Difference between class & type

Kok Keong Chua

Hi all,
I'm a newbie in C#, appreciated if someone can clear my doubt on the class & type:

DirectoryInfo is a .net class, we need to instantiate an object to use it as below:

string path = @"C:\My Folder";
DirectoryInfo di = new DirectoryInfo(path);

However, why we also can use this class as a type as shown below:
DirectoryInfo parent = Directory.GetParent(path);

4 Replies

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Vulpes15y ago
Accepted answer
You need to differentiate between instance and static members of a class.

As their name suggests, 'instance' members apply to a particular instance of the class. However, static members apply to the class as a whole and know nothing about particular instances.

When you access an instance member, you precede the member by a reference to a particular instance.

In contrast, when you access a static member, you precede the member by the class name.

So in your example, you could do:

DateTime dt = di.LastAccessTime;

and because LastAccessTime is an instance property, you precede it by 'di' which is a reference to a DirectoryInfo instance.

Although the DirectoryInfo has no static members itself, there is an associated class called Directory which contains only static members. So in this line:

DirectoryInfo parent = Directory.GetParent(path);

you're calling Directory's static method, GetParent, which returns a DirectoryInfo instance and so you can therefore assign it to the DirectoryInfo variable, parent.

As you gain more experience of C#, this sort of thinking will become automatic :)

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Kok Keong Chua15y ago
thanks for all the replied, now i get better idea of it! :)
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Sam Hobbs15y ago
Vulpes answered your question but his explanation is so extensive it might be confusing. The important thing is that Directory.GetParent is static.

One way to determine that Directory.GetParent is static is to go to the MSDN and search for it, as in:
http://social.msdn.microsoft.com/Search/en-US?query=directory.getparent&ac=1

Then click on the Directory.GetParent Method (System.IO) . See where it says that it is static? Another way to get to the documentation is to put the cursor over the method or property and press F1.
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Ankit Nandekar15y ago
Class is also a user define type. Class instances are of the type of the associated class. For example, an instance of the class "Fruit" (a "Fruit" object) would be of the type "Fruit".
in ur above example string is also a class but it is inbuild class provided by visual stdio,to check this weather string is a class or not in ur program right click on string and u can see Go to definition option is there click on it will show u how string class implemented in visual stdio.when we r writing like string path="c:/My Folder"; it is automaticly create instance of string class.
We r creating instance of a class because class is a type and type not taking any memory space in memory to use class members we have to create instance which is taking memory space in memory for example
int 5;//error because we can not store directly value into types
int i=5;//work

int is a value type so it is define as a structure in vs,all value types define as a structure in vs and all reference type like string define as a class in visual studio.
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