How do I go about if I need to initialize an object's base with existing object? For example, in this scenario:
public class A
{
public string field1;
public string field2;
}
public class B : A
{
public string field3;
public void Assign(A source)
{
this.base = source; // <-- will not work, what can I do here?
}
}
Assign() method can, obviously assign values to the base class field-by-field, but isn't there a better solution? Since class B inherits from A, there must be a way to just assign A to the B.base
In C++ this would be a trivial thing to do, but I can't seem to grasp how to do this in .NET
Unfortunately base is readonly.
[Edit]
Well perhaps not so unfortunate. The relationship between a base class and a child class is IS-A not HAS-A. By allowing a child class to change the instance of the base class you are allowing the child class to change its own reference since it IS-A base class. If you truly need this functionality then I would suggest you change your inheritance model to reflect what you truly want to do.
Something like this:
public class A
{
public string field1;
public string field2;
}
public class B
{
public string field3;
public A a;
public void Assign(A source)
{
this.a = source;
}
}
seems more appropriate and has clearer meaning and functionality.
public Assign(A a)
{
foreach (var prop in typeof(A).GetProperties())
{
this.GetType().GetProperty(prop.Name).SetValue(this, prop.GetValue(a, null),null);
}
}
Basically, it uses reflection to get all the properties of the base and assign the values of this, to all the values that exist in A.
EDIT: To all you naysayers out there, I quickly tested this now with a base class that had 100 integer variables. I then had this assign method in a subclass. It took 46 milliseconds to run. I don't know about you, but I'm totally fine with that.
While there are many excellent answers here, I think the proper way to do this is by chaining the constructors:
public class A
{
public string field1;
public string field2;
public A(string field1, string2 field2)
{
this.field1 = field1;
this.field2 = field2;
}
}
public class B : A
{
public string field3;
public B(string field1, string2 field2, string field3)
: base(field1, field2)
{
this.field3 = field3;
}
}
No, the syntax you are trying is not possible. In C# .NET you need to do:
public void Assign(A source) {
field1 = source.field1;
field2 = source.field2;
}
Is the intent that these fields will be initialized once during object construction, or could "Assign" be called multiple times during an object's lifetime? If the latter, you can disregard the rest of this :)
Andrew's distinction between IS-A and HAS-A is an important one; if the relationship really is a HAS-A, his composition solution is the way to go.
If an IS-A relationship makes more sense (and you are able to modify A), a copy constructor might be a good idea:
public class A
{
public string field1;
public string field2;
public A(A copyFrom)
{
this.field1 = copyFrom.field1;
this.field2 = copyFrom.field2;
}
}
public class B : A
{
public string field3;
public B(A source)
: base(source)
{
}
}
You end up having to copy each of A's properties, but the responsibility for doing so resides in A where it belongs.
Why would you need to? By declaring a new B, the CLR automatically calls the constructors for both classes.
B myB = new B();
B new has the fields of both classes. However, you should declare them with an initializer unless you like nulls:
public string field1 = "";
public string field2 = string.Empty;
I hope I'm not the only one who thinks swapping out your base class is a bad design pattern. Another approach is to replace inheritance with composition:
public class A
{
public string Field1 { get; set; }
public string Field2 { get; set; }
}
public class B
{
public A A { get; set; }
public string Field3 { get; set; }
public B(A a) { this.A = a; }
}
Now its trivial to write something like this:
B b = new B ( new A { Field1 = "hello", Field2 = "world" } );
b.A = new A { Field1 = "hola", Field2 = "luna" };
Wrong question.
You're obviously abusing inheritance here.
Try to refactor it, so that you keep a reference to A as a member field.
If you need polymorphism, consider having common base class or better yet - an interface.
