I am attempting to create an application which interacts with a database.
I program in many languages and I loved how easy it was to use Models in an MVC based application.
So my question is, trying to replicate this functionality, I have 2 classes as follows:
Base Class:
public class BaseModel
{
protected string TableName { get; set; }
public BaseModel()
{
}
public void Save()
{
// Save data in derived class to table stored in TableName
}
}
Derived Class:
public class UserModel : BaseModel
{
public string Field1 { get; set; }
public string Field2 { get; set; }
public UserModel()
{
base.TableName = "user";
}
}
From my main application, i want to be able to do the following:
public class Class1
{
public Class1()
{
UserModel model = new UserModel();
model.Field1 = "Value1";
model.Field2 = "Value2";
model.Save();
}
}
Here is where i have hit a problem. I cannot for the life in me figure out, how i would be able to access the properties in UserModel so they can be saved to the database table specified in in the constructor of UserModel to the BaseModel.
Hope this made sense. :)
This code is not my working code, it is a very simple representation of what i would like to achieve so Fields and properties (validators etc) have been dumbed down for ease of reading.
I think that a more "real" example would actually help understand better, but I think that you should create a virtual (or even abstract) method in the base class to get the data to save in the database:
public class BaseModel {
protected virtual string GetDataToSaveInDB() {
// if it makes sense to have the data as a string...
}
public void Save() {
string data = GetDataToSaveInDB();
// do something with data...
}
}
Then you return the data in the derived classes:
public class UserModel : BaseModel {
protected override string GetDataToSaveInDB() {
return "Field1=" + Field1 + ";Field2=" + Field2;
}
}
This is just to illustrate the concept, if you provide more information it will be easier to provide a real answer.
How do you save the data in your DB? How is the structure to the table?
This is in general solved by utilize abstract functions at the base level and then overriding them with specifics in the derived class. So would would set up a call tree in the base but functions that need to know the details of the derived class would be implemented in the derived class.
public class BaseModel
{
protected string TableName { get; set; }
public BaseModel()
{
}
public abstract void Save();
}
In this case where you have one concrete data representation manifested in a base class it may be better to create a DTO object which your business classes take in a utilize. This is the approch of most of the frameworks like entity.
Related
Before I explain my problem, keep in mind that I went for an architecture like this because this is going to be used in an inventory system under Unity, so I had to separate the Item which is a MonoBehavior which is just something in the world from its Data which are just values used for Inventory purposes... If that makes sense.
I have an architecture that goes like this:
public class ItemData
{
// some fields...
public string _name; //should be private with its properties but it doesn't matter in the current example
}
public class EquipmentData : ItemData
{
// some additional fields...
public float _weight;
}
public class Item
{
private ItemData _data;
//Properties
public virtual ItemData Data { get; set; }
}
public class Equipment : Item
{
//Properties
public override ItemData Data
{
get { return _data as EquipmentData; }
set { _data = value as EquipmentData; }
}
}
So, basically I have an item hierarchy that goes even deeper but 1 level is enough to explain myself. (It keeps going like Weapon : Equipment)...
The thing is, if I leave private ItemData _data; in Item class, and add a private EquipmentData _eData; in Equipment class, I will have the ItemData fields twice since EquipmentData inherits ItemData and so on for the other derived classes... getting it a third time if I have a class that derives from Equipment etc...
Something like:
public class Item
{
private ItemData _data;
}
public class Equipment : item
{
private EquipmentData _eData;
}
The fields of ItemData such as _name will appear twice in Equipment and I don't want that...
So I am guessing there is something wrong with my architecture, and there might be a way around this method that looks kind of "dirty", but I couldn't find anything specific for this problem online and I have reached my limits.
What I have tried:
I have tried using the keyword new in Equipment thinking I could hide the initial protected ItemData _data; that is in my Item class, and then have protected new EquipmentData _data; in Equipment but it obviously does not let me since to Shadow _data it needs to be the same type, doesn't seem to work with derived types.
Also, as shown in my code sample, I have tried overriding the property to return the proper type depending on the class it is called in, the casts always return null...
I still find it strange that I ended up trying to implement something like that, so I am open to new ideas to restructure things in a better way, or if someone has a solution I haven't thought of to keep things that way but make them work, it'd be very nice.
I hope I was detailed enough in my problem, if not I can clear things up if needed.
What You need is Generic Classes. By this, You can assign each Item its proper type of ItemData. So Equipment will have its EquipmentData assigned.
//TItemData is a name of this generic type.
//It could be T for example, just like variable name.
//These type names start from T by convention.
//This is not a new class or something like that.
//where TItemData : ItemData is a constraint,
//that assumes that this type should be a subtype of ItemData
public abstract class Item<TItemData> where TItemData : ItemData
{
protected TItemData Data;
}
//EquipmentData is a subtype of ItemData, so it fits here.
//No chances to write, for example, IntEquipment : Item<int> ,
//because int does not derive from ItemData.
public class Equipment : Item<EquipmentData>
{
//here Data will be of EquipmentData type, without any casting.
}
By implementing the above You will achieve Type Safety.
EDIT
To make a class that properly extends Equipment (let's call it a Weapon), and has its proper ItemData (let's call it WeaponData), You need to write something like this:
Edit Equipment, and make it abstract:
public abstract class Equipment<TEquipmentData>
: Item<TEquipmentData>
//this constraint is VERY important, as EquipmentData derives from ItemData, thus fulfill Item<