Create an object in the constructor or at top of the class

which declaration/instantiation is better and WHY ?

public class MainWindow
{
   private Test _test;

   public MainWindow()
   {
       _test = new Test();
   }
}

OR

public class MainWindow
{
   pri开发者_JAVA技巧vate Test _test = new Test();

   public MainWindow()
   {

   }
}

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Ask yourself this question: what happens when you add other constructors to MainWindow? Do you want to then have to remember to invoke other constructors to ensure that _test is correctly initialized? Or is it okay for _test to not be initialized if another constructor is used?

Personally when creating a UI component I would move as many points of failure out of the constructor as possible, and I would move something like that to either the Loaded event handler, or keep it as you did in option 2 (which keeps the initialization nicely localized in the file).

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I'd go for a third option:

public class MainWindow
{
   private Test _test;

   public MainWindow(Test test)
   {
       _test = test;
   }
}

By injecting the dependency you make your code easier to unit test.

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It's better to do it inside the constructor to make it plain what's happening when the object is created, especially when you go to write subclasses.

Generally though it's a matter of taste and the most important thing is to be consistent in your choice.

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This is the same as the difference between

int i;
...
i = 0;

and

int i = 0;

My opinion is that the initialization should be close to the declaration if possible, in ideal case a part of it. Beside the bonus in readability you get smaller chances to forget the initialization. So the 2nd variant is a better one.

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I don't think you can say that one declaration is better then the other, it's all about the logic of your form, if you don't want to initiate Test on the form start-up, but on a button click then the first declaration is better.

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The latter, because declaration and initialization occur on the same line... easier to read, harder to make a mistake.

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The second is cleanest imho. I usually only create objects in the constructor when they need to be initialized with parameters.

No parameters

public class MainWindow
{
   private Test _test = new Test();

   public MainWindow()
   {

   }
}

with parameters:

public class MainWindow
{
   private Test _test;

   public MainWindow()
   {
       _test = new Test("abc")
   }
}

As Jackson Pope said, it can be a good idea to add a constructor for the object to make it easier to start using DI's later on. You can read about DI/IoC here: http://www.codeproject.com/KB/architecture/DependencyInjection.aspx

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In the example that you gave, neither of the to choices is better. Both of these snippets instantiate a member variable during construction of the class. The only real difference is that, in the second case, the member is initialized before the constructor is executed.

The only time that it really makes a difference is when the member variable needs information passed into its constructor that the Main class gets in it's constructor. Then you have no choice but to use the second option.

For example:

public class MainWindow  
{  
   private Test _test;  

   public MainWindow(int i)  
   {  
       _test = new Test(i);  
   }  
} 

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It doesn't make a difference at all, because the compiler generates exactly the same IL. In terms of readability it is just a matter of personal taste. I prefer the latter version but with the readonly modifier:

public class MainWindow
{
   private readonly Test _test = new Test();

   public MainWindow()
   {

   }
}

This is especially easier to read and to maintain when the class has more than one constructor.