[TestMethod]
public void TestMethod()
{
A a = new A();
a.field1 = "test";
string xml = Serialize(a);
xml = xml.Replace("A", "B");
B b = Deserialize(xml);
Assert.AreEqual("test", b.field1);
}
public string Serialize(A a)
{
System.IO.StreamReader streamReader = null;
System.IO.MemoryStream memoryStream = null;
try
{
memoryStream = new System.IO.MemoryStream();
XmlSerializer serializer = new XmlSerializer(typeof(A));
serializer.Serialize(memoryStream, a);
memoryStream.Seek(0, System.IO.SeekOrigin.Begin);
streamReader = new System.IO.StreamReader(memoryStream);
return streamReader.ReadToEnd();
}
finally
{
if ((streamReader != null))
{
streamReader.Dispose();
}
if ((memoryStream != null))
{
memoryStream.Dispose();
}
}
}
public static B Deserialize(string xml)
{
System.IO.StringReader stringReader = null;
try
{
stringReader = new System.IO.StringReader(xml);
XmlSerializer serializer = new XmlSerializer(typeof(B));
return ((B)(serializer.Deserialize(System.Xml.XmlReader.Create(stringReader))));
}
finally
{
if ((stringReader != null))
{
stringReader.Dispose();
}
}
}
According to MSDN, "base" can only be used for the following operations:
Call a method on the base class that has been overridden by another method.
Specify which base-class constructor should be called when creating instances of the derived class.
Related
So what I'm trying to do here is pass the same copy of a class (class A) to another class (class B), but class B is instanced in class A.
Using a new statement in class B won't work because it would cause an infinite loop, as well as creating a new instance of it, when I want to be able to use variables from the 1st instance of class A.
I know about object.equals() but I can't use it until I define the class A's object in class B. Just using object.equals results in a null reference.
public partial class class_A : Form
{
public class_B _class_B = new class_B;
public Int32 var;
private void setclassA()
{
_class_B._class_A.equals(this);
}
}
public class class_B
{
public class_A _class_A; // I know this is null
// code
}
Like I said I want to avoid instancing a new copy of class A because I want the values in class A to be set.
I've tried using a method to do it but still get a null reference.
Pass A in the constructor of B:
public class A
{
private B _b;
public A()
{
_b = new B(this);
}
}
public class B
{
private A _a;
public B(A a)
{
_a = a;
}
}
As mentioned in the comments you're completely misunderstanding .Equals(). It's used to compare whether two objects are equal not clone / pass references.
Answer
You're 75% there. As Ron mentioned, just change .equals(this) to = this as follows:
_class_B._class_A = this;
Explanation
If we want to pass the same copy of ClassA to ClassB, and to do that when we instantiate ClassB inside of ClassA, then we use the this keyword, which represents the current instance of a class.
Though there are other ways to give this to ClassB, the below example shows two:
pass this to the constructor and do the property assignment in ClassB or
assign this directly to a property in ClassB.
The key for you is that = is an assignment operator whereas Equals() checks whether two variables refer to the same object or not.
Example
public class ClassA
{
public ClassB MyClassB1 { get; set; }
public ClassB MyClassB2 { get; set; }
public ClassA()
{
// pass `this` to the constructor
this.MyClassB1 = new ClassB(this);
// pass `this` directly to a property in `ClassB`
this.MyClassB2 = new ClassB();
this.MyClassB2.MyClassA = this;
}
}
public class ClassB
{
public ClassA MyClassA { get; set; }
public ClassB() { }
public ClassB(ClassA classA)
{
// do property assignment in the constructor
this.MyClassA = classA;
}
}
Proof of Concept
This runs as a Fiddle here and prints out "They are the same object," and some other things too in the more recent version.
using System;
public class Program
{
public static void Main()
{
var classA = new ClassA();
if(classA.Equals(classA.MyClassB1.MyClassA) &&
classA.Equals(classA.MyClassB2.MyClassA) &&
classA.MyClassB1.MyClassA.Equals(classA.MyClassB2.MyClassA))
{
Console.WriteLine("They are the same object.");
}
}
}
An important note is that, when we use this, we're giving ClassB a reference to ClassA not a copy of it. These are very different things.
