Before reading any further, please read the disclaimer.
The C# compiler is kind enough to give you a "field is never assigned to" warning if you forget to initialize a readonly member which is private or internal, or if the class in which it is being declared is internal. But if the class is public, and the readonly member is public, protected or protected internal, then no warning for you! Why, oh why?
namespace Test1
{
class Test1
{
#if TRY_IT
public readonly int m; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
protected readonly int n; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
internal readonly int o; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
private readonly int p; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
protected internal readonly int q; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
Test1()
{
if( p != 0 ) //To avoid warning 'The field is never used'
return;
}
#endif
}
public class Test2
{
#if TRY_IT
private readonly int m; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
internal readonly int n; //OK: warning CS0649: Field is never assigned to, and will always have its default value 0
Test2()
{
if( m != 0 ) //To avoid warning 'The field is never used'
return;
}
#endif
public readonly int o; //Blooper: no warning about field never assigned to.
protected readonly int p; //Blooper: no warning about field never assigned to.
protected internal readonly int q; //Blooper: no warning about field never assigned to.
}
public sealed class Test3
{
public readonly int m; //Blooper: no warning about field never assigned to.
}
}
For a moment you might think "well, a descendant might initialize that member", but that theory does not hold any water, for a number of reasons:
- Internal classes may also be subclassed, but the compiler does not fail to issue the warning in their case.
- Sealed classes may not be subclassed, but the compiler fails to issue the warning in their case, as Test3 in the sample code demonstrates.
- The warning makes sense for the sake of the integrity of the base class regardless of what a derived class may or may not do.
- Lastly but most importantly, the C# specification expressly prohibits a derived class from initializing a readonly member of a base class. You get Error CS0191: A readonly field cannot be assigned to (except in a constructor or a variable initializer) which, incidentally, is a little bit misleading, because you may be trying to assign the field from within a constructor, only it is the constructor of the wrong class.
Compiler Warning (level 4) CS0649:
Field 'field' is never assigned to, and will always have its default value 'value'
The compiler detected an uninitialized private or internal field declaration that is never assigned a value.
The question is: why?
UPDATE:
I posted this question on StackOverflow, and Eric Lippert himself answered it. The short answer is that it is an oversight of the compiler, but the long answer is also quite interesting and worth reading.
-
UPDATE:
I posted this question on StackOverflow, and Eric Lippert himself answered it. The short answer is that it is an oversight of the compiler, but the long answer is also quite interesting and worth reading.
-
