i have a structure defined, which contains a public field and a public property named _one and One respectively, now i instantiate the struct in the main function (not creating new object), and called the Property from the struct, i am getting the compile time error saying use of unassigned local variable One, however when i called the field _one, it works pretty expected here what i am doing:
public struct myStruct
{
public int _one;
public int One
{
get { return _one; }
set { _one = value; }
}
public void Display()
{
Console.WriteLine(One);
}
}
static void Main(string[] args)
{
myStruct _struct;
_struct.One = 2; // Does not works
_struct._one = 2; // Works fine
}
can anyone explain whats the reason behind this, could not understand the concept.
You need to initialize the struct in order for the property to be accessible - _struct has a default value otherwise:
myStruct _struct = new myStruct();
By the way - mutable value types are evil.
This is unintuitive behavior, but it is permitted by the rules of Definite assignment checking. Described in excruciating detail in section 5.3 of the C# Language Specification. The key phrase, early in the chapter is:
In additional to the rules above, the following rules apply to struct-type variables and their instance variables:
- An instance variable is considered definitely assigned if its containing struct-type variable is considered definitely assigned.
- A struct-type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.
It is the latter rule that permits this. In other words, you can also initialize a struct by assigning all of its variables. You can see this by trying these snippets:
myStruct _struct = new myStruct();
_struct.Display(); // fine by the 1st bullet
myStruct _struct;
_struct.Display(); // bad
myStruct _struct;
_struct._one = 2;
_struct.Display(); // fine by the 2nd bullet
So you don't get CS0165 by assigning the field because that would disallow initializing the structure by assigning its variables.
The reasons which would favor using read-write properties instead of exposed fields in class definitions do not apply to structures, since they can support neither inheritance nor update notifications, and the mutability of a struct's field depends upon the mutability of the struct instance, regardless of whether the field is exposed or not. If a struct is supposed to represent a group of related but freely-independently-modifiable variables, it should simply expose those variables as fields. If a property with a backing field is supposed to be read-only, the constructor should set the backing field directly, rather than via property setter.
Related
If I define structs at the module level, I can reference not-yet defined structs.
struct S {
ComesLater c;
}
struct ComesLater {}
But If I do the same inside an unittest or a function block, it doesn't work:
unittest {
struct S {
ComesLater c;
}
struct ComesLater {}
}
Error: undefined identifier 'ComesLater'
Why is that? How can I get order-independent declarations inside functions? Is there some kind of forward-declaration in d? I need this because I generate structs using mixin and ordering the declarations in the order of their inner-dependencies would be quite some effort, sometimes impossible, if there are circularly referencing structs. (using pointers.)
Declarations inside functions, unittests, or anywhere else that statements can actually be executed are indeed order-dependent because their values may depend on the code before them running. Think of a local variable:
int a;
writeln(a);
a = b;
int b = get_user_line();
If order wasn't important there, when would the two functions get called? Would the user be asked for a line before the writeln as the declarations are rewritten?
The current behavior of making b an undefined variable error keeps it simple and straightforward.
It works independent of order in other contexts because there is no executable code that it can depend on, so there's no behavior that can change if the compiler needs to internally think about it differently.
So:
How can I get order-independent declarations inside functions?
Change the context such that there is no executable code... put it all inside another struct!
void main() { // or unittest { }
struct Holder {
static struct S {
C c;
}
static struct C {}
}
}
Since execution happens around the holder and doesn't happen inside it, the order of declaration inside doesn't matter again. Since you can define almost anything inside a struct, you can use this for variables, functions, other structs, and so on. Basically all you have to do is wrap your existing code inside the struct Holder {} brackets.
By making everything static inside, you can just use it like a container and reference the stuff with Holder.S, etc., on the outside.
I have just come across an interesting gotcha where optional arguments on an interface and the implementing class can conflict.
I found this out the hard way (school boy error) whilst experimenting. You cannot spot it in the debugger and I assumed it was me messing up the dependency injection.
I'm guessing this is so an alternative interface can give a differing view on what default behaviour should be?
Is there a compiler warning or style cop rule to help point this out?
public interface MyInterface
{
MyStuff Get(bool eagerLoad = true); //this overrules the implementation.
