Avoid Default Constructors in Domain Models

In C#, a default constructor (a constructor with no parameters) is silently added by the compiler when there is no explicit constructor available in a class. All the fields of the object will be set to their default values when a default constructor is invoked at runtime.

In the example below, you can observe that when an object of the LawFirm class is allowed to be created using a default constructor, the Name field of type string on the object is set to null as an initial value, which is the default value for a reference type. This forces consumers to check for null before accessing it because the object allowed itself to be created in an inconsistent state.

public class LawFirm
{
    public string Name { get; set; }

    public LawFirm()
    {

    }
}

To avoid this, always ensure that the object is initialized in a consistent state through a parameterized constructor or a factory method. Design your classes in such a way that if an object of a class exists, it is in a consistent state. As shown in the example below, an ArgumentNullException is thrown if null is passed in the constructor, terminating the object creation.

public class LawFirm
{
    public string Name { get; private set; }

    public LawFirm(string name)
    {
        this.Name = name ?? throw new ArgumentNullException(nameof(name));
    }
}

Always design your classes with the aim of making life easier for the consumers. Think twice before you allow a default constructor to sneak into your domain model.