Try this:
public class A
{
public B Instance_B;
public A(B b)
{
Instance_B = b;
}
}
public class B
{
public A Instance_A;
public B()
{
Instance_A = new A(this);
}
}
Use get; set;
In Class A:
public partial class class_A : Form
{
Class_B B = new Class_B();
B.Class_A = this;
public Int32 var;
}
Then in Class B:
public class class_B
{
Class_A A { get; set; }
// code
}
I've been searching for a bit and found similar things but nothing that quite answers my question. Here it is- I have two classes. A, B. I want class C to have all the properites of those two classes. This is in the Object Layer, c#.net.
Class A:
public class A
{
public string property1 { get; set; }
}
public class B
{
public string anotherproperty { get; set; }
}
public interface IB
{
B b { get; set; }
}
public class C : A, IB
{
//nothing here cause hopefully it inherits everything
}
I have no idea if my implementation is even close. I do get error message of "C does not implement interface member IB"
Anyone know how to do this?
I have two classes. A, B. I want class C to have all the properites of those two classes.
...then you will have to manually add either A's or B's properties to C. C# does not support multiple inheritance, sorry.
However, in most cases multiple inheritance is not needed. A common alternative is to use composition instead of inheritance: For example, you could have your class C provide references to instances of A and B:
public class C
{
private A a = new A();
private B b = new B();
public A A { get { return a; } }
public B B { get { return b; } }
}
This would allow you to access all properties via myC.A.property1 and myC.B.anotherproperty.
If you really need to combine implementations from both A and B in C, you might need to have a look at Mixin libraries for C#.
You can do something like this:
public class ClassA
{
public string AProp { get { return "AProp";} }
}
public class ClassB : ClassA
{
public string BProp { get { return "BProp"; } }
}
public class ClassC : ClassB
{
}
And then:
ClassC c = new ClassC();
Console.WriteLine(c.AProp);
Console.WriteLine(c.BProp);
Prints:
AProp
BProp
You can't override anything but you can access the inherited properties.
I implemented 2 classes:
public class A
{
public string GetName()
{
return "Class A";
}
}
public class B
{
public string GetName()
{
return "Class B";
}
}
I also created an interface that is not been assign to class A and B:
public interface TellMyNameInterface
{
string GetName();
}
I want to use the interface with class A and B:
TellMyNameInterface a = new A();
TellMyNameInterface b = new B();
string aName= a.GetName();
Is there any way to use an instance of class A or B with that interface without declaring it in the class declaration?
You cannot, but you can write adapter classes to make it more convenient to do the conversions, and then use extension methods to make creating the adapter classes look more natural (and in fact hide the adapter classes behind the interface).
You would normally only do this if you are unable to change the original class definitions to implement the required interface directly.
So given these classes that you cannot edit:
public class A
{
public string GetName()
{
return "Class A";
}
}
public class B
{
public string GetName()
{
return "Class B";
}
}
And this interface that you really want them to implement, but can't:
public interface ITellMyNameInterface
{
string GetName();
}
You can write a couple of adapter classes that do implement the interface, like this:
public class AAdapter: ITellMyNameInterface
{
public AAdapter(A a)
{
_a = a;
}
public string GetName()
{
return _a.GetName();
}
private readonly A _a;
}
public class BAdapter: ITellMyNameInterface
{
public BAdapter(B b)
{
_b = b;
}
public string GetName()
{
return _b.GetName();
}
private readonly B _b;
}
Then write extension methods to make it more natural to create the adapter classes:
public static class ABExt
{
public static ITellMyNameInterface AsITellMyNameInterface(this A self)
{
return new AAdapter(self);
}
public static ITellMyNameInterface AsITellMyNameInterface(this B self)
{
return new BAdapter(self);
}
}
Once you've done all that, it does at least make it simpler to get the ITellMyNameInterface for an instance of A or B, like so:
ITellMyNameInterface a = new A().AsITellMyNameInterface();
ITellMyNameInterface b = new B().AsITellMyNameInterface();
No you can not. The only way i can see is using object to store it and then call the functions via reflection.
