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C#: Calling parent (Base) constructor from child's constructor body

I have an abstract class Parent and a derived class Child. I know that I can call Parent's constructor inside Child's constructor in the following way:
abstract class Parent
{
protected int i;
protected Parent(int i)
{
this.i = i;
}
}
class Child : Parent
{
public Child(int i) : base(i)
{
}
}
However, I don't want to pass some parameters to the Parent constructor right away. I would like to perform some calculations and then call Parent's constructor using the result of such calculation as input parameters. The code "would look" something like this:
public class Child : Parent
{
public Child(int i)
{
int calculation = i * 2; // Complicated operation
base(calculation); // This line will break
}
}
The second snippet is not valid C# code. Is there any way of postponing the call to Parent's constructor to mimic the sentiment expressed on the second code snippet?

This would do the same trick assuming u can access the properties directly
abstract class Parent
{
protected int i;
protected Parent()
{
//default constructor
}
}
class Child : Parent
{
public Child(int i)
{
Int calculation = i * 2
base.i = calculation
}
}
however if u cant do that because of restricted access to the properties my personal preference is to outsource the logic of the calculation in separate function and call the base class like following:
abstract class Parent
{
protected int i;
protected Parent(int i)
{
this.i = i;
}
}
class Child : Parent
{
public Child(int i) : base(Child.dosomework(i))
{
}
public static int dosomework(int i){
int calculation = i * 2
return calculation
}
}

abstract class Parent
{
protected int i;
protected Parent(int i)
{
this.i = i;
}
protected Parent(Func<int> param)
{
i = param();
}
}
class Child : Parent
{
public Child(int i) : base(() => i * 2)
{
}
}

Create a static method and use base(MyStaticMethod(params))

If you were allowed to call a base constructor in the child class in that way, you can face weird problems in your programs. Means that you can leave that instance in an inconsistent state because you could make a try-catch to handle some input-parameter errors and bypass the constructor. That is not the idea, imagine if you were allowed to create a Date in that way.
class MyDate : DateTime
{
public int(int year, int month, int day)
{
try
{
base(-1, -1, -1)
}
catch
{
}
}
}
The funny thing is that Java allows that with the super keyword.

The point of a constructor is to construct the instance, i.e. get it into a valid state. It should do that and nothing else. And if your class can exist without having i set already, then the act of setting i is not essential to its validity and therefore doesn't belong in a constructor.
Perhaps you don't want inheritance, you want composition.
class Inner
{
protected readonly int _i;
public Inner(int i)
{
_i = i;
}
}
class Outer
{
protected Inner _inner = null;
public Outer()
{
//Construct
}
public void SetI(int i)
{
_inner = new Inner(i); //Call constructor of Inner
}
}

How can I access private members from other classes?

I'm not that new to C# but don't have as much experience as in Java.
As you know, in Java, we can access all the private members from outer classes.
So I tried the same thing in C# because I had some fields and methods needed to be accessed from only inside my plugin library and didn't want it to be shown to users. A simple example can be like this.
public static class StaticClass {
public class InstanceClass {
private int oldValue;
public int Value;
}
public static void Backup(InstanceClass ic) {
ic.oldValue = ic.Value;
}
public static void Restore(InstanceClass ic) {
ic.Value = ic.oldValue;
}
}
If I make the field oldValue public, then it'll be mess and look dirty when end users use the plugin. It doesn't have to be an Inner class or in a some specific form. I just want to know if there is any way to control or access private members of an instance from other static classes in the same assembly only by me.

