Cannot create an instance of the variable type 'Item' because it does not have the new() constraint

You can't initialize Generic type object unless you mark it as implementing default constructor using new keyword:

public void CountTestHelper<Item>() where Item : IHasRect, new()
 {
    Rectangle rectangle = new Rectangle(0, 0, 100, 100); 
    SomeClass<Item> target = new SomeClass<Item>(rectangle);            
    Point p = new Point(10,10);
    Item i = new Item();    // constructor has to be parameterless!
    ...
 }

On the other hand, if you're trying to initialize an Item type object defined somewhere else in the application try using the namespace before it:

MyAppNamespace.Item i = new MyAppNamespace.Item(p, 10);

Because many people get here by the question tilte (which is very generic and matches the compiler message) let me give a more detailed answer about the compiling error itsef.

You are using generics in a method. The compiler doesn´t know which type it will receive and thus it is not warranted that your type has a parameterless construtor. For ex:

class A {
    A(int i){ ... }
}

class B { ... }

public void MyMethod<T>(){
    T t = new T(); //This would be fine if you use 'MyMethod<B>' but you would have a problem calling 'MyMethod<A>' (because A doesn´t have a parameterless construtor;
}

To resolve this, you can tell the compiler that your generic parameter has a parameterless construtor. This is done by defining constraints:

public void MyMethod<T>()  where T: new(){
    T t = new T(); //Now it's ok because compiler will ensure that you only call generic method using a type with parameterless construtor;
}

More information about constructor constraints may be found here: https://msdn.microsoft.com/en-us/library/bb384067.aspx

The Item in the line:

Item i = new Item(p, 10);

refers to the generic type parameter Item of the CountTestHelper method, not the class Item. Change the generic parameter name e.g.

public void CountTestHelper<TItem>() where TItem : IHasRect
{
    Rectangle rectangle = new Rectangle(0, 0, 100, 100); 
    SomeClass<TItem> target = new SomeClass<TItem>(rectangle);            
    Point p = new Point(10,10);
    Item i = new Item(p, 10);    
    ...
}

alternatively you can fully qualify the name of the Item class you want to create:

public void CountTestHelper<Item>() where Item : IHasRect
{
    Rectangle rectangle = new Rectangle(0, 0, 100, 100); 
    SomeClass<Item> target = new SomeClass<Item>(rectangle);            
    Point p = new Point(10,10);
    SomeNamespace.Item i = new SomeNamespace.Item(p, 10);  
}