}
public class MyClass : MyInterface
{
public MyStuff Get(bool eagerLoad = false) //will still be true
{
//stuff
}
}
Remember default arguments are a compile-time feature. The compiler picks up the default argument based on the static type of the reference in question and inserts the appropriate default argument. I.e. if you reference is of the interface type you get one behavior but if the reference is of the class type you get the other in your case.
I used the below to see how dart calls methods passed in to other methods to see what context the passed in method would/can be called under.
void main() {
var one = new IDable(1);
var two = new IDable(2);
print('one ${caller(one.getMyId)}'); //one 1
print('two ${caller(two.getMyId)}'); //two 2
print('one ${callerJustForThree(one.getMyId)}'); //NoSuchMethod Exception
}
class IDable{
int id;
IDable(this.id);
int getMyId(){
return id;
}
}
caller(fn){
return fn();
}
callerJustForThree(fn){
var three = new IDable(3);
three.fn();
}
So how does caller manager to call its argument fn without a context i.e. one.fn(), and why does callerJustForThree fail to call a passed in fn on an object which has that function defined for it?
In Dart there is a difference between an instance-method, declared as part of a class, and other functions (like closures and static functions).
Instance methods are the only ones (except for constructors) that can access this. Conceptually they are part of the class description and not the object. That is, when you do a method call o.foo() Dart first extracts the class-type of o. Then it searches for foo in the class description (recursively going through the super classes, if necessary). Finally it applies the found method with this set to o.
In addition to being able to invoke methods on objects (o.foo()) it is also possible to get a bound closure: o.foo (without the parenthesis for the invocation). However, and this is crucial, this form is just syntactic sugar for (<args>) => o.foo(<args>). That is, this just creates a fresh closure that captures o and redirects calls to it to the instance method.
This whole setup has several important consequences:
You can tear off instance methods and get a bound closure. The result of o.foo is automatically bound to o. No need to bind it yourself (but also no way to bind it to a different instance). This is way, in your example, one.getMyId works. You are actually getting the following closure: () => one.getMyId() instead.
It is not possible to add or remove methods to objects. You would need to change the class description and this is something that is (intentionally) not supported.
var f = o.foo; implies that you get a fresh closure all the time. This means that you cannot use this bound closure as a key in a hashtable. For example, register(o.foo) followed by unregister(o.foo) will most likely not work, because each o.foo will be different. You can easily see this by trying print(o.foo == o.foo).
You cannot transfer methods from one object to another. However you try to access instance methods, they will always be bound.
Looking at your examples:
print('one ${caller(one.getMyId)}'); //one 1
print('two ${caller(two.getMyId)}'); //two 2
print('one ${callerJustForThree(one.getMyId)}'); //NoSuchMethod Exception
These lines are equivalent to:
print('one ${caller(() => one.getMyId())}');
print('two ${caller(() => two.getMyId())}');
print('one ${callerJustForThree(() => one.getMyId())}';
Inside callerJustForThree:
callerJustForThree(fn){
var three = new IDable(3);
three.fn();
}
The given argument fn is completely ignored. When doing three.fn() in the last line Dart will find the class description of three (which is IDable) and then search for fn in it. Since it doesn't find one it will call the noSuchMethod fallback. The fn argument is ignored.
If you want to call an instance member depending on some argument you could rewrite the last example as follows:
main() {
...
callerJustForThree((o) => o.getMyId());
}
callerJustForThree(invokeIDableMember){
var three = new IDable(3);
invokeIDableMember(three);
}
I'll try to explain, which is not necessarily a strength of mine. If something I wrote isn't understandable, feel free to give me a shout.
Think of methods as normal objects, like every other variable, too.
When you call caller(one.getMyId), you aren't really passing a reference to the method of the class definition - you pass the method "object" specific for instance one.
In callerJustForThree, you pass the same method "object" of instance one. But you don't call it. Instead of calling the object fn in the scope if your method, you are calling the object fn of the instance three, which doesn't exist, because you didn't define it in the class.
Consider this code, using normal variables:
void main() {
var one = new IDable(1);
var two = new IDable(2);
caller(one.id);
caller(two.id);
callerJustForThree(one.id);
}
class IDable{
int id;
IDable(this.id);
}
caller(param){
print(param);
}
callerJustForThree(param){
var three = new IDable(3);
print(three.id); // This works
print(param); // This works, too
print(three.param); // But why should this work?
}
It's exactly the same concept. Think of your callbacks as normal variables, and everything makes sense. At least I hope so, if I explained it good enough.
