Hi i'm writing a simple ORM DLL. The library works fine and i would like add some addone - Lazy Loading.
But i don't know how to implements that. I have one proposition.
In my orm (i have creator but never mind) User should create DB class who implements IMapper and set mapper class.
Somethink linke this.
public class Person
{
public virtual string Name {get;set;}
public virtual int Age {get; set;}
}
public class DataBase : IMapper
{
MapperSet<Person> Persons = new MapperSet<Person>();
}
If we have that class, and setup connectionString, we can take the data from DB. It's very similar to Entity Framework
Person p = Persons.Single(x=>x.Name == "John");
In that moment, i will check the all properties in mapping class and if are virtual then not return a that class, but Lazy Loading class.
I think out one conception. Not return (in that example) Person class but the class who extends Person class, and override all properties.
public class PersonReturn : Person
{
//here i must create a method who really take the data from db
private string Query = "SELECT TOP(1) FROM Person WHERE Name = 'John'";
private Execute()
{
p = (Person)Db.TableToObject(Query);
}
Person p;
public override string Name
{
get
{
if(p == null)
p = Execute();
return p.Name;
}
set {}
}
//same
public override int Age {get; set;}
}
User shouldn't see any change with using that class (only in debug it maybe see other class) it's should work as magic :P
My questions are :
1. How is implement Lazy Loading in for example Entity Framework, anybody know ?
2. Is simpler way from my proposition ? In my idea i must use TypeBuilder and Emit with IL source code - i hear it's problems with properties
they aren't be use in normaly way.
Use Castle.DynamicProxy (same proxy that nhibernate uses)
Lazy loading is implemented by generating inheritor at runtime and override all methods.
public class A
{
virtual protected string name { get; set; }
}
public interface IInterceptor
{
object Invoke(MethodInfo method, params object[] arguments);
}
public sealed class AProxy : A
{
static private readonly MethodInfo getname = typeof(A).GetProperty("name", ...).GetGetMethod(true);
static private readonly MethodInfo setname = typeof(A).GetProperty("name", ...).GetSetMethod(true);
private readonly IInterceptor interceptor;
public AProxy(IInterceptor interceptor)
{
this.interceptor = interceptor;
}
override protected string name
{
get { return this.interceptor.Invoke(AProxy.getname); }
set { this.interceptor.Invoke(AProxy.setname, value); }
}
}
Proxy factory have to to
return new AProxy(custominterceptor);
custominterceptor must take id of your entity and in first usage instantiate A, query database to populate A and delegate call to A.
AProxy must be generated (postbuild or at runtime using TypeBuilder)
Related
I implemented generic repository pattern and unitofwork. I used basic patterns and they work great. In project I have requirement which says, every table has several fields which contains long, really long text, and user should have ability chose and open any of it. As each field named differently i decided to use power ov generics with reflection, to write method which resieves table name and field name and returns it.
Method, in generic Repository class, i wrote looks like this, it seems work properly
public interface IRepository<T> where T : class
{
//other methods
string GetPropertyByName(int id, string property);
}
public class Repository<T> : IRepository<T> where T : class
{
// other methods. add, edit, delete...
public string GetPropertyByName(int id, string property)
{
T model = this.Get(id);
var obj = model.GetType().GetProperty(property).GetValue(model, null);
return obj != null ? obj.ToString() : null;
}
}
I creted model classes for tables with help EF. Some tables binds directly genric repository, while other have separate interface and its implementation, as they require additional method. Example:
public interface ICompanyRepo : IRepository<COMPANY>
{
//some methods
}
public class CompanyRepo : Repository<COMPANY>, ICompanyRepo
{
//implementations of interface methods
}
And UOW implementation:
public interface IUnitOfWork
{
ICompanyRepo Company { get; }
IRepository<CURRENCY> Currency { get; }
}
public class UnitOfWork : IUnitOfWork
{
static DBEntities _context;
private UZMEDEXPORTEntities context
{
get
{
if (_context == null)
_context = new DBEntities();
return _context;
}
}
public UnitOfWork()
{
_context = context;
Company = new SP_CompanyRepo();
Currency = new Repository<CURRENCY>();
}
public ICompanyRepo Company { get; private set; }
public IRepository<CURRENCY> Currency { get; private set; }
}
I have problem on invoking GetPropertyByName() method in business layer.
I tried this:
public string GetHistory(string tableName, string fieldName, int id)
{
var prop = unitOfWork.GetType().GetProperty(tableName);
MethodInfo method;
method = prop.PropertyType.GetMethod("GetPropertyByName"); //try to find method
if(method == null) //if method not found search for interface which contains that method
method = prop.PropertyType.GetInterface("IRepository`1").GetMethod("GetPropertyByName");
var res = method.Invoke(prop, new object[] { id, fieldName });
return (string)res;
}
Which returns System.Reflection.TargetException. As I understood the problem is whith unitofwork implementation. In my invoke method "prop" is type of interface (ICompanyRepo), but invoke's target should be interface implementation class, in this case "CompanyRepo".
I could not find how to identify type of implementetion class, and solve this problem. Any help is appropriated
I am not sure that this is best option, but problem solved with use of ToExpando() extension given here. With this extension i could loop throw all properties of unitofwork and find required property by its name.
var propValue = unitOfWork.ToExpando().Single(x => x.Key == prop.Name).Value;
var res = method.Invoke(propValue, new object[] { id, fieldName });
Now method is invoking properly. May be there is cleaner solution, and I still hope to find this. For now i am going to use this solution, and just realised that I must read and practice a lot about reflections, dynamics and generics.
P.S Special thanks to Alexei for important notes and advices
Short version:
I have an abstract class A. It has a method that needs to know the value of a static class property that is specific to each subclass. The name and type is the same, just the value can be unique for each subclass.
Can I define this static property in the base class A to be able to access it with methods defined in A, but keeping the property's values of different subclasses unrelated?
Or how would I implement something like that?
Long version:
Let's say I have an abstract base class for data models. It has a public property Id (Int32).
I would like to implement a constructor in the base class that generates a new ID based on the last assigned ID for objects of a subclass.
The reason is that the real IDs are assigned automatically by the database, but each data model object already has to have a unique ID when it gets constructed without being written to the database yet. As the database assigns only positive integers as ID, my plan is to assign newly created data model objects a temporary, unique negative ID. As soon as the object gets written, the ID will get changed to the real one.
As I have quite a few different data model classes all deriving from my abstract base class, I thought it would be good to include that functionality there to not duplicate it. But each subclass has to have their own counter that points to the next free negative ID, as the different classes' IDs are unrelated.
So I need a static property in each subclass storing this class' last assigned temporary ID, but the mechanism to assign it is always the same and could be implemented into the abstract base class' constructor. However, I can't access a property from the base class that has to be implemented by the subclasses, which means I have to define it in the base class. But will this static property then be global for all subclasses, which is not what I want?
How can I implement this temporary ID counter the most elegant way?
Simplified code example:
public abstract class ModelBase
{
public Int32 Id { get; set; }
protected static Int32 LastTempId { get; set; } = 0;
public ModelBase()
{
Id = --LastTempId;
}
}
public class Model1 : ModelBase
{
public Model1 () : base ()
{
// do something model1-specific
}
}
public class Model2 : ModelBase
{
public Model2() : base()
{
// do something model2-specific
}
}
If I implement it like this, I fear that for both subclasses model1 and model2, the inherited static property LastTempId will be the same instance. But I want a separate counter for each subclass while still using it in the base class constructor.
Short answer
The sub-classes cannot have different values for the static property because the static property is a property of the class, not of it's instances, and it's not inherited.
Long answer
You could implement a single counter on the abstract class as a static property and have one constructor of the abstract class using it.
EDIT: To save different counters for each sub-class you could use a static dictionary mapping a Type (sub-class) to a counter.
public abstract class A<T>
{
public static Dictionary<Type, int> TempIDs = new Dictionary<Type, int>();
public int ID { get; set; }
public A()
{
if (!TempIDs.ContainsKey(typeof(T)))
TempIDs.Add(typeof(T), 0);
this.ID = TempIDs[typeof(T)] - 1;
TempIDs[typeof(T)]--;
}
}
public class B : A<B>
{
public string Foo { get; set; }
public B(string foo)
: base()
{
this.Foo = foo;
}
}
public class C : A<C>
{
public string Bar { get; set; }
public C(string bar)
: base()
{
this.Bar = bar;
}
}
B b1 = new B("foo");
B b2 = new B("bar");
C c1 = new C("foo");
C c2 = new C("foo");
b1.ID would be -1, b2.ID would be -2, c1.ID would be -1 and c2.ID would be -2
First of all, my humble opinion is entities shouldn't be responsible of assigning their own unique identifier. Keep a clear separation of concerns.
There should be another player in that game that should assign those temporary unique identifiers (either if they're negative or positive integers).
Usually, that so-called other player is an implementation of repository design pattern which is responsible of translating the domain (your models) into the definitive representation of your data and vice versa.
Usually a repository has a method to add objects. And this should be the point where you set these temporary identifiers:
public void Add(Some some)
{
some.Id = [call method here to set the whole id];
}
And, most repository implementations are per entity.
CustomerRepository
InvoiceRepository
...
...but this doesn't prevent you from defining a base repository class which could implement what can be in common when handling some entity types:
public interface IRepository<TEntity> where TEntity : EntityBase
{
// Other repository methods should be defined here
// but I just define Add for the convenience of this
// Q&A
void Add(TEntity entity);
}
public class Repository<TEntity> : IRepository<TEntity>
where TEntity : EntityBase
{
public virtual void Add(TEntity entity)
{
entity.Id = [call method here to set the whole id];
}
}
...and now any class deriving Repository<TEntity> will be able to generate a temporary identifier for their specialized entities:
public class CustomerRepository : Repository<Customer> { }
public class InvoiceRepository : Repository<Invoice> { }
How you could implement the unique and temporary entity identifier as part of the abstract repository class and being able to do so for each specific entity type?
Use a dictionary to store per-entity last assigned identifier implementing a property to Repository<TEntity>:
public Dictionary<Type, int> EntityIdentifiers { get; } = new Dictionary<Type, int>();
...and a method to decrease next temporary identifier:
private static readonly object _syncLock = new object();
protected virtual void GetNextId()
{
int nextId;
// With thread-safety to avoid unwanted scenarios.
lock(_syncLock)
{
// Try to get last entity type id. Maybe the id doesn't exist
// and out parameter will set default Int32 value (i.e. 0).
bool init = EntityIdentifiers.TryGetValue(typeof(TEntity), out nextId);
// Now decrease once nextId and set it to EntityIdentifiers
nextId--;
if(!init)
EntityIdentifiers[typeof(TEntity)] = nextId;
else
EntityIdentifiers.Add(typeof(TEntity), nextId);
}
return nextId;
}
Finally, your Add method could look as follows:
public virtual void Add(TEntity entity)
{
entity.Id = GetNextId();
}
One way to go is reflection, but it takes run-time and is prone to runtime errors. As others mentioned: you cannot force inheriting classes to redeclare some static field and be able to use this field in ancestor class. So I think minimal code redundancy is necessary: each inheriting class should provide it's own key generator. This generator can be kept in static field of the class of course.
(Note this is not necessarily thread-safe.)
class KeyGenerator
{
private int _value = 0;
public int NextId()
{
return --this._value;
}
}
abstract class ModelBase
{
private KeyGenerator _generator;
public ModelBase(KeyGenerator _generator)
{
this._generator = _generator;
}
public void SaveObject()
{
int id = this._generator.NextId();
Console.WriteLine("Saving " + id.ToString());
}
}
class Car : ModelBase
{
private static KeyGenerator carKeyGenerator = new KeyGenerator();
public Car()
: base(carKeyGenerator)
{
}
}
class Food : ModelBase
{
private static KeyGenerator foodKeyGenerator = new KeyGenerator();
public Food()
: base(foodKeyGenerator)
{
}
}
class Program
{
static void Main(string[] args)
{
Food food1 = new Food();
Food food2 = new Food();
Car car1 = new Car();
food1.SaveObject();
food2.SaveObject();
car1.SaveObject();
}
}
This produces:
Saving -1
Saving -2
Saving -1
Just generate a GUID for each object before it gets added to your database. You could have an isAdded flag that tells you the object should be referred to be GUID, or clear the GUID once the object is added. With a GUID you never have to worry that two objects will clash. Also it obviates the need for separate IDs per subclass. I would not reuse the same property for two states as you propose.
https://msdn.microsoft.com/en-us/library/system.guid(v=vs.110).aspx
Well, static classes aren't inherited, so that's out,m and you can't force subclasses to implement a static method, so that's out too.
Rather than putting that method in the class itself, why not have a base interface that you can implement. Then you can have an instance method that can be abstract:
public interface IDataModelFactory<T> where T:ModelBase
{
int GetLastTempId();
}
public Model1Factory : IDataModelFactory<Model1>
{
public int GetLastTempId()
{
// logic for Model1
}
}
public Model2Factory : IDataModelFactory<Model2>
{
public int GetLastTempId()
{
// logic for Model2
}
}
Or if the logic is common to all classes, have an abstract base class with (or without) the interface:
public DataModelFactory<T> : IDataModelFactory<T>
{
public virtual int GetLastTempId()
{
// common logic
}
// other common logic
}
You could even make the factories singletons so you don't have to create instances all the time, and they can even be sub-classes of the model classes so they're closely linked.
As a side note, if you're uncertain what the inheritance/interface relationship would be, I often find it's quicker start with copy/paste reuse and refactor your code to introduce base classes and interfaces. That way you know what the common code is and can refactor that into common methods. Otherwise you are tempted to try and put everything in the base class and use switches or other constructs to change logic based on the derived type.
I have an interface that represents a table in a 3rd party API. Each instance provides the ability to search a single table using forward-only cursors:
public interface ITable
{
string TableName { get; }
ICursor Search(string whereClause);
}
I have written a wrapper class to handle searching an ITable and returning an enumerable instead (it's a little more complex than that in reality, but sufficient for showing my issue):
public interface ITableWrapper
{
IEnumerable<object> Search(string whereClause);
}
public class TableWrapper : ITableWrapper
{
private ITable _table;
public TableWrapper(ITable table)
{
_table = table;
}
public IEnumerable<Row> Search(string whereClause)
{
var cursor = _table.Search(whereClause);
while(cursor.Next())
{
yield return cursor.Row;
}
}
}
I then have several repository classes that should have a table wrapper injected:
public class Table1Repository
{
private ITableWrapper _table;
public Table1Reposiroty(ITableWrapper table)
{
_table = table;
}
//repository methods to actually do things
}
Since each table will have its own wrapper, and repositories need the correct table injecting, my thought was to use named bindings on the tables and wrappers so that ninject provides the correct instance. Thus the above class would have NamedAttribute applied to the constructor argument, and the binding would be as follows:
public void NinjectConfig(IKernel kernel, ITableProvider provider)
{
Bind<ITable>().ToMethod(ctx => provider.OpenTable("Table1")).Named("Table1").InSingletonScope();
Bind<ITableWrapper>().ToMethod(ctx => new TableWrapper(ctx.ContextPreservingGet<ITable>("Table1"))).Named("Table1Wrapper").InSingletonScope();
}
My questions are:
Is there a cleaner way to express this binding? I was thinking maybe a way to bind ITableWrapper once and have a new instance returned for each named ITable, with the repository constructor parameter attribute picking the named ITable for which it wants the ITableWrapper.
If the ITable should never be used by anything, and everything should always use ITableWrapper, is it ok (or even recommended) to bind just ITableWrapper and have that combine both ToMethod contents:
public void NinjectConfig(IKernel kernel, ITableProvider provider)
{
Bind<ITableWrapper>().ToMethod(ctx => new TableWrapper(provider.OpenTable("Table1"))).Named("Table1Wrapper").InSingletonScope();
}
There's no Ninject-built-in way to provide metadata to Ninject by attribute. The only thing it supports is the ConstraintAttribute (and the NamedAttribute as a subclass). This can be used to select a specific binding, but it can't be used to provide parameters for a binding.
So, in case you don't want to add a lot of code, the easiest and most concise way is along of what you suggested yourself:
public static BindTable(IKernel kernel, ITableProvider tableProvider, string tableName)
{
kernel.Bind<ITableWrapper>()
.ToMethod(ctx => new tableWrapper(tableProvider.OpenTable(tableName))
.Named(tableName);
}
(I've used the same string-id here for both table name and ITableWrapper name - this way you don't need to map them).
Also, i think it's better not to create a binding for ITable if you're not going to use it, anyway.
note: If you were to create the ITableWrapper by a factory (instead of ctor-injecting it), you could use parameters and a binding which reads the table-id from the parameter. Meaning a single binding would suffice.
Generic Solution
Now in case you're ok with adding some custom code you can actually achieve a generic solution. How? you add a custom attribute to replace the NamedAttribute which provides the table name. Plus you create a binding which reads the table name from this custom attribute. Let's say:
[AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
public class TableIdAttribute : Attribute
{
public TableIdAttribute(string tableName)
{
TableName = tableName;
}
public string TableName { get; private set; }
}
let's implement an IProvider to capsule the added binding complexity (it would also work with a ToMethod binding):
internal class TableWrapperProvider : Provider<ITableWrapper>
{
private readonly ITableProvider _tableProvider;
public TableWrapperProvider(ITableProvider tableProvider)
{
_tableProvider = tableProvider;
}
protected override ITableWrapper CreateInstance(IContext context)
{
var parameterTarget = context.Request.Target as ParameterTarget;
if (parameterTarget == null)
{
throw new ArgumentException(
string.Format(
CultureInfo.InvariantCulture,
"context.Request.Target {0} is not a {1}",
context.Request.Target.GetType().Name,
typeof(ParameterTarget).Name));
}
var tableIdAttribute = parameterTarget.Site.GetCustomAttribute<TableIdAttribute>();
if (tableIdAttribute == null)
{
throw new InvalidOperationException(
string.Format(
CultureInfo.InvariantCulture,
"ParameterTarget {0}.{1} is missing [{2}]",
context.Request.Target,
context.Request.Target.Member,
typeof(TableIdAttribute).Name));
}
return new TableWrapper(_tableProvider.Open(tableIdAttribute.TableName));
}
}
and here's how we use it (example classes):
public class FooTableUser
{
public FooTableUser([TableId(Tables.FooTable)] ITableWrapper tableWrapper)
{
TableWrapper = tableWrapper;
}
public ITableWrapper TableWrapper { get; private set; }
}
public class BarTableUser
{
public BarTableUser([TableId(Tables.BarTable)] ITableWrapper tableWrapper)
{
TableWrapper = tableWrapper;
}
public ITableWrapper TableWrapper { get; private set; }
}
and here's the bindings plus a test:
var kernel = new StandardKernel();
kernel.Bind<ITableProvider>().ToConstant(new TableProvider());
kernel.Bind<ITableWrapper>().ToProvider<TableWrapperProvider>();
kernel.Get<FooTableUser>().TableWrapper.Table.Name.Should().Be(Tables.FooTable);
kernel.Get<BarTableUser>().TableWrapper.Table.Name.Should().Be(Tables.BarTable);
I am trying to accomplish the following scenario that the generic TestClassWrapper will be able to access static properties of classes it is made of (they will all derive from TestClass). Something like:
public class TestClass
{
public static int x = 5;
}
public class TestClassWrapper<T> where T : TestClass
{
public int test()
{
return T.x;
}
}
Gives the error:
'T' is a 'type parameter', which is not valid in the given context.
Any suggestions?
You can't, basically, at least not without reflection.
One option is to put a delegate in your constructor so that whoever creates an instance can specify how to get at it:
var wrapper = new TestClassWrapper<TestClass>(() => TestClass.x);
You could do it with reflection if necessary:
public class TestClassWrapper<T> where T : TestClass
{
private static readonly FieldInfo field = typeof(T).GetField("x");
public int test()
{
return (int) field.GetValue(null);
}
}
(Add appropriate binding flags if necessary.)
This isn't great, but at least you only need to look up the field once...
Surely you can just write this:
public int test()
{
return TestClass.x;
}
Even in a nontrivial example, you can't override a static field so will always call it from your known base class.
Why not just return TestClass.x?
Generics do not support anything related to static members, so that won't work. My advice would be: don't make it static. Assuming the field genuinely relates to the specific T, you could also use reflection:
return (int) typeof(T).GetField("x").GetValue(null);
but I don't recommend it.
Another solution is to simply not make it static, and work with the new() constraint on T to instantiate the object. Then you can work with an interface, and the wrapper can get the property out of any class that implements that interface:
public interface XExposer
{
Int32 X { get; }
}
public class TestClass : XExposer
{
public Int32 X { get { return 5;} }
}
public class XExposerWrapper<T> where T : XExposer, new()
{
public Int32 X
{
get { return new T().X; }
}
}
In fact, you can change that to public static Int32 X on the TestClassWrapper and simply get it out as Int32 fetchedX = XExposerWrapper<TestClass>.X;
Though since whatever code calls this will have to give the parameter T those same constraints, the wrapper class is pretty unnecessary at this point, since that calling code itself could also just execute new T().X and not bother with the wrapper.
Still, there are some interesting inheritance models where this kind of structure is useful. For example, an abstract class SuperClass<T> where T : SuperClass<T>, new() can both instantiate and return type T in its static functions, effectively allowing you to make inheritable static functions that adapt to the child classes (which would then need to be defined as class ChildClass : SuperClass<ChildClass>). By defining protected abstract functions / properties on the superclass, you can make functions that apply the same logic on any inherited object, but customized to that subclass according to its implementations of these abstracts. I use this for database classes where the table name and fetch query are implemented by the child class. Since the properties are protected, they are never exposed, either.
For example, on database classes, where the actual fetching logic is put in one central abstract class:
public abstract class DbClass<T> where T : DbClass<T>, new()
{
protected abstract String FetchQuery { get; }
protected abstract void Initialize(DatabaseRecord row);
public static T FetchObject(DatabaseSession dbSession, Int32 key)
{
T obj = new T();
DatabaseRecord record = dbSession.RetrieveRecord(obj.FetchQuery, key);
obj.Initialize(record);
return obj;
}
}
And the implementation:
public class User : DbClass<User>
{
public Int32 Key { get; private set;}
public String FirstName { get; set;}
public String LastName { get; set;}
protected override String FetchQuery
{ get { return "SELECT * FROM USER WHERE KEY = {0}";} }
protected override void Initialize(DatabaseRecord row)
{
this.Key = DbTools.SafeGetInt(row.GetField("KEY"));
this.FirstName = DbTools.SafeGetString(row.GetField("FIRST_NAME"));
this.LastName = DbTools.SafeGetString(row.GetField("LAST_NAME"));
}
}
This can be used as:
User usr = User.FetchObject(dbSession, userKey);
This is a rather simplified example, but as you see, this system allows a static function from the parent class to be called on the child class, to return an object of the child class.
T is a type, not parameter or variable so you cannot pick any value from any members. Here is a sample code.
public class UrlRecordService
{
public virtual void SaveSlug<T>(T entity) where T : ISlugSupport
{
if (entity == null)
throw new ArgumentNullException("entity");
int entityId = entity.Id;
string entityName = typeof(T).Name;
}
}
public interface ISlugSupport
{
int Id { get; set; }
}
cjk and Haris Hasan have the most-correct answers to the question as asked. However in this comment the OP implies that he is after something else not quite possible in C#: a way to define a contract for a static member in a derived class.
There isn't a way to strictly define this, but it is possible to set up a pattern that may be implied by a base class (or interface); e.g.:
public class TestClass
{
private static int x;
public virtual int StaticX => x;
}
or if not intended to be used directly
public abstract class AbstractTestClass
{
public abstract int StaticX {get;}
}
or (my preference in this contrived example)
public interface ITest
{
int StaticX {get;}
}
Elsewhere, this pattern of a StaticXxx member may be (loosely) associated with implementations that should back the member with static fields (as in TestClass above).
What's kind of fun is that this can be (re)exposed as static by the generic wrapper, because generic statics are isolated to each type used.
public class TestClassWrapper<T> where T : ITest, new()
{
private readonly static T testInstance = new T();
public static int test() => testInstance.x;
}
This uses a new() condition, but an associated static, generic factory pattern for creating ITest (or TestClass or AbstractTestClass) instances may also be used.
However this may not be feasible if you can't have long-lived instances of the class.
In this situation you assume that T is a subclass of TestClass. Subclasses of TestClass will not have the static int x.
In my current project I need to be able to have both editable and read-only versions of classes. So that when the classes are displayed in a List or PropertGrid the user is not able to edit objects they should not be allowed to.
To do this I'm following the design pattern shown in the diagram below. I start with a read-only interface (IWidget), and then create an edtiable class which implements this interface (Widget). Next I create a read-only class (ReadOnlyWidget) which simply wraps the mutable class and also implements the read only interface.
I'm following this pattern for a number of different unrelated types. But now I want to add a search function to my program, which can generate results that include any variety of types including both mutable and immutable versions. So now I want to add another set of interfaces (IItem, IMutableItem) that define properties which apply to all types. So IItem defines a set of generic immutable properties, and IMutableItem defines the same properties but editable. In the end a search will return a collection of IItems, which can then later be cast to more specific types if needed.
Yet, I'm not sure if I'm setting up the relationships to IMutable and IItem correctly. Right now I have each of the interfaces (IWidget, IDooHickey) inheriting from IItem, and then the mutable classes (Widget, DooHickey) in addition also implement IMutableItem.
Alternatively, I was also thinking I could then set IMutableItem to inherit from IItem, which would hide its read-only properties with new properties that have both get and set accessors. Then the mutable classes would implement IMutableItem, and the read-only classes would implement IItem.
I'd appreciate any suggestions or criticisms regarding any of this.
Class Diagram
Code
public interface IItem
{
string ItemName { get; }
}
public interface IMutableItem
{
string ItemName { get; set; }
}
public interface IWidget:IItem
{
void Wiggle();
}
public abstract class Widget : IWidget, IMutableItem
{
public string ItemName
{
get;
set;
}
public void Wiggle()
{
//wiggle a little
}
}
public class ReadOnlyWidget : IWidget
{
private Widget _widget;
public ReadOnlyWidget(Widget widget)
{
this._widget = widget;
}
public void Wiggle()
{
_widget.Wiggle();
}
public string ItemName
{
get {return _widget.ItemName; }
}
}
public interface IDoohickey:IItem
{
void DoSomthing();
}
public abstract class Doohickey : IDoohickey, IMutableItem
{
public void DoSomthing()
{
//work it, work it
}
public string ItemName
{
get;
set;
}
}
public class ReadOnlyDoohickey : IDoohickey
{
private Doohickey _doohicky;
public ReadOnlyDoohickey(Doohickey doohicky)
{
this._doohicky = doohicky;
}
public string ItemName
{
get { return _doohicky.ItemName; }
}
public void DoSomthing()
{
this._doohicky.DoSomthing();
}
}
Is it OK to create another object when you need a readonly copy? If so then you can use the technique in the included code. If not, I think a wrapper is probably your best bet when it comes to this.
internal class Test
{
private int _id;
public virtual int ID
{
get
{
return _id;
}
set
{
if (ReadOnly)
{
throw new InvalidOperationException("Cannot set properties on a readonly instance.");
}
}
}
private string _name;
public virtual string Name
{
get
{
return _name;
}
set
{
if (ReadOnly)
{
throw new InvalidOperationException("Cannot set properties on a readonly instance.");
}
}
}
public bool ReadOnly { get; private set; }
public Test(int id = -1, string name = null)
: this(id, name, false)
{ }
private Test(int id, string name, bool readOnly)
{
ID = id;
Name = name;
ReadOnly = readOnly;
}
public Test AsReadOnly()
{
return new Test(ID, Name, true);
}
}
I would suggest that for each main class or interface, there be three defined classes: a "readable" class, a "changeable" class, and an "immutable" class. Only the "changeable" or "immutable" classes should exist as concrete types; they should both derive from an abstract "readable" class. Code which wants to store an object secure in the knowledge that it never changes should store the "immutable" class; code that wants to edit an object should use the "changeable" class. Code which isn't going to write to something but doesn't care if it holds the same value forever can accept objects of the "readable" base type.
The readable version should include public abstract methods AsChangeable(), AsImmutable(), public virtual method AsNewChangeable(), and protected virtual method AsNewImmutable(). The "changeable" classes should define AsChangeable() to return this, and AsImmutable to return AsNewImmutable(). The "immutable" classes should define AsChangeable() to return AsNewChangeable() and AsImmutable() to return this.
The biggest difficulty with all this is that inheritance doesn't work terribly well if one tries to use class types rather than interfaces. For example, if one would like to have an EnhancedCustomer class which inherits from BasicCustomer, then ImmutableEnhancedCustomer should inherit from both ImmutableBasicCustomer and ReadableEnhancedCustomer, but .net doesn't allow such dual inheritance. One could use an interface IImmutableEnhancedCustomer rather than a class, but some people would consider an 'immutable interace' to be a bit of a smell since there's no way a module that defines an interface in such a way that outsiders can use it without also allowing outsiders to define their own implementations.
Abandon hope all ye who enter here!!!
I suspect that in the long run your code is going to be very confusing. Your class diagram suggests that all properties are editable (or not) in a given object. Or are your (I'm)mutable interfaces introducing new properties that are all immutable or not, separate from the "core"/inheriting class?
Either way I think you're going to end up with playing games with property name variations and/or hiding inherited properties
Marker Interfaces Perhaps?
Consider making all properties in your classes mutable. Then implement IMutable (I don't like the name IItem) and IImutable as a marker interfaces. That is, there is literally nothing defined in the interface body. But it allows client code to handle the objects as a IImutable reference, for example.
This implies that either (a) your client code plays nice and respects it's mutability, or (b) all your objects are wrapped by a "controller" class that enforces the given object's mutability.
Could be too late :-), but the cause "The keyword 'new' is required on property because it hides property ..." is a bug in Resharper, no problem with the compiler. See the example below:
public interface IEntityReadOnly
{
int Prop { get; }
}
public interface IEntity : IEntityReadOnly
{
int Prop { set; }
}
public class Entity : IEntity
{
public int Prop { get; set; }
}
[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestMethod1()
{
var entity = new Entity();
(entity as IEntity).Prop = 2;
Assert.AreEqual(2, (entity as IEntityReadOnly).Prop);
}
}
Same for the case without interfaces. The only limitation, you can't use auto-properties
public class User
{
public User(string userName)
{
this.userName = userName;
}
protected string userName;
public string UserName { get { return userName; } }
}
public class UserUpdatable : User
{
public UserUpdatable()
: base(null)
{
}
public string UserName { set { userName = value; } }
}
[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestMethod1()
{
var user = new UserUpdatable {UserName = "George"};
Assert.AreEqual("George", (user as User).UserName);
}
}