Difference between "var objectName= new SomeObject()" and "interfaceName objectName= new SomeObject()"

Question

I was doing some code analysis via code inspections from resharper and i got the next warning:

Only implementations of property 'propertyname' are used

That didn't helped me at all because then i wouldn't have an interface anymore...

So I started testing how to get rid of that warning. On the folowing dummy interface and class:

public interface ITest
{
    string Name { get; set; }
}

public class Test: ITest
{
    public string Name { get; set; }
}

if i use:

var newTest = new Test();
newTest.Name= "newName";

Then the warning occurs. But when I use the next line, the warning will disappear.

ITest newTest = new Test();
newTest.Name = "newName";

I'm not an interface Ninja, so i was wondering, what is the difference between both manners..?

Thanks!

whether can [this](http://stackoverflow.com/questions/296783/resharper-vars?rq=1) article help you? It seems to me the same is asked there. — Mr Zak, Mar 28 '13 at 12:25
checkout this article this may be helpfull to you http://msdn.microsoft.com/en-us/library/64syzecx(v=vs.80).aspx — navya, Mar 28 '13 at 12:32

James · Accepted Answer · 2013-03-28T12:32:26.870

3

This infers the concrete type:

//X is of type X
var x = new X();

This does not

//X is of type IX
IX x = new X();

I'm guessing the warning will still occurt if you write

//X is of type X
X x = new X();

Also does the following remove the warning?

public void TestX()
{
    //X is of type X
    var x = new X();
    UseIX(x);
}

public void UseIX(IX interfaceParam)
{
    interfaceParam.Foo = 1;
}

edited Mar 28 '13 at 12:32

answered Mar 28 '13 at 12:26

James

indeed, the warning is shown with the third one. And with the last one it is not shown. But is there and advantage of using 'IX x = new X();" instead of the var one.? Or is there an best practice...? – bruno Mar 28 '13 at 12:55
@bruno The only difference I can think of is that declaring the variable as IX would prevent you from using non-interface methods in the code and force you to case IX to X in order to use them. Not necesserily an advantage. I think the usage of it comes down more to personal preference. – James Mar 28 '13 at 13:10

score 2 · Answer 2 · answered Mar 28 '13 at 12:25

var newTest = new Test() will set the type of newTest to Test. ITest newTest = new Test() will set the type of newTest to ITest.

So with the former ITest is not required as it is never used.

So resharper is just saying you don't need ITest. It's up to you whether you agree. Do you need ITest?

2 Answers2