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

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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?

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.


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);

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);

How to check a type for parameterless constructor?, GetConstructor(Type.EmptyTypes) will return the parameterless constructor, or null if one does not exist, so you can have: return typeof(TT). Validation in constructor is completely fine. This "no logic in constructor" rule does not apply on it, guess it's worded bit unfortunately. Task of constructor is to take dependencies from outside (dependency injection), make sure they are valid and then probably store them in instance attributes - creating valid instance in short.

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.

Type.GetConstructors Method (System), Constructors for struct types resemble class constructors, but structs cannot contain an explicit parameterless constructor because one is provided As you know, in C++ when you declare any constructor, the compiler will no longer generate� Constructors look a little strange because they have no return type, not even void. This is because the implicit return type of a class constructor is the class type itself. It is the constructor's job to initialize the internal state of an object so that the code creating an instance will have a fully initialized, usable object immediately.

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) ...

Gcc default constructor, 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� Being only able to only check that a type as a public parameterless as answered here would not be enough as it would not allow calling the target methods. The objective is to have the following logic, where IInteresting objects do not implement a public parameterless constructor and need to be converted before calling Save1:

This should work:

   myClass.GetType().GetConstructors()
                    .All(c=>c.GetParameters().Length == 0)

How do I check if a type provides a parameterless, Could the new JSON API check if the constructor parameters have of reference types without parameterless constructor is not supported. The C# parameterless default constructor. Mar 29, 2015 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:

Depending on your situation, you could also use a generic type restriction:

public void DoSomethingWith<T>(T myObject) where T:new() {...}

The above method declaration will restrict the parameter type to any Object that can be instantiated with a parameterless constructor. The advantage here is that the compiler will catch any attempt to use the method with a class that doesn't have a parameterless constructor, so as long as the type is known SOMEWHERE at compile-time, this will work and will alert you to a problem earlier.

Of course if the type really is known only at runtime (i.e. you're using Activator.CreateInstance() to instantiate an object based on a string or a constructed Type) then this won't help you. I generally use reflection as the absolute last option, because once you've gone to dynamic land you pretty much have to stay in dynamic land; it's usually difficult or even messier to dynamically instantiate something and then start dealing with it statically.

NotSupportedException when Deserializing object that has default , A default constructor is a constructor that either has no parameters, or if it has parameters, is needed, the compiler implicitly declares a default parameterless constructor A::A() . The compiler will implicitly define A::A() when the compiler uses this constructor to create an object of type A . The Provide feedback to IBM. No parameterless constructor defined for this object. Structs cannot contain explicit parameterless constructors The ixmlserializable type 'system.xml.linq.xelement' cannot be deserialized in partial trust because it does not have a public parameterless constructor.

Default constructors (C++ only), To quote the spec, "An instance constructor is a member that implements the actions required will implicitly try to call a parameterless constructor from the base type. as otherwise it won't know what it's meant to be providing information on. Most likely, you're running Visual Studio 2015, which doesn't have full .NET Core support. Basically, in previous versions of EF, DbContext had a parameterless constructor, and this version of the scaffold generator is depending on that. In EF Core, DbContext does not have a parameterless constructor, so the generator is choking on that.

Constructors in C#, It says that it needs an parameter-less constructor to do so. If there's a mismatch between the interface you want and the interface This does limit the usefulness of the framework you are using, but gives you more flexibility. You might find them easier when constructing an object representing the� It turns out that if you write your value type in IL, you can provide your own parameterless constructor with custom code without ilasm complaining at all. It’s possible that other languages targeting the CLI allow you to do this as well, but as I don’t know any, I’ll stick to IL.

Can I use Dependency Injection to test my class when a , CreateInstance(type); //Requires parameterless constructor. GetConstructor( new[] { typeof(string) }); // Don't forget to check if such constructor exists if (c == null) throw Length]; // array that will contains parameter expessions for (var i = 0 ;� If the valid state of your object depends on some private members, then XmlSerializer can not help you; and if you do not provide a parameterless constructor, XmlSerializer would not know how to create the object when it tries to deserialize, since it would not know what value to pass to the parameters.

Comments
  • Sidenote: There is a generic constraint for parameterless constructors.
  • is the question whether the type only provides a parameterless constructor or if it provides one at all?
  • The latter. I don't mind the presence of additional ctors.
  • this wont find private ctors fyi
  • Addressed comments above.
  • Short and Sweet. +1 from the future.
  • Not what I meant, but please don't delete - it's a related issue and a nice information.
  • Indeed, the correct expression for the OP's request should be myClass.GetType().GetConstructors().Any(c=>c.GetParameters().Length == 0). Note the use of Any instead of All.
  • Actually, there's a very useful pattern for bridging the lands of static and dynamic invocation: a static generic caching class [e.g. EqualityComparer.Default<T>]. For any given type T, one will only have to use Reflection once to build an object whose instance type will be statically known to qualify for any required constraints, and store a reference to it in a field that does not impose any constraints that a caller might not be able to supply.