How do I check if a type provides a parameterless constructor?

Question

I'd like to check if a type that is known at runtime provides a parameterless constructor. The Type class did not yield anything promising, so I'm assuming I have to use reflection?

Answer 1

The Type class is reflection. You can do:

Type theType = myobject.GetType(); // if you have an instance
// or
Type theType = typeof(MyObject); // if you know the type

var constructor = theType.GetConstructor(Type.EmptyTypes);

It will return null if a parameterless constructor does not exist.

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If you also want to find private constructors, use the slightly longer:

var constructor = theType.GetConstructor(
  BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, 
  null, Type.EmptyTypes, null);

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There's a caveat for value types, which aren't allowed to have a default constructor. You can check if you have a value type using the Type.IsValueType property, and create instances using Activator.CreateInstance(Type);

Answer 2

type.GetConstructor(Type.EmptyTypes) != null

would fail for structs. Better to extend it:

public static bool HasDefaultConstructor(this Type t)
{
    return t.IsValueType || t.GetConstructor(Type.EmptyTypes) != null;
}

Succeeds since even enums have default parameterless constructor. Also slightly speeds up for value types since the reflection call is not made.

Answer 3

Yes, you have to use Reflection. But you already do that when you use GetType()

Something like:

var t = x.GetType();
var c = t.GetConstructor(new Type[0]);
if (c != null) ...