QUESTION IS CLARIFIED
new thread : subclass properties accessed in generic method with superclass input
I have a base class A
subClasses B, C, and D inherit from A.
Each subClass has 3 subClasses a, b, and c.
a, b, and c have a list of unique properties.
However, now I want to build a generic function to access those properties, so how would I do this without switching on Type?
Clarification : I do not want a : B to have abstract C methods/properties
Example:
public void Method(A a){
if(a.isSubClassOf(B))
{Console.WriteLine(a.BaProperty);}
if(a.isSubClassOf(C))
{Console.WriteLine(a.CbProperty);}
if(a.isSubClassOf(D))
{Console.WriteLine(a.DcProperty);}
}
You can't define a member in derived class and access it via the reference to base class without casting to derived class:
class A {}
class B
{
public int i;
}
A a = new B();
a.i = 0; // error
((B)a).i = 0; // OK
Either declare virtual property in any of base types in your hierarchy, or use casting to the concrete derived type. Of course, in the second case your method doesn't make any sense.
Typically, you would use a virtual or abstract method defined in A and overridden in the subclasses.
public abstract class A
{
protected abstract PropertyType PropertyValue {get;}
public void Method()
{
Console.WriteLine(PropertyValue);
}
}
public class B : A
{
protected override PropertyType Property { get { return PropertyType.B; } }
}
// etc...
An elegant solution is to override ToString
public abstract class A { }
public class B : A {
public int b { get; set; }
public override string ToString()
{
return b.ToString();
}
}
// Do the same with C and D ....
A[] array = new A[] { new B(), new C(), new D() };
foreach (A a in array) {
Console.WriteLine(a);
}
Note that Console.WriteLine does not need to know about a special method or property in A. It also works for types not deriving from A.
It depends greatly in what you really want to achieve. In some cases what Steve Czetty suggests is the best option.
In others you could just keep all the properties different and have a virtual method in the base class that returns for example in this case a "string" that you can then write in the console or anything you wish.
Edit: You could override ToString as Olivier suggested. But only if you feel what yo are going to retun is the "String representation of the object".
public abstract class A
{
public string PropertyA { get; set; }
public virtual string GetString() //
{
return PropertyA;
}
}
public class B:A
{
public string PropertyB { get; set; }
public override string GetString()
{
return PropertyB;
}
}
public class C:A
{
public string PropertyC { get; set; }
public override string GetString()
{
return string.Format("{0} - {1}", base.GetString(), PropertyC) // for example if you wanted to do something more complex
}
}
Now if what you need can not be solved like this, you could cast as Dennis Suggested.
There is another posibility: you could use the visitor pattern. Here you can find several ways to implement it.
Just so you have an idea you would wind up having a class similar to this: (it will depend on what you really want to achieve)
You have to implement some other basic things (interface and some methods), but from a Reference to the base class you will be able to call the corresponding "Visit" method easily. There is a lot of detail in the link i added.
