使用和不使用构造函数声明 C# 变量

Question

I have this doubt in my mind for a very long time. May be this is the right forum to ask..

Is there any difference between the two types of variable declaration in C#. If so what is the difference and which is the best practice.

  DataTable dt;

vs

  DataTable dt=new Datatable();

Answer 1

First one you are only declaring and in second you declaring and initializing. If you use any instance of a class without initializing, then you will get null reference exception as Object reference not set to an instance of an object

Answer 2

http://msdn.microsoft.com/en-us/library/ms173109.aspx

if DataTable is a struct they are the same

if not DataTable dt; means create empy reference to a DataTable named dt; 'new Datatable();' means init the reference using Datatable object from the garbage collector

Answer 3

In my opinion the best practice is to always initialize the variable in the constructor and never in the declaration. This is for consistency mainly, if you want to see how a type is initialized you should always only have to look in the constructor.

Answer 4

The first dt can hold a DataTable or object that inherites DataTable , the value of the first dt will be null after the first one is done.

The second dt can hold a DataTable or object that inherites DataTable, You initiated it with empty constructor of DataTable therefore it will not be null .

"Best Parctice" is depends on what you want to do with dt. for example, if you have a method on the next line that returns a new initiated DataTable it is not nessesery to initiate an instance that will not be used.