Question

I am trying to test a method – and getting an error:

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

Required information for below:

public interface IHasRect
{
    Rectangle Rectangle { get; }
}

Helper class:

class Item : IHasRect
{
    public Item(Point p, int size)
    {
        m_size = size;
        m_rectangle = new Rectangle(p.X, p.Y, m_size, m_size); 
    }
}

For the function to be tested, I need to instantiate an object…

public class SomeClass<T> where T : IHasRect

The test:

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);
    Item i = new Item(p, 10);      // error here        
    ...
}
[TestMethod()]
public void CountTest()
{
    CountTestHelper<Item>();
}   

I am trying to understand what this error means, or how to fix it, by reading http://msdn.microsoft.com/en-us/library/d5x73970.aspx and http://msdn.microsoft.com/en-us/library/x3y47hd4.aspx – but it doesn’t help.

I don’t understand this error – I have already constrained the “SomeClass” to be of type. I cannot constrain the entire Test class (the unit test class generated by Visual Studio, which contains all the tests) – I will get a number of other errors otherwise. The Item class doesn’t have any template…

Answer 1

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);  
}