Why should member variables be initialized in constructors?

Question

When I first started working with object-oriented programming languages, I was offered the following rule:

When declaring a field in a class, do not initialize it yet. Do it in the constructor.

Example in C #:

public class Test
{
    private List<String> l;

    public Test()
    {
        l = new List<String>();
    }
}

But when someone recently asked me why to do this, I could not think of a reason. I am not very familiar with the internal workings of C # (or other programming languages, since, in my opinion, this can be done in all OO languages).

So why is this done? Is it safety? Properties?

Answer 1

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Answer 2

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class Test
{
    Object o = new Object();
}

:

class Test
{
    Object o;

    public Test()
    {
        this.o = new Object();
    }
}

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Answer 3

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Ex. ( 2)

  var foo = new Foo { Bar = "baz" };

  public class Foo
  {
       public string Bar { get; set; }

       public Foo() { }
  }

Answer 4

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Answer 5

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Answer 6

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