor>
<sponsor>
<name>Triple A Commercial</name>
<spcode>9</spcode>
</sponsor>
<sponsor>
<name>Mont Blanc Associates</name>
<spcode>1</spcode>
</sponsor>
</sponsors>
</mySample>
out.xml
<mySample xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" >
<Phase1 startDate="2013-01-01" endDate="2014-12-31" capital="100000.00">
<fundings>
<funding registrar="Genentech Pty Ltd" assets="100000.00" miscellany="150000.00"/>
<funding registrar="Acme Pty Ltd" assets="25000.00" miscellany="25000.00"/>
</fundings>
</Phase1>
<Phase2 capital="150000.00">
<fundings>
<funding registrar="Mont Blanc Associates" assets="50000.00" miscellany="25000.00"/>
<funding registrar="Acme Pty Ltd" assets="10000.00" miscellany="10000.00"/>
<funding registrar="Triple A Commercial" assets="10000.00" miscellany="10000.00"/>
</fundings>
</Phase2>
<integration value="6"/>
<years value="1,2,6,3,7,4,8,5"/>
<bandWidths value="1,2"/>
<regions>
<region name="Trevae, Srt, 3241" state="3" zodex="3241"/>
<region name="Silua, Rev, 2940" state="2" zodex="2940"/>
</regions>
<sponsors>
<sponsor name="Genentech Pty Ltd" spcode="7"/>
<sponsor name="Acme Pty Ltd" spcode="15"/>
<sponsor name="Triple A Commercial" spcode="9"/>
<sponsor name="Mont Blanc Associates" spcode="1"/>
</sponsors>
</mySample>
----------------------------------------------------------
Lorem ipsum dolor sit amet.
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An xslt would be a simple way to achieve this with a well defined schema. If you're not comfortable with xslt though, you could easily do this with something like XLinq as well.
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Thanks.
Do you agree the following summarizes what I am trying to achieve?
Unless the child element has a child element of its own, make it an attribute of the parent.
----------------------------------------------------------
Lorem ipsum dolor sit amet.
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It certainly looks like it.
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Yeah, time to practice some XSLT-Fu. 
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i hate you. your reply is funny but limited in usefulness.
----------------------------------------------------------
Lorem ipsum dolor sit amet.
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I disagree. I feel his post was the most usefull of all, instructing you to increase your skillset. This would have the effect of you being able to solve this particular problem in a far more efficient method than posting a question in a discussion forum.
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Dave Kreskowiak wrote: increase your skillset
Actually, I assumed he was already at least somewhat familiar with XSLT; he seems to know what it is and that it is applicable to the situation.
I think what I meant to imply was that he should maybe become more confortable with XSLT.
Oh, and OP: we do have an XML/XSLT forum if you need assistance group therapy.
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I was just joking around. Either way, it wouldn't hurt to increase his skillset anyway.
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swjam wrote: funny but limited in usefulness
Ah, good, you know XSLT already. 
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I thought the I hate you was more indicative of XSLT kowledge!
Never underestimate the power of human stupidity
RAH
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To quote Billy Joel: "You may be right."
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