IOC vs New guidelines

Question

Recently I was looking at some source code provided by community leaders in their open source implementations. One these projects made use of IOC. Here is sample hypothetical code:

public class Class1
{
    private ISomeInterface _someObject;

    public Class1(ISomeInterface someObject)
    {
        _someObject = someObject;
    }

    // some more code and then

    var someOtherObject = new SomeOtherObject();
}

My question is not about what the IOCs are for and how to use them in technical terms but rather what are the guidelines regarding object creation. All that effort and then this line using "new" operator. I don't quite understand. Which object should be created by IOC and for which ones it is permissible to be created via the new operator?

Answer 1

As a general rule of thumb, if something is providing a service which may want to be replaced either for testing or to use a different implementation (e.g. different authentication services) then inject the dependency. If it's something like a collection, or a simple data object which isn't providing behaviour which you'd ever want to vary, then it's fine to instantiate it within the class.