Instantiating objects in the constructor

Question

Is there any benefit to doing the following:

public class Foo
{
    private Bar bar;

    public Foo()
    {
        bar = new Bar();
    }
}

Instead of doing it like so:

public class Foo
{
    private Bar bar = new Bar();

    public Foo()
    {
    }
}

Given that at instantiation, the private member variable in either example will be instantiated, I don't believe there is a difference, but I've seen it enough times to where I'm curious.

Answer 1

In the exact case you've given, there isn't a difference - but in general there is.

Variable initializers are executed before the base class constructor is called. If that base constructor calls a virtual method which uses some of the instance variables, you can see that difference.

For spec fans, it's in section 10.11.2 of the C# 4 spec:

When an instance constructor has no constructor initializer, or it has a constructor initializer of the form base(...), that constructor implicitly performs the initializations specified by the variable-initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor.

Here's an example demonstrating this:

using System;

public class Base
{
    public Base()
    {
        Dump();
    }

    public virtual void Dump() {}    
}

class Child : Base
{
    private string x = "initialized in declaration";
    private string y;

    public Child()
    {
        y = "initialized in constructor";
    }

    public override void Dump()
    {
        Console.WriteLine("x={0}; y={1}", x, y);
    }
}

class Test
{
    static void Main(string[] args)
    {
        new Child();
    }
}

Result:

x=initialized in declaration; y=

Now having said the above, I would try hard to avoid calling virtual methods from a constructor. You're basically asking the derived class to work in a partially-initialized fashion. However, it's a difference you should be aware of.

As for where to initialize variables... I have to admit I'm not particularly consistent, and I don't find that's actually a problem. If I have any specific bias, it's probably to initialize anything which won't depend on any parameters at the point of declaration, leaving the variables which I can't initialize without extra information to the constructor.