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C# instantiate class with varying "child" class

I have a class that has some derived classes. That works.
I want to instantiate the "parent" class. Got that far...
But I want to instantiate it with one of the "child" classes, and then possibly change that "child" class later. Maybe the deriving is inappropriate here.
Take the following example:
public class Unicorn {
public string Horn { get; set; }
public Unicorn(){
}
}
public class BadUnicorn : Unicorn{
public string Rainbow()
{
return "dark rainbow";
}
}
public class GoodUnicorn : Unicorn{
public string Rainbow()
{
return "light rainbow";
}
}
I could instantiate one of the children, but then if I change one from "good" to "bad", I would have to re-instantiate. And maybe that's just the way it is, and that would be an acceptable answer if that's all there is to it.
I rather want to instantiate a Unicorn, and then be able to change it from Good to Bad to maintain information stored on that Unicorn, but have access to the current methods and properties of the "child" class.
That way when I call Unicorn.Rainbow() it calls the desired method of the "child" class.
I'm a little new to C#, is there a pattern that fits this bill?

You can't do what you want with polymorphism. You cannot change an instance of class from one to another. Once it is created it is always the same type.
You can use composition though.
Here's what you'd need to do:
public class Unicorn
{
public string Horn { get; set; }
public Unicorn(Rainbow rainbow)
{
_rainbow = rainbow;
}
public void SetRainbow(Rainbow rainbow)
{
_rainbow = rainbow;
}
private Rainbow _rainbow;
public string Rainbow()
{
return _rainbow.Colour();
}
}
public abstract class Rainbow
{
public abstract string Colour();
}
public class BadRainbow : Rainbow
{
public override string Colour()
{
return "dark rainbow";
}
}
public class GoodRainbow : Rainbow
{
public override string Colour()
{
return "light rainbow";
}
}
You can test like this:
var unicorn = new Unicorn(new GoodRainbow());
Console.WriteLine(unicorn.Rainbow());
unicorn.SetRainbow(new BadRainbow());
Console.WriteLine(unicorn.Rainbow());
This outputs:
light rainbow
dark rainbow
The instance of Unicorn stays the same, but you can change the rainbow.

Here's my take on delegate dictionary. While it seems superfluous to use Func instead of just string, if the method have additional functionality like calculation or need parameters, you're covered with Func.
public class Unicorn
{
static Dictionary<Attitude, Func<string>> RainbowByAttitude =
new Dictionary<Attitude, Func<string>>()
{
[Attitude.Bad] = new Func<string>(() => "dark rainbow"),
[Attitude.Good] = new Func<string>(()=>"light rainbow")
};
public string Horn { get; set; }
public enum Attitude
{
Good,Bad
}
public Attitude attitude;
public Unicorn(Attitude attitude)
{
this.attitude = attitude;
}
public string Rainbow() => RainbowByAttitude[attitude].Invoke();
}
class Program
{
static void Main(string[] args)
{
Unicorn unicorn;
unicorn = new Unicorn(Unicorn.Attitude.Bad);
Console.WriteLine(unicorn.Rainbow());
unicorn.attitude = Unicorn.Attitude.Good;
Console.WriteLine(unicorn.Rainbow());
}
}

It seems like a state pattern to me like this:
public abstract class UnicornState
{
public abstract UnicornState Change();
public abstract string Rainbow();
}
public sealed class GoodUnicornState : UnicornState
{
public override UnicornState Change()
{
return new BadUnicornState();
}
public override string Rainbow()
{
return "light rainbow";
}
}
public sealed class BadUnicornState : UnicornState
{
public override UnicornState Change()
{
return new GoodUnicornState();
}
public override string Rainbow()
{
return "dark rainbow";
}
}
public class Unicorn
{
public string Horn { get; set; }
public UnicornState State { get; set; }
public string Rainbow => State.Rainbow();
}
Usage:
var u = new Unicorn();
u.State = new GoodUnicornState();
Console.WriteLine(u.Rainbow);
u.State = u.State.Change();
Console.WriteLine(u.Rainbow);

Static method call in generic manner

I have multiple classes like:
public class Base { }
public class Base1: Base { public static List<Base1> LoadFromXml(string path) }
public class Base2: Base { public static List<Base2> LoadFromXml(string path) }
Then I want to have a method like this:
public List<T> PrepareBase<T>() where T: Base { return T.Load("C:\test.xml"); }
So that I don't have to make a method for every type.
But I don't know how to accomplish this or something similar.
The problem is that I can't make the LoadFromXml method known to the base class because static inheritance is not a thing. Neither is creating a seperate interface with a static method.
Is there a way to do this or am I expecting too much?
Edit:
An example of the LoadFromXml method:
public class Base1
{
public int ID { get; set; }
public string PropertyOnlyInBase1 { get; set; }
public static List<Base1> LoadFromXml(string path)
{
List<Base1> baseList = new List<Base1>();
XDocument doc = XDocument.Load(path);
foreach(var node in doc.Descendants("Base1"))
{
Base 1 base = new Base1() { ID = node.Attributes["id"] };
base.PropertyOnlyInBase1 = node.Element("PropertyOnlyInBase1");
baseList.Add(base);
}
return baseList;
}
}
So the Base classes also have some unique properties. That's why I needed the inheritance thing in the first place.

One option is to add a GenericBase:
public abstract class Base
{
}
public static class GenericBase<T>
where T : Base
{
public static List<T> LoadFromXml(string path)
{
//Load from XML
}
}
public class Base1 : Base { }
public class Base2 : Base { }
public class Test //MainForm.cs class or whatever you want
{
public void Tester() //Load event handler or whatever you want
{
List<Base1> base1List = PrepareBase<Base1>();
}
public List<T> PrepareBase<T>() where T : Base
{ return GenericBase<T>.LoadFromXml("C:\test.xml"); }
}
Edit:
As D Stanley mentioned, it's not possible; but I made some work-around that could be helpful for you:
public abstract class Base
{
public static List<T> LoadFromXml<T>(string path) where T : Base, new()
{
List<T> baseList = new List<T>();
XDocument doc = XDocument.Load(path);
foreach (var node in doc.Descendants(typeof(T).Name))
{
T t = new T();
Dictionary<string, string> d = new Dictionary<string, string>();
foreach (var item in node.Elements())
d.Add(item.Name.ToString(), item.Value);
t.Load(d);
baseList.Add(t);
}
return baseList;
}
protected internal abstract void Load(Dictionary<string, string> elements);
}
public class Base1 : Base
{
public string CustomProp1 { get; set; }
public string CustomProp2 { get; set; }
public string CustomProp3 { get; set; }
protected internal override void Load(Dictionary<string, string> elements)
{
if (elements.ContainsKey("CustomProp1"))
CustomProp1 = elements["CustomProp1"];
if (elements.ContainsKey("CustomProp2"))
CustomProp2 = elements["CustomProp2"];
if (elements.ContainsKey("CustomProp3"))
CustomProp3 = elements["CustomProp3"];
}
}
public class Base2 : Base
{
public string CustomProp1 { get; set; }
public string CustomProp2 { get; set; }
public string CustomProp3 { get; set; }
protected internal override void Load(Dictionary<string, string> elements)
{
if (elements.ContainsKey("CustomProp1"))
CustomProp1 = elements["CustomProp1"];
if (elements.ContainsKey("CustomProp2"))
CustomProp2 = elements["CustomProp2"];
if (elements.ContainsKey("CustomProp3"))
CustomProp3 = elements["CustomProp3"];
}
}
public class Test //MainForm.cs class or whatever you want
{
public void Tester() //Load event handler or whatever you want
{
List<Base1> base1List = PrepareBase<Base1>();
}
public List<T> PrepareBase<T>() where T : Base, new()
{
return Base.LoadFromXml<T>("C:\test.xml");
}
}

I think you're correct that the class that loads these from XML should be separate from the class that's being loaded. As you said, it has no real connection to the instance.
Perhaps what you need is a separate class that loads those instances for you.
public class BaseXmlLoader<TBase> where TBase : Base
{
public List<TBase> LoadFromXml(string filePath)
{
var serializer = new XmlSerializer(typeof(TBase));
// Load your file and deserialize.
}
}
The benefits aren't huge because it's not saving you that much code. But if the LoadFromXml methods are essentially the same except for the type then you're getting something out of it.

I changed my approach to the problem and solved it using the Factory pattern. I also provided each class with an instance method SetPropertiesFromXml to handle the custom properties. Unlike the previously used method, a method like that made sense as an instance method.
Factory:
public static class BaseFactory
{
public static Base GetBase(string id)
{
switch(id) { case '1': return new Base1(); ... }
}
public static T GetBaseList<T>(string xml, string tagName) where T: Base
{
List<T> list = new List<T>();
var nodes = XDocument.Load(xml).Descendants(tagName);
foreach(XElement node in nodes)
{
var base = GetBase(node.Attribute("id").Value);
base.SetPropertiesFromXml(node);
list.Add(base as T);
}
}
}
Bases
public abstract class Base
{
public virtual void SetPropertiesFromXml(XElement node)
{
//<Set Properties like: this.Property = node.Element("key");>
}
}
public class Base1
{
public override void SetPropertiesFromXml(XElement node)
{
//<Set Custom Properties for Base1>
//Call Base to add the normal properties as well
base.SetPropertiesFromXml(node);
}
}
Call
List<Base1> list = BaseFactory.GetBaseList<Base1>("test.xml", "Base1");

Access const with generics C#

I have the following base class:
public class Base
{
public string LogicalName { get; set; }
public int NumberOfChars { get; set; }
public Base()
{
}
public Base(string logicalName, int numberOfChars)
{
LogicalName = logicalName;
NumberOfChars = numberOfChars;
}
}
and the following derived classes:
public class Derived1 : Base
{
public const string EntityLogicalName = "Name1";
public const int EntityNumberOfChars = 30;
public Derived1() : base(EntityLogicalName, EntityNumberOfChars)
{
}
}
public class Derived2 : Base
{
public const string EntityLogicalName = "Name2";
public const int EntityNumberOfChars = 50;
public Derived2()
: base(EntityLogicalName, EntityNumberOfChars)
{
}
}
and I also have this function that is provided by a service:
public IEnumerable<T> GetEntities<T>(string entityName, int numberOfChars) where T : Base
{
//Some code to get the entities
}
My problem is how can I call this function generically? I want to call it with something that looks like this:
public void TestEntities<T>() where T : Base
{
var entities = GetEntities<T>(T.EntityLogicalName, T.EntityNumberOfChars);
//some other code to test the entities
}
This of course doesn't work because at this point T is not known. How can I accomplish something similar to this? EntityLogicalName and EntityNumberOfChars are characteristics that all Base derived classes have and they never change for each derived class. Can I get them from the Base class without instantiating objects or some other way that I am not seeing?

Replace constants with getter abstract properties
public abstract class Base
{
public abstract string LogicalName { get; }
public abstract int NumberOfChars { get; }
public Base()
{
}
}
public class Derived1 : Base
{
public string LogicalName { get { return "Name1"; } }
public int NumberOfChars { get { return 30; } }
public Derived1() : base()
{
}
}
Also, you will be able to put some logic into overriden getter, e.g. :
...
public string LogicalName { get { return this.EntityMap.Name; } }
...
UPDATE: The fact that you do not want to instantiate object from class but want to be able to get that string in a strongly typed manner can be handled in one more way. It is totally separate from answer above ( Since you can't override static props in c#). Consider the following code. We are adding one more class here, but LocatorInner can be a member of BaseClass. We are using this approach a lot in several existing apps.:
public class Locator
{
public static class LocatorInner<T> where T : BaseClass
{
public static string Name { get; set; }
}
public static string GetName<T>() where T : BaseClass
{
return LocatorInner<T>.Name;
}
public static void SetName<T>(string name) where T : BaseClass
{
LocatorInner<T>.Name = name;
}
}
public class BaseClass
{
}
public class DerivedClass: BaseClass
{
static DerivedClass()
{
Locator.LocatorInner<DerivedClass>.Name = "me";
}
}
public class TestClass<T> where T : BaseClass
{
public void Method()
{
var name = Locator.GetName<T>();
}
}

IMHO, I believe using constants here is a bad design decision.
You can either solve the issue using #vittore approach, but for me it sounds like you should use meta-programming with attributes if you're looking to get data from the T generic argument
For example, what about:
public class LogicalNameAttribute : Attribute
{
public LogicalNameAttribute(string name)
{
Name = name;
}
public string Name { get; private set; }
}
public class NumberOfCharsAttribute : Attribute
{
public NumberOfCharsAttribute (int number)
{
Number = number;
}
public string Number { get; private set; }
}
[LogicalName("Name1"), NumberOfChars(30)]
public class Derived1 : Base
{
public Derived1() : base()
{
}
}
Now your service method can extract attribute metadata as follows:
public void TestEntities<T>() where T : Base
{
LogicalNameAttribute logicalNameAttr = typeof(T).GetCustomAttribute<LogicalNameAttribute>();
NumberOfCharsAttribute numberOfCharsAttr = typeof(T).GetCustomAttribute<NumberOfCharsAttribute >();
Contract.Assert(logicalNameAttr != null);
Contract.Assert(numberOfCharsAttr != null);
string logicalName = logicalNameAttr.Name;
int numberOfChars = numberOfCharsAttr.Number;
// Other stuff
}
There's a performance penalty because you need to use reflection to get attributes applied to T, but you gain the flexibility of not forcing derived classes to provide this static info.

As #vittore mentioned, move the properties to base,pass the hard coded values from derived and in creation use just defautl(T)
public IEnumerable<T> GetEntities<T>(string entityName, int numberOfChars) where T : Base
{
yield return default(T); //Is its always class use new constraint and return new T();
}

Overriding property with subclass

Say I have a property like this in an abstract class AbstractClass:
protected MyClass MyProperty { get; set; }
And this property is used in a number of (virtual) methods in AbstractClass. In a subclass of the AbstractClass I want MyProperty to be a subclass of MyClass, is this possible or do I have to cast it?
MyProperty = new SubclassOfMyClass();
((SubclassOfMyClass)MyProperty).Method();
Doesn't look very nice... I have tried to use the 'new' keyword to hide MyProperty like this:
protected new SubclassOfMyClass MyProperty { get; set; }
But this did not work out as I thought it would, as it seems to creates a second MyProperty, and results in the one in AbstractClass always being null.
So I came up with something that may seem like a bit of a hack:
protected new SubclassOfMyClass MyProperty
{
get { return base.MyProperty as SubclassOfMyClass; }
set { base.MyProperty = value; }
}
Is there a better way to do this?

You can use generics:
public abstract class AbstractClass<T> where T : MyClass, new() {
protected T MyProperty { get; set; }
}
public class SubClass : AbstractClass<SubclassOfMyClass> {
}
EDIT:
This is in response to your "hack":
Consider this code:
public class MyClass { }
public class SubclassOfMyClass : MyClass { }
public abstract class AbstractClass
{
public MyClass MyProperty { get; set; }
}
public class Subclass : AbstractClass
{
public new SubclassOfMyClass MyProperty
{
get { return base.MyProperty as SubclassOfMyClass; }
set { base.MyProperty = value; }
}
}
public class Test
{
public static void Main()
{
AbstractClass sub = new Subclass();
sub.MyProperty = new MyClass();
Subclass sub2 = (Subclass)sub;//casting succeeds since sub is Subclass
if (sub2.MyProperty == null)
Console.WriteLine("sub2.MyProperty is null!");
}
}
This actually prints "sub2.MyProperty is null!". After casting sub to sub2 and then accessing sub2.MyProperty base.MyProperty is still of type MyClass and base.MyProperty as SubclassOfMyClass returns null as expected. So if you're going to use the new keyword you should really know what's going on behind the scenes.

Get properties from derived class in base class

How do I get properties from derived class in base class?
Base class:
public abstract class BaseModel {
protected static readonly Dictionary<string, Func<BaseModel, object>>
_propertyGetters = typeof(BaseModel).GetProperties().Where(p => _getValidations(p).Length != 0).ToDictionary(p => p.Name, p => _getValueGetter(p));
}
Derived classes:
public class ServerItem : BaseModel, IDataErrorInfo {
[Required(ErrorMessage = "Field name is required.")]
public string Name { get; set; }
}
public class OtherServerItem : BaseModel, IDataErrorInfo {
[Required(ErrorMessage = "Field name is required.")]
public string OtherName { get; set; }
[Required(ErrorMessage = "Field SomethingThatIsOnlyHereis required.")]
public string SomethingThatIsOnlyHere{ get; set; }
}
In this example - can I get the "Name" property from ServerItem class while in BaseModel class?
EDIT:
I'm trying to implement model validation, as described here:
http://weblogs.asp.net/marianor/archive/2009/04/17/wpf-validation-with-attributes-and-idataerrorinfo-interface-in-mvvm.aspx
I figured that if I create some base model with (almost) all of the validation magic in it, and then extend that model, it will be okay...

If both classes are in the same assembly, you can try this:
Assembly
.GetAssembly(typeof(BaseClass))
.GetTypes()
.Where(t => t.IsSubclassOf(typeof(BaseClass))
.SelectMany(t => t.GetProperties());
This will give you all the properties of all the subclasses of BaseClass.

If you require that a derived class must implement a method or property, you should introduce that method or property into the base class as an abstract declaration.
For example, for your Name property, you would add to the base class:
public abstract string Name { get; set; }
Then any derived classes must implement it, or be abstract classes themselves.
Once you have added the abstract version of the Name property to the base class, you will be able to access it in the base class anywhere except in the base class's constructor.

If you must do literally fetch property of derived class from within base class, you can use Reflection, for example - like this...
using System;
public class BaseModel
{
public string getName()
{
return (string) this.GetType().GetProperty("Name").GetValue(this, null);
}
}
public class SubModel : BaseModel
{
public string Name { get; set; }
}
namespace Test
{
class Program
{
static void Main(string[] args)
{
SubModel b = new SubModel();
b.Name = "hello";
System.Console.Out.WriteLine(b.getName()); //prints hello
}
}
}
This is not recommended, though, and you most probably should rethink your design like Matthew said.
As for not throwing properties to your base classes -- you can try to decouple base and deriving classes into unrelated objects and pass them via constructors.

Another way to solve this issue by create virtual property in base class and override it to derived class.
public class Employee
{
public virtual string Name {get; set;}
}
public class GeneralStaff
{
public override string Name {get; set;}
}
class Program
{
static void Main(string[] args)
{
Employee emp = new GeneralStaff();
emp.Name = "Abc Xyz";
//---- More code follows----
}
}

Okay, I solved this problem slightly different than the author of this post: http://weblogs.asp.net/marianor/archive/2009/04/17/wpf-validation-with-attributes-and-idataerrorinfo-interface-in-mvvm.aspx
public abstract class BaseModel : IDataErrorInfo {
protected Type _type;
protected readonly Dictionary<string, ValidationAttribute[]> _validators;
protected readonly Dictionary<string, PropertyInfo> _properties;
public BaseModel() {
_type = this.GetType();
_properties = _type.GetProperties().ToDictionary(p => p.Name, p => p);
_validators = _properties.Where(p => _getValidations(p.Value).Length != 0).ToDictionary(p => p.Value.Name, p => _getValidations(p.Value));
}
protected ValidationAttribute[] _getValidations(PropertyInfo property) {
return (ValidationAttribute[])property.GetCustomAttributes(typeof(ValidationAttribute), true);
}
public string this[string columnName] {
get {
if (_properties.ContainsKey(columnName)) {
var value = _properties[columnName].GetValue(this, null);
var errors = _validators[columnName].Where(v => !v.IsValid(value)).Select(v => v.ErrorMessage).ToArray();
return string.Join(Environment.NewLine, errors);
}
return string.Empty;
}
}
public string Error {
get { throw new NotImplementedException(); }
}
}
Maybe it will help somebody.

Scan your assembly for all inherited classes from BaseModel and create dictionary like this:
Dictionary<Type, Dictionary<string, Func<BaseModel, object>>>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace TESTNEW
{
public abstract class BusinessStructure
{
public BusinessStructure()
{ }
public string Name { get; set; }
public string[] PropertyNames{
get
{
System.Reflection.PropertyInfo[] Pr;
System.Type _type = this.GetType();
Pr = _type.GetProperties();
string[] ReturnValue = new string[Pr.Length];
for (int a = 0; a <= Pr.Length - 1; a++)
{
ReturnValue[a] = Pr[a].Name;
}
return ReturnValue;
}
}
}
public class MyCLS : BusinessStructure
{
public MyCLS() { }
public int ID { get; set; }
public string Value { get; set; }
}
public class Test
{
void Test()
{
MyCLS Cls = new MyCLS();
string[] s = Cls.PropertyNames;
for (int a = 0; a <= s.Length - 1; a++)
{
System.Windows.Forms.MessageBox.Show(s[a].ToString());
}
}
}
}