UPDATE
I have to apologize for confusing the readers. After I got totally lost in the code, I reverted all my changes from Mercurial repo, carefully applied the same logic as before -- and it worked. The answers below helped me understand the (new to me) concept better, and for that I gave them upvotes.
Bottom line: if a call to a missing method happens within a closure, and resolution set to DELEGATE_FIRST, methodMissing() will be called on the delegate. If it doesn't -- check you own code, there is a typo somewhere.
Thanks a lot!
Edit:
OK, now that you've clarified what your are doing (somewhat ;--))
Another approach (one that I use for DSLs) is to parse your closure group to map via a ClosureToMap utility like this:
// converts given closure to map method => value pairs (1-d, if you need nested, ask)
class ClosureToMap {
Map map = [:]
ClosureToMap(Closure c) {
c.delegate = this
c.resolveStrategy = Closure.DELEGATE_FIRST
c.each{"$it"()}
}
def methodMissing(String name, args) {
if(!args.size()) return
map[name] = args[0]
}
def propertyMissing(String name) { name }
}
// Pass your closure to the utility and access the generated map
Map map = new ClosureToMap(your-closure-here)?.map
Now you can iterate through the map, perhaps adding methods to applicable MCL instance. For example, some of my domains have dynamic finders like:
def finders = {
userStatusPaid = { Boolean active = true->
eq {
active "$active"
paid true
}
}
}
I create a map using the ClosureToMap utility, and then iterate through, adding map keys (methods, like "userStatus") and values (in this case, closure "eq") to domain instance MCL, delegating the closure to our ORM, like so:
def injectFinders(Object instance) {
if(instance.hasProperty('finders')) {
Map m = ClosureToMap.new(instance.finders).map
m?.each{ String method, Closure cl->
cl.delegate = instance.orm
cl.resolveStrategy = Closure.DELEGATE_FIRST
instance.orm.metaClass."$method" = cl
}
}
}
In this way in controller scope I can do, say:
def actives = Orders.userStatusPaid()
and "eq" closure will delegate to the ORM and not domain Orders where an MME would occur.
Play around with it, hopefully I've given you some ideas for how to solve the problem. In Groovy, if you can't do it one way, try another ;--)
Good luck!
Original:
Your missingMethod is defined on string metaclass; in order for it to be invoked, you need "someString".foo()
If you simply call foo() by itself within your closure it will fail, regardless of delegation strategy used; i.e. if you don't use the (String) delegate, good luck. Case in point, do "".foo() and it works.
I don't fully understand the issue either, why will you not have access to the closure's delegate? You are setting the closure's delegate and will invoke the closure, which means you will have access to the delegate within the closure itself (and can just delegate.foo())
nope, you will not catch a missing method and redirect it to the delegate with metaclass magic.
the closure delegate is the chance to capture those calls and adapt them to the backing domain.
that means...
you should create your own delegate with the methods required by the dsl.
do not try to force a class to do delegate work if it's not designed for the task, or the code will get really messy in not time.
keep everything dsl related in a set of specially designed delegate classes and everything will suddenly become ridiculously simple and clear.
I am running into a problem when trying to implicitly convert one of my dynamic types. There are two assemblies with definitions similar to the following:
Configuration.dll:
public class ConfigurationValue : DynamicObject
{
public ConfigurationValue(string val)
{
//...
}
//...
public static implicit operator string(ConfigurationValue val)
{
return val.ToString();
}
}
There is another class in this dll called Configuration with a member variable called Instance (to make the class singleton). This variable holds the ConfigurationValue instances in a dictionary and is of type dynamic. This allows me to do this following:
Server.dll:
//...
if (Configuration.Instance.SecurityLevel != "Insecure")
{
//...
}
Assuming that SecurityLevel is in the dictionary.
This if statement appears verbatim in my code and always fails with the following error:
{"Operator '!=' cannot be applied to operands of type 'System.Dynamic.DynamicObject' and 'string'"}
Previously, when these two classes were in the same assembly, this code worked fine. Can anyone tell me what I'm doing wrong here?
Thanks,
Max
Solved the problem, a little embarrassing actually, I forgot to change the container class for ConfigurationValue (e.g. the type of Configuration.Instance) from internal to public when I moved it to the new assembly, so of course the type couldn't be resolved and the implicit conversion was not found
Try
var SecurityLevel = new ConfigurationValue("Insecure");