Why throwing exception in constructor results in a null reference?
Solution 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;
Solution 2
Cause you're checking the referencies.
try
{
teacher = new Teacher( "", st ); //this line raises an exception
// so teacher REMAINS NULL.
// it's NOT ASSIGNED to NULL,
// but just NOT initialized. That is.
}
catch ( Exception e )
{
Console.WriteLine( e.Message );
}
but
public Teacher( string name, Student student )
{
student.teacher = this; //st.Teacher is assigned BEFORE exception raised.
if ( name.Length < 5 )
throw new ArgumentException( "Name must be at least 5 characters long." );
}
Solution 3
When you throw an exception in a constructor, you break object's construction. So it's never finished and hence, there's no object to return. In fact, that assignment operator (teacher = new Teacher( "", st );
) is never executed since exception breaks the calling stack.
And the Teacher constructor still writes a reference to itself (the object being constructed) into the Student object's property. But you should never try using this Teacher object afterwards, since it has not been constructed. It may result in undefined behavior.
Solution 4
If Foo
is a reference type, the statement Foo = new FooType();
will construct an object and then, after the constructor has completed, store a reference into Foo
. If the constructor throws an exception, the code which would store the reference into Foo
will be skipped without Foo
having been written.
In cases where:
- A statement like the above occurs within a
try
/catch
block - The statement can be reached without
Foo
having been written beforehand. Foo
a local variable defined in a context surrounding thecatch
block.- It is possible for execution starting at the catch to reach a statement which reads
Foo
without its having written in after thecatch
.
The compiler will assume that the latter attempt to read Foo
could be executed without Foo
having been written, and will refuse compilation in that case. The compiler will allow Foo
to be read without having been written, however, if:
Foo
is a class field, or a field of a struct stored in a class field, a field of a struct stored in a field of a struct stored in a class field, etc.Foo
is passed as anout
parameter to a method (written in a language other than C#) that doesn't store anything to it, and the statement which readsfoo
would only be reachable if the method had returned normally rather than via exception.
In the former case, Foo
will have a defined value of null
. In the latter case, the value of Foo
will likely be null the first time it is created during the execution of a method; if re-created within a loop, it may contain null
or the last value to written to it after the last time it was created; the standard is not specific about what will happen in that situation.
Note that if FooType
has anything resembling a normal constructor, Foo = new FooType();
will never cause Foo
to become null if it wasn't before. If the statement completes normally, Foo
will hold a reference to an instance of exact type FooType
to which no reference had previously existed anywhere in the universe; if it throws an exception, it will not affect Foo
in any way.
Setyo N
Updated on June 04, 2020Comments
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Setyo N about 4 years
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; } }
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Setyo N about 12 yearsthanks for the great explanation, I edited "null object" in the question to "null reference".
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Asherah almost 11 yearsThis is not correct; it doesn't result in anything at all, as pointed out in other answers.
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Hi-Angel over 9 yearsGreat explanation, also you're just proved that not initialized object in C# always holds «null». I began doubt in that because when I tried to use in Visual Studio an object that could be uninitialized in a certain conditions, but anyway it was checked checked for «null», and next used, the compiler shown an error about uninitialized variable. After I explicitly initialized the object with «null», the error disappeared. Thanks to you, now I know that it is just a bug of Visual Studio.
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Jon Skeet over 9 years@Hi-Angel: Nope, it's not a bug. It's the difference between a field and a local variable. A field has a default value, and can be used without ever having been set - a local variable cannot be read until it's definitely assigned.