c# - Why throwing exception in constructor results in a null reference?

Question

Why throwing exception in constructor results in a null reference? For example, if we run the codes below the value of teacher is null, while st.teacher is not (a Teacher object is created). Why?

using System;

namespace ConsoleApplication1
{
  class Program
  {
    static void Main( string[] args )
    {
      Test();
    }

    private static void Test()
    {
      Teacher teacher = null;
      Student st = new Student();
      try
      {
        teacher = new Teacher( "", st );
      }
      catch ( Exception e )
      {
        Console.WriteLine( e.Message );
      }
      Console.WriteLine( ( teacher == null ) );  // output True
      Console.WriteLine( ( st.teacher == null ) );  // output False
    }
  }

  class Teacher
  {
    public string name;
    public Teacher( string name, Student student )
    {
      student.teacher = this;
      if ( name.Length < 5 )
        throw new ArgumentException( "Name must be at least 5 characters long." );
    }
  }

  class Student
  {
    public Teacher teacher;
  }

}

Answer 1

The constructor never completes, therefore the assignment never occurs. It's not that null is returned from the constructor (or that there's a "null object" - there's no such concept). It's just that you never assign a new value to teacher, so it retains its previous value.

For example, if you use:

Teacher teacher = new Teacher("This is valid", new Student());
Student st = new Student();
try
{
    teacher = new Teacher("", st);
}
catch (... etc ...)

... then you'll still have the "This is valid" teacher. The name variable still won't be assigned a value in that Teacher object though, as your Teacher constructor is missing a line such as:

this.name = name;