The C# parameterless default constructor

There’s not many things in C# that I wish were different, but this is one of them. If you define a class, it comes with a default parameterless constructor if you don’t define one:

Unless the class is static, classes without constructors are given a public default constructor by the C# compiler in order to enable class instantiation.

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/using-constructors

So this class:

public class Foo
{
    private string Bar;
}

is equivalent to this one:

public class Foo
{
    private string Bar;
    public Foo() : base() { }
}

So far so good. Now if you have a constructor with a parameter taking a default value, this rule doesn’t apply anymore.

In the following example, the parameterless contructor is not automatically provided:

public class Foo
{
    private string Bar;
    public Foo(string bar = null)
    {
        Bar = bar;
    }
}

In most cases, you would be creating an instance using new as in var foo = new Foo() and shouldn’t care much about the missing default constructor. But if you are using reflection as in var o = Activator.CreateInstance(typeof(Foo));, you will be out of luck. Doing so will give you a System.MissingMethodException, No parameterless constructor defined for this object.

This is a problem I often run into when creating a custom resolver for AutoMapper (implementing the IValueResolver interface) for example.

Your only option is to recreate the parameterless constructor:

public class Foo
{
    private readonly string Bar;
    public Foo(string bar = null)
    {
        Bar = bar;
    }
    public Foo() : this(null) { } //default constructor
}

Updated in April 2018:

I have posted a new post about why the last line was actually a wrong assumption. You can read it here.

See something wrong or want to ask a question? Get in touch on Twitter

Next: You can find more articles on this blog by going to the homepage.