How to write a unit test without interface implementation

I'm new to unit testing.

I have to test the following method:

private readonly int id;

private readonly IService service;

public MyClass(int id, IService service)

{

id= id;

service= service;

}

public double Number{ get; private set; }

public void RefreshAmount()

{

Number= service.GetTotalSum(id);

}

What would be a correct unit test to write?