Parameterless Constructors

Question

In C#, is there way to enforce that a class MUST have a parameterless constructor?

Answer 1

If you're talking about generic constraints, yes:

class SomeContainer<T> where T : new() {
    ...
}

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If you're talking about inheritance. It is not possible to require that every class that implements your interface or inherits your base class has a parameterless constructor.

The best you can do is use reflection in your base constructor to throw an exception (at runtime), like this:

abstract class MyBase {
    protected MyBase() {
        if (GetType().GetConstructor(Type.EmptyTypes) == null)
            throw new InvalidProgramException();
    }
}

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If you're talking about a single class, yes; just put one in.

Answer 2

Generics can enforce this, but we aren't always using generics ;p

If you are using unit tests, you could use reflection to find all the types that meet the pattern you want to have parameterless constructors (for example, everything derived from MyBaseObject, or everything in the Foo.Bar namespace), and verify that way (by finding the parameterless constructor).

If you want to assert this at runtime too (perhaps in #DEBUG), things like static constructors can be useful points to inject extra type checks.