Create instance from class in c# via reflection

Question

Consider this code:

class Program
{
    static void Main(string[] args)
    {
        var instance = Activator.CreateInstance<Person>();//No parameterless constructor defined for this object.

    }
}

public class Person
{
    public Person(string name = "Shahrooz") { }
}

When use this code: Activator.CreateInstance<Person>();I get this error:

No parameterless constructor defined for this object.

Note that my constructor has default parameter:string name = "Shahrooz"

Why we can not create instance from a class that has constructor,despite the constructor has default value parameter?

Sergey Kalinichenko · Accepted Answer · 2013-09-14 10:38:01Z

8

We can not create instance from a class that has constructor because the constructor still needs one parameter, even though it is defaulted. You can get the default value, though, by calling GetParameters() and accessing the initial element:

ConstructorInfo constr = typeof(Person).GetConstructor(new[] {typeof(string)});
ParameterInfo p0 = constr.GetParameters()[0];
object defaultValue = p0.DefaultValue;
Person p = (Person)constr.Invoke(new[] {defaultValue});
// ...or using Activator
Person p = (Person)Activator.CreateInstance(typeof(Person), defaultValue);

answered Sep 14, 2013 at 10:38

Sergey Kalinichenko

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3 Comments

user1968030 Over a year ago

But why we can create by this code: object o = Activator.CreateInstance(typeof(Person), "Ali");

Karel Frajták Over a year ago

because the method requires array of objects/parameters

Sergey Kalinichenko Over a year ago

@ShahroozJefriㇱ You can do that because you pass the parameter explicitly. The generic version of Create<T> requires a parameterless constructor, which is not the same as a constructor will all its parameters defaulted to some values.

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