For allowing access only within assembly use internal modifier.
public class InstanceClass {
internal int oldValue;
public int Value;
}

This is not possible in C#. The container class has no special access over the nested class.
You can access private members of the container from the nested class, but not vice versa. The pattern you're trying to use simply isn't used in C# - it's a violation of member accessibility. There are some hacks to force the Java pattern on C# (using reflection or abusing interfaces), but they are just that - hacks.
The "cleanest" approach might look something like this:
public static class StaticClass
{
private interface IInstanceClassInternal
{
int OldValue { get; set; }
}
public sealed class InstanceClass : IInstanceClassInternal
{
int IInstanceClassInternal.OldValue { get; set; }
public int Value;
}
public static void Backup(InstanceClass ic)
{
((IInstanceClassInternal)ic).OldValue = ic.Value;
}
public static void Restore(InstanceClass ic)
{
ic.Value = ((IInstanceClassInternal)ic).OldValue;
}
}
It's obvious that you're trying to write Java in C# - the patterns, the coding style... That's probably a bad idea. Those static methods should probably be extension methods. The "hidden functionality in an object" doesn't quite sit with C#'s notion of OOP - your parent shouldn't have free access to your guts, it should only really have the same public interface everyone else has. After all, that's the whole point of LSP - such tight coupling is quite tricky for any extensibility. Why separate StaticClass from InstanceClass in the first place, if you want StaticClass to mess with InstanceClasses privates? Just make Backup and Restore public members of InstanceClass - or even a part of an interface (perhaps even through explicit implementation, if you want to "hide" it from users of InstanceClass).

You can use the internal access modifier, see https://msdn.microsoft.com/en-us/library/ms173121.aspx
Internal is only visible from inside the assembly
Example: https://dotnetfiddle.net/FNavfE

Have you tried to make it "internal"? It will be available in same dll but not external dll.
public class InstanceClass {
internal int oldValue;
public int Value;
}

Technically, you can use Reflection (if you insist on private field and a static class methods):
using System.Reflection;
...
public static void Backup(InstanceClass ic) {
if (null == ic)
throw new ArgumentNullException("ic");
ic.GetType()
.GetField("oldValue", BindingFlags.NonPublic | BindingFlags.Instance)
.SetValue(ic, ic.Value);
}
public static void Restore(InstanceClass ic) {
if (null == ic)
throw new ArgumentNullException("ic");
ic.Value = (int) (ic.GetType()
.GetField("oldValue", BindingFlags.NonPublic | BindingFlags.Instance)
.GetValue(ic));
}
however, a much better approach is to change access modifier from private to internal:
public class InstanceClass {
internal int oldValue;
public int Value;
}
Even better solution is to move both Backup and Restore methods into InstanceClass:
public class InstanceClass {
private int oldValue;
public int Value;
public void Backup() {
oldValue = Value;
}
public void Restore() {
Value = oldValue;
}
}

This field oldValue is an implementation detail of both StaticClass and InstanceClass. Lets make InstanceClass an implementation detail of StaticClass and export an interface StaticClass.IInstance to external clients:
public static class StaticClass {
public interface IInstance {
int Value { get; set; }
}
private class InstanceClass: IInstance {
public int oldValue;
public Value { get; set; }
}
// Static class becomes responsible for handing out `IInstance` objects
public static IInstance GetInstance() {
return new InstanceClass();
}
public static void Backup(IInstance i) {
if (i is InstanceClass ic) {
ic.oldValue = ic.Value;
}
else {
throw new InvallidOperationException("Can only Backup IInstance objects that were created by GetInstance");
}
}
public static void Restore(IInstance i) {
if (I is InstanceClass ic)
{
ic.Value = ic.oldValue;
}
else {
throw new InvallidOperationException("Can only Restore IInstance objects that were created by GetInstance");
}
}
This solution is similar to the one Luaan proposes. But instead of using an interface to export private data, it uses an interface to limit the publicly available data; to my opinion this is a cleaner design with less surprises.
It does change Value from a field to a property; so when you really need a field, this pattern does not work.
The static class in the example of OP makes it a bit awkward and having better solutions, but imagine this in a regular class, perhaps in a repository. Working on a repository, where observers should be notified when properties of items in the repository are set and not wanting the items to contain a reference to the repository or to the repositories observers, led me to searching for "method only accessible to container class?" which led me to this question.
I intend to solve it as follows:
public class Repo
{
public interface IItem
{
int Id { get; }
string MyProperty { get; }
}
private class Item
{
public int Id { get; }
public string MyProperty { get; private set; }
public bool TrySetMyProperty(string newValue)
{
if (!Equals(MyProperty, newValue) &&
IsPreconditionValid())
{
MyProperty = newValue;
return true;
}
else
{
return false;
}
IsPreconditionValid() => true;
}
}
public event EventHandler<EventArgs> OnChanged;
private readonly ConcurrentDictionary<int, Item> items = new ConcurrentDictionary<int, Item>();
public IItem GetOrCreateItemById(int id)
{
bool changed = false;
IItem result = items.GetOrAdd(int, CreateItem);
if (changed)
{
OnChanged?.Invoke(this, EventArgs.Empty);
}
return result;
IItem CreateItem(int key)
{
changed = true;
return new Item() { Id = key };
}
}
public bool TrySetItemMyProperty(int id, string newValue)
{
if (items.TryGet(id, out Item i))
{
if (i.TrySetMyProperty(newValue))
{
OnChanged?.Invoke(this, EventArgs.Empty);
return true;
}
}
return false;
}
}