class ClassicVisitor : IAVisitor
{
public string SuperComplexStringResult { get;set; }
public void Visit(B b) { SuperComplexStringResult = String.Format("Super Complex Stuff + {0}", b.PropertyB); }
public void Visit(C c) { SuperComplexStringResult = String.Format("Super Complex Stuff + {0}", c.PropertyC); }
}
I have a BL class named:A , DTO class named:DTO . Now assume I want to add some more properties in my DTO. So I derive a new DTO class from my existing DTO and add properties to it.Below is the code:
namespace TestConsole
{
class test
{
static void Main(string[] args)
{
B b = new B();
b.D.ID = 1;
b.D.Name = "4";
MyBLMethod(b);
}
static void MyBLMethod(A b)
{
MyDALMethod(b.D);
}
static void MyDALMethod(DTO dto)
{
int i = dto.ID;
string name = ((MyDTO)dto).Name;//I could not do this
//because i will get object cast error as i can't cast from
//parent to child
}
}
public class DTO
{
public int ID = 99;
public DTO()
{
}
public DTO(DTO source)
{
ID = source.ID;
}
}
public class MyDTO : DTO
{
public string Name = "";
public MyDTO() { }
public MyDTO(MyDTO source)
: base(source)
{
Name = source.Name;
}
}
public class A
{
private DTO _d;
public A()
{
D = new DTO();
}
public DTO D
{
get { return _d; }
set { _d = value; }
}
}
public class B : A
{
private MyDTO _md;
public B()
{
_md = new MyDTO();
}
public MyDTO D
{
get { return _md; }
set { _md = value; }
}
}
}
From Main (you can think it as UI) i am calling MyBLMethod (present in BL repository) and passing class object to it , and from BL repository i am calling my DAL. In DAL i have written this:
static void MyDALMethod(DTO dto)
{
int i = dto.ID;
string name = ((MyDTO)dto).Name;//I could not do this
//because i will get object cast error as i can't cast from
//parent to child
}
Could you suggest me how could i get the newly extended property (name in the example) in my DAL.
When B inherits A, it already owns a DTO attribute. So the problem is really that you are hidding this inheritance. You don't need a new property inside B class, just set it in your class constructor.
public class B : A
{
public B()
{
this.D = new MyDTO();
}
}
But, in your main class you will need a explicit cast in your property, just like below, since DTO does not have a "Name" property.
static void Main(string[] args)
{
B b = new B();
b.D.ID = 1;
((MyDTO)b.D).Name = "4";
MyBLMethod(b);
}
If the object is ACTUALLY of the base type, you can't just tack on the additional properties. Doesn't work that way, sorry.
What you want is to CONVERT the object (maybe). Make a constructor in your child class that can take a parent and copy all of it's stuff into itself - then you'll have the additional properties.
The reason that the cast fails, is that you are not passing a MyDTO object to the method, but a DTO object. The MyBLMethod method always sends the DTO object to the DAL even if there is a MyDTO object.
You haven't made the D property virtual. That means that when you use the D property on an A reference, you get the DTO object that the A class contains even if the actual object happens to the a B instance so that it also has a MyDTO object.
You can make the D property virtual to access the D property of the actual object instead of the one specified by the type of the reference.
Or you can cast the reference to B so that you can access it's MyDTO object instead of it's DTO object:
static void MyBLMethod(A b) {
MyDALMethod(((B)b).D);
}
Note that the B class contains both a DTO and a MyDTO object, which might not be what you really want.
It sounds like you are losing "resolution" because you are passing through a static business logic method. I would suggest revisiting that part rather than struggling with the DAL method first.
There may be a reason you're stuck with that though, so if you are, you can consider using reflection to find the properties you need or using an "as" cast and then testing for null in your dal method. If you aren't stuck with this design, I'd refactor my way out of the static method though. Static seems so easy and unfortunately there's a lot of code 'quality' tools pushing you to make methods static which forget to remind you can't easily change static methods to virtual methods later.
Allow B to pass A a DTO object in A's constructor. If needed, make the constructor protected. Then, have B.D cast A.D.
public class A
{
private DTO _d;
// New constructor.
protected A(DTO d)
{
_d = d;
}
// Old constructor calls new constructor.
public A() : this(new DTO())
{
}
public DTO D
{
get { return _d; }
set { _d = value; }
}
}
public class B : A
{
// Old B constructor calls new A constructor.
public B() : base(new MyDTO())
{
}
new public MyDTO D
{
// Getting D casts A.D instead of using an object exclusive to B.
get { return (MyDTO)base.D; }
set { base.D = value; }
}
}