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Am I understanding encapsulation/properties correctly? (C#)


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#1 Bill Fountaine   Members   -  Reputation: 193

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Posted 05 October 2012 - 06:43 AM

For starters, it is my understanding that almost all fields should be private. And if they need to be accessed outside of the class, to use properties.

It is also my understanding properties sets it up so your code is only used how you intend it to be used. Like setting a Month property where you can only have 12 months, and external classes can't set it to say, 13, etc.

Only other thing I think I understand is,when setting a field private, and adding a property. It allows that field to behave the way it was supposed to in that class, while the property enables that field to be used/changed outside the class, while not changing the original field. Am I getting this correctly?

Is that really all there is to, it, as well as the obvious hiding data that doesn't need to be seen outside of that class? Or is there more to it that I am missing?

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#2 Telastyn   Crossbones+   -  Reputation: 3726

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Posted 05 October 2012 - 07:24 AM

Describing encapsulation as data hiding is one of the great failings of OO teaching. Encapsulation exists to protect class invariance.

Otherwise, your second paragraph is key. How that happens isn't really important. Different languages use different constructs to achieve that.

#3 turch   Members   -  Reputation: 590

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Posted 05 October 2012 - 07:36 AM

while the property enables that field to be used/changed outside the class, while not changing the original field.


Not quite. If you just declare a data member public, there is no way to ensure that the data contained is valid, because a user can set it to any value they want. So one pattern that emerged from early oo design is something like this:

Class Foo
{
	 private:
	 string name;
	
	 public:
	 void setName(string newName)
	 {
		  name = newName;
	 }

	 string getName()
	 {
		  return name;
	 }
};

You would define a private data member and get/set functions. In the code above, the data member is effectively public. You could have a read-only data member - for example, if you were implementing some sort of array wrapper, you probably want a data member called Count, and you want to let people read it but not set it. In that case you would make setCount private. You can also do data validation - make sure that the int month passed in is between 1 and 12, etc.

Those functions are called accessors, and C# Properties are just a easier way to get the same result - a private variable with 2 functions, without having to type in as much. So to get the same effect as above in C#, you would write something like this;

class Foo
{
	 public string Name { get; set; }
}

// meanwhile, in another file
Foo foo;
foo.Name = "Roger Wilco";
Console.WriteLine(foo.Name);

This is simply shorthand for saying "I want a string property called Name with default get and set behavior, and I don't care about the actual variable". You don't have to define the actual variable to store the data, and you don't need to define the get and set bodies. As you can see, the accessors are used just like they were a variable, but behind the scenes it calls the get or set function. Note that the only reason you do this is to be consistent with stuff where you actually need custom set or get behavior, because otherwise this is no different than a public string.

Here's another example with actual set and get methods defined:

class Foo
{
	 private int month;

	 public int Month
	 {
		  get
		  {
			   return month;
		  }

		  set
		  {
			   if (value >= 1 && value <= 12)
					month = value;
		  }

		  public string MonthName
		  {
			   get
			   {
					switch (month)
					{
						 case 1:
							  return "January";
							  break;
						 // etc...
					}
			   }
			   private set {}
		  }
}

So here you have a class with an int property that can be set to any number from 1 - 12, and can be read. There is also a string property which returns the string representation of the month, and has a private set.

Edit: the variable value that I use in the set method is automatically defined as the value passed into the function.
Edit 2: much more info presented more coherently at msdn

Edited by turch, 05 October 2012 - 07:56 AM.





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