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

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() constr

Answer 1

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 = new T(); //This would be fine if you use 'MyMethod' but you would have a problem calling 'MyMethod' (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()  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