Automatically calling an init function whenever an object is used for the 1st time

I have an object that only initializes itself with barebones data when constructed (fast), and loads itself for real (slow) when first accessed. The idea is that I'm creating a lot of these barebones objects at startup and hash them into a map, then fully load each object whenever it is individually accessed for the first time. The problem is that I cannot guarantee how clients will interact with this object, there are multiple public methods that might be invoked.
Is there a good pattern to support this kind of situation? The obvious (and my current) solution is to track state with an internal bool, check against that bool in every function that might be invoked, and load that way. But that requires code duplication of that behavior across all public functions, and is vulnerable to errors.
I can imagine a single point-of-entry method that then dishes out behaviors based on a client request type etc., but before I go consider going down that road I want to see if there's a commonly accepted approach/pattern that I might not be aware of. I'm doing this in C#, but any insight is appreciated.

If I understood what you want to achieve, you are looking for the Proxy Design Pattern, more specifically, a virtual Proxy.
Refer to http://www.dofactory.com/net/proxy-design-pattern
A small example would be something like:
public abstract class IObjectProvider
{
public abstract IObjectProvider Object{get;}
public abstract void doStuff();
}
public class RealObject : IObjectProvider
{
public RealObject()
{
//Do very complicated and time taking stuff;
}
public override IObjectProvider Object
{
get { return this; }
}
public override void doStuff()
{
//do this stuff that these objects normally do
}
}
public class ObjectProxy : IObjectProvider
{
private IObjectProvider objectInstance = null;
public override IObjectProvider Object
{
get
{
if (objectInstance == null)
objectInstance = new RealObject();
return objectInstance;
}
}
public override void doStuff()
{
if(objectInstance!=null)
objectInstance.doStuff();
}
}
public class SkeletonClass
{
public IObjectProvider Proxy1 = new ObjectProxy();
public IObjectProvider Proxy2 = new ObjectProxy();
}
static void Main(String[] args)
{
//Objects Not Loaded
SkeletonClass skeleton = new SkeletonClass();
//Proxy1 loads object1 on demand
skeleton.Proxy1.Object.doStuff();
//Proxy2 not loaded object2 until someone needs it
}

Here's an example of dynamic proxy approach.
using System;
using System.Diagnostics;
using Castle.DynamicProxy; //Remember to include a reference, too. It's nugettable package is Castle.Core
namespace ConsoleApp
{
public class ActualClass
{
//Have static instances of two below for performance
private static ProxyGenerator pg = new ProxyGenerator();
private static ActualClassInterceptor interceptor = new ActualClassInterceptor();
//This is how we get ActualClass items that are wrapped in the Dynamic Proxy
public static ActualClass getActualClassInstance()
{
ActualClass instance = new ActualClass();
return pg.CreateClassProxyWithTarget<ActualClass>(instance, interceptor);
}
//Tracking whether init has been called
private bool initialized = false;
//Will be used as evidence of true initialization, i.e. no longer null
private int? someValue = null;
public void Initialize()
{
if (!initialized)
{
//do some initialization here.
someValue = -1; //Will only get set to non-null if we've run this line.
initialized = true;
}
}
//Any methods you want to intercept need to be virtual!
public virtual int replaceValue(int value)
{
//below will blow up, if someValue has not been set to -1 via Initialize();
int oldValue = someValue.Value;
someValue = value;
return oldValue;
}
//block off constructor from public to enforce use of getActualClassInstance
protected ActualClass() { }
}
public class ActualClassInterceptor : ActualClass, IInterceptor
{
public void Intercept(IInvocation invocation)
{
//Call initialize before proceeding to call the intercepted method
//Worth noting that this is the only place we actually call Initialize()
((ActualClass)invocation.InvocationTarget).Initialize();
invocation.Proceed();
}
}
class Program
{
static void Main(string[] args)
{
ActualClass instance1 = ActualClass.getActualClassInstance();
ActualClass instance2 = ActualClass.getActualClassInstance();
int x1 = instance1.replaceValue(41);
int x2 = instance2.replaceValue(42);
int y1 = instance1.replaceValue(82);
Debug.Assert(y1 == 41);
int y2 = instance2.replaceValue(84);
Debug.Assert(y2 == 42);
var read = Console.ReadKey();
}
}
}

How to implement Lazy loading with PostSharp?

I would like to implement lazy loading on properties with PostSharp.
To make it short, instead of writing
SomeType _field = null;
private SomeType Field
{
get
{
if (_field == null)
{
_field = LongOperation();
}
return _field;
}
}
I would like to write
[LazyLoadAspect]
private object Field
{
get
{
return LongOperation();
}
}
So, I identify that I need to emit some code in the class to generate the backing field, as well as inside the getter method in order to implement the test.
With PostSharp, I was considering overriding CompileTimeInitialize, but I am missing the knowledge to get a handle over the compiled code.
EDIT:
The question can be extended to any parameterless method like:
SomeType _lazyLoadedField = null;
SomeType LazyLoadableMethod ()
{
if(_lazyLoadedField ==null)
{
// Long operations code...
_lazyLoadedField = someType;
}
return _lazyLoadedField ;
}
would become
[LazyLoad]
SomeType LazyLoadableMethod ()
{
// Long operations code...
return someType;
}

After our comments, I think I know what you want now.
[Serializable]
public class LazyLoadGetter : LocationInterceptionAspect, IInstanceScopedAspect
{
private object backing;
public override void OnGetValue(LocationInterceptionArgs args)
{
if (backing == null)
{
args.ProceedGetValue();
backing = args.Value;
}
args.Value = backing;
}
public object CreateInstance(AdviceArgs adviceArgs)
{
return this.MemberwiseClone();
}
public void RuntimeInitializeInstance()
{
}
}
Test code
public class test
{
[LazyLoadGetter]
public int MyProperty { get { return LongOperation(); } }
}

Thanks to DustinDavis's answer and comments, I could work on my own implementation, and I just wanted here to share it to help other people.
The main differences from the original answer are:
Implement the suggested "only run the operation once" (purpose of the lock)
Made the initialization status of the backing field more reliable by passing this responsibility to a boolean.
Here is the code:
[Serializable]
public class LazyLoadAttribute : LocationInterceptionAspect, IInstanceScopedAspect
{
// Concurrent accesses management
private readonly object _locker = new object();
// the backing field where the loaded value is stored the first time.
private object _backingField;
// More reliable than checking _backingField for null as the result of the loading could be null.
private bool _hasBeenLoaded = false;
public override void OnGetValue(LocationInterceptionArgs args)
{
if (_hasBeenLoaded)
{
// Job already done
args.Value = _backingField;
return;
}
lock (_locker)
{
// Once the lock passed, we must check if the aspect has been loaded meanwhile or not.
if (_hasBeenLoaded)
{
args.Value = _backingField;
return;
}
// First call to the getter => need to load it.
args.ProceedGetValue();
// Indicate that we Loaded it
_hasBeenLoaded = true;
// store the result.
_backingField = args.Value;
}
}
public object CreateInstance(AdviceArgs adviceArgs)
{
return MemberwiseClone();
}
public void RuntimeInitializeInstance() { }
}

I think the requirement cannot be accurately described as 'lazy loading', but is a special case of a more general caching aspect with in-AppDomain storage but without eviction. A general caching aspect would be able to handle method parameters.

How to access to the parent object in c#

I have a "meter" class. One property of "meter" is another class called "production".
I need to access to a property of meter class (power rating) from production class by reference. The powerRating is not known at the instantiation of Meter.
How can I do that?
public class Meter
{
private int _powerRating = 0;
private Production _production;
public Meter()
{
_production = new Production();
}
}

Store a reference to the meter instance as a member in Production:
public class Production {
//The other members, properties etc...
private Meter m;
Production(Meter m) {
this.m = m;
}
}
And then in the Meter-class:
public class Meter
{
private int _powerRating = 0;
private Production _production;
public Meter()
{
_production = new Production(this);
}
}
Also note that you need to implement an accessor method/property so that the Production class can actually access the powerRating member of the Meter class.

I wouldn't reference the parent directly in the child objects. In my opinion the childs shouldn't know anything about the parents. This will limits the flexibility!
I would solve this with events/handlers.
public class Meter
{
private int _powerRating = 0;
private Production _production;
public Meter()
{
_production = new Production();
_production.OnRequestPowerRating += new Func<int>(delegate { return _powerRating; });
_production.DoSomething();
}
}
public class Production
{
protected int RequestPowerRating()
{
if (OnRequestPowerRating == null)
throw new Exception("OnRequestPowerRating handler is not assigned");
return OnRequestPowerRating();
}
public void DoSomething()
{
int powerRating = RequestPowerRating();
Debug.WriteLine("The parents powerrating is :" + powerRating);
}
public Func<int> OnRequestPowerRating;
}
In this case I solved it with the Func<> generic, but can be done with 'normal' functions.
This why the child(Production) is totally independent from it's parent(Meter).
But! If there are too many events/handlers or you just want to pass a parent object, i would solve it with an interface:
public interface IMeter
{
int PowerRating { get; }
}
public class Meter : IMeter
{
private int _powerRating = 0;
private Production _production;
public Meter()
{
_production = new Production(this);
_production.DoSomething();
}
public int PowerRating { get { return _powerRating; } }
}
public class Production
{
private IMeter _meter;
public Production(IMeter meter)
{
_meter = meter;
}
public void DoSomething()
{
Debug.WriteLine("The parents powerrating is :" + _meter.PowerRating);
}
}
This looks pretty much the same as the solution mentions, but the interface could be defined in another assembly and can be implemented by more than 1 class.
Regards,
Jeroen van Langen.

You would need to add a property to your Production class and set it to point back at its parent, this doesn't exist by default.

Why not change the constructor on Production to let you pass in a reference at construction time:
public class Meter
{
private int _powerRating = 0;
private Production _production;
public Meter()
{
_production = new Production(this);
}
}
In the Production constructor you can assign this to a private field or a property. Then Production will always have access to is parent.

You could maybe add a method to your Production object called 'SetPowerRating(int)' which sets a property in Production, and call this in your Meter object before using the property in the Production object?

I would give the parent an ID, and store the parentID in the child object, so that you can pull information about the parent as needed without creating a parent-owns-child/child-owns-parent loop.

something like this:
public int PowerRating
{
get { return base.PowerRating; } // if power inherits from meter...
}