Re: How to resolve class type using enum - C# Discussion Boards

lukeer23-Jan-12 4:24

23-Jan-12 4:24

Chose one forum and stick to it[^].
Posting to multiple forums annoys the people that you want to get help from, so it's not a good idea.

Ciao,

luker

How to resolve class type using enum

emmmatty122-Jan-12 20:05

emmmatty1

22-Jan-12 20:05

I have a group of classes (following strategy pattern) in my project. In the main function, I receive an enum value from the server and based on that I create an object of the base class type.

I am using switch/case statement to achieve this. I read somewhere that the Open/Closed principle does not allow opening a function to add a new case statement whenever a new class is added.

I am thinking of using a Activator.CreateInstance(). Is there any drawback to it.

Is there any other way to create an object from the enum type?

Adding example below even though it is not a full fledged Strategy pattern

abstract public class Mammal
{
public abstract void MakeSound()
}

class Cat:Mammal
{      
    public override  void MakeSound()
    {
        Console.WriteLine("Meow");        
    }    
}

class Dog:Mammal
{

    public override void MakeSound()
    {
         Console.WriteLine("Bow");        
    }    
}

Main()
{

    MammalTypes mammalType = RecieveValueFromServer();
    Mammal mammalBase
    switch(mammalType) // need to make this dynamic depending upon Enum type
    {
        case MammalTypes.Cat:mammalBase = new Cat()
                             break;
        case MammalTypes.Dog:mammalBase = new Dog()
                             break;            
    }

    mammalBase.MakeSound()
}

Re: How to resolve class type using enum

OriginalGriff22-Jan-12 22:05

OriginalGriff

22-Jan-12 22:05

If you qualify it with the namespace, you can use Reflection to create an object from it's string name:

string myType = "MyNamespace.MyClass";
Assembly asm = Assembly.GetExecutingAssembly();
Type t = asm.GetType(myType);
Object o = Activator.CreateInstance(t, null, null);

I would combine this with this tip: Human readable strings for enum elements[^] to replace the human readable bit with the full namespec on each enum element, and you are done.

Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water

Re: How to resolve class type using enum

emmmatty122-Jan-12 22:37

emmmatty1

22-Jan-12 22:37

Thanks,

Is there any drawback if this method replaces

switch/case statements for determinging type at run time?

Re: How to resolve class type using enum

OriginalGriff22-Jan-12 22:38

OriginalGriff

22-Jan-12 22:38

Speed probably - I haven't checked this specific case, but Reflection is never quick!

Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water

Re: How to resolve class type using enum

PIEBALDconsult23-Jan-12 5:13

PIEBALDconsult

23-Jan-12 5:13

OriginalGriff wrote:
Reflection is never quick

True, but often you can cache the results, I think this is one such situation.

Re: How to resolve class type using enum

jschell23-Jan-12 8:17

jschell

23-Jan-12 8:17

OriginalGriff wrote:
but Reflection is never quick!

That certainly requires qualification.

Regardless of whether one uses reflection or not the .Net code is still loaded via the same mechanism and called via the same mechanism.

So if there is a measurable difference then it will be due to the proxy nature of the reflection semantics. I would suspect that using an interface/base class would entirely eliminate that.

But even if not I would suppose that one is going to have a very unusual plugin hierarchy for the reflection call itself to have an significant (above noise level) impact on performance.

Re: How to resolve class type using enum

BobJanova24-Jan-12 10:07

BobJanova

24-Jan-12 10:07

The class resolution is not through the same mechanism if you use reflection, and it is much slower. But like you say, it's not like you use it enough to matter in most cases (and if you do, you can save the results in a Dictionary<string, Type> on first use).

Re: How to resolve class type using enum

jschell25-Jan-12 14:50

jschell

25-Jan-12 14:50

BobJanova wrote:
The class resolution is not through the same mechanism if you use reflection,
and it is much slower.

The same mechanism is used to load the assembly and class into the AppDomain regardless of whether reflection or application reference is used.

Class loading is a one time operation so that will not have an impact on any general application.

So best you can be referring to is method invocation. If you use indirect method invocation, every time, then that is proxy mechanism where the proxy mechanism itself has a impact.

Again however you would be hard pressed to measure that with code that actually does anything.

However if you use a direct implementation, such as interface/base class invocation then the call semantics are the same. A class instance in memory is the same regardless of whether the class/assembly was loaded by reflection or via that application loading mechanism.

Re: How to resolve class type using enum

Dean Oliver23-Jan-12 18:20

Dean Oliver

23-Jan-12 18:20

Yes. but that depends if you willing to violate the Open/Close principle in the process. It is because you would have to modify the class everytime you wanted to change its implementation. You should use the factory pattern to make the functionality more dynamic. This can either be achieved by way of reflection or being database driven. As for performance with reflection, from tests that I have done. The quickest method of execution is using the DynamicMethod class and generating IL code, and wrapping it in a delegate.

Re: How to resolve class type using enum

BobJanova24-Jan-12 10:32

BobJanova

24-Jan-12 10:32

I think it's obvious that the posted code would be used in a method taking the name as a parameter, not hard coded in a string :P.

Task cancellation

michaelgr122-Jan-12 3:59

michaelgr1

22-Jan-12 3:59

Hi,

If i just cancel the CancellationTokenSource token (that is a parameter of the Task.Factory.StartNew) , does it cancells automatically the task? or i need to check in the code of the task if the IsCancellationRequested of this cancellationToken and stop the task manually there?

Re: Task cancellation

Dave Kreskowiak22-Jan-12 8:15

Dave Kreskowiak

22-Jan-12 8:15

You need code in your Action that checks to see if the IsCancellationRequested property of the token is True.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak

how to retrieve a single value from datatable

mschotamaster21-Jan-12 22:01

mschotamaster

21-Jan-12 22:01

winforms sql2008
I have a table

SQL

Login(id(int),EmailId(varchar(35),connType(varchar))

where conntype has values like pop3 or imap. consider an user is loggedin. i want to fetch connType value of the logged user to do like this

SQL

if(conntypeValue == imap)
{
 //code for imap connection
}else
{
//code for pop3 connection
}

how can i do it

modified 22-Jan-12 4:10am.

Re: how to retrieve a single value from datatable

Richard MacCutchan21-Jan-12 23:01

Richard MacCutchan

21-Jan-12 23:01

//code for imap connection

What exactly are you trying to do in here?

Unrequited desire is character building. OriginalGriff
I'm sitting here giving you a standing ovation - Len Goodman

Re: how to retrieve a single value from datatable

mschotamaster22-Jan-12 3:12

mschotamaster

22-Jan-12 3:12

that code has been done. just i want to retrieve the single value from the table

Re: how to retrieve a single value from datatable

Richard MacCutchan22-Jan-12 3:15

Richard MacCutchan

22-Jan-12 3:15

What value? What table? You need to provide some more details about your problem.

Unrequited desire is character building. OriginalGriff
I'm sitting here giving you a standing ovation - Len Goodman

Re: how to retrieve a single value from datatable

mschotamaster22-Jan-12 3:17

mschotamaster

22-Jan-12 3:17

winforms sql2008
I have a table

Login(id(int),EmailId(varchar(35),connType(varchar))

where conntype has values like pop3 or imap. consider an user is loggedin. i want to fetch connType value of the logged user to do like this

if(conntypeValue == imap)
{
//code for imap connection
}else
{
//code for pop3 connection
}

how can i do it

i want to fetch connType column value from the table login where id = userid

Re: how to retrieve a single value from datatable

Richard MacCutchan22-Jan-12 3:23

Richard MacCutchan

22-Jan-12 3:23

Brilliant! I asked you to explain your problem in more detail and you have just repeated your original question.

Unrequited desire is character building. OriginalGriff
I'm sitting here giving you a standing ovation - Len Goodman

Re: how to retrieve a single value from datatable

mschotamaster22-Jan-12 3:38

mschotamaster

22-Jan-12 3:38

I want to connect with pop3 server or imap server on the basis of the account configured by the user. that account values are in the login table. when the user logged in. i want to fetch the value from the login table column servertype. if that value is pop3 the code of pop3 server connectivity will run.

Re: how to retrieve a single value from datatable

Nathan Stiles23-Jan-12 10:45

Nathan Stiles

23-Jan-12 10:45

Login(id(int),EmailId(varchar(35),connType(varchar))

execute a scalar
select connType from Login where id = 5
or
select connType from EmailId where id = 'noone@example.com'

Re: how to retrieve a single value from datatable

Eddy Vluggen22-Jan-12 3:41

Eddy Vluggen

22-Jan-12 3:41

mschotamaster wrote:
sql2008

I have a table

Aight, first you'd need a Sql-statement to select the value you're interested in. Something like this

SQL

SELECT [connType]
  FROM [Login]
 WHERE [Id] = @Id

mschotamaster wrote:
i want to fetch connType value of the logged user to do like this

Open a

<a href="http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.aspx">SqlConnection</a>[<a href="http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.aspx" target="_blank" title="New Window">^</a>]

(using a correct connectionstring[^]), create a

<a href="http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.aspx">SqlCommand</a>[<a href="http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.aspx" target="_blank" title="New Window">^</a>]

, put the Sql-statement in the CommandText property, add the parameter, open the connection, and catch the result from the ExecuteScalar[^]-method.

Bastard Programmer from Hell Suspicious | :suss: