C# do nested classes need to be instantiated?

Question

In the following scenario:

public class outerclass
{
   public innerClass Ic
     {get;set}

   public class innerClass
   {

   }
}

Do you need to instantiate the inner class property before assigning values to it, like this?

public class outerclass
{
   public outerclass()
     {
        this.Ic = new innerClass(); 
     }

   public innerClass Ic
     {get;set}

   public class innerClass
   {

   }
}

Answer 1

在声明的范围类中无关紧要 - 您应该始终以相同的方式使用类：在与特定类实例交互之前，您必须使用new运算符创建它。

Answer 2

Yes, unlike a base class you need to instantiate an inner class if you wish to use it.

You can prove this to yourself quite easily by trying it:

public class OuterClass
{
    public InnerClass Ic { get; set; }

    public class InnerClass
    {
        public InnerClass()
        {
            Foo = 42;
        }

        public int Foo { get; set; }
    }
}

public class Program
{
    static void Main()
    {
        Console.WriteLine(new OuterClass().Ic.Foo);
    }
}

The above code throws a NullReferenceException because Ic has not been assigned.

I would also advise you to follow the Microsoft naming convention and use pascal case for type names.

Answer 3

In this case the answer doesn't depend on the fact that the class is defined inside your outer class. Because you used the automatic getter/setter, a hidden backing field is used for the property Ic . Like all fields of reference type, this field has a default value of null . Thus if you try to access the members of Ic without setting it to refer to some instance, you can expect a NullReferenceException .

Everything I just said would still be true even if innerClass was defined somewhere else.

Answer 4

No, the property is an instance of the class. The set would set a new instance anyway. No need to construct one unless you want to make sure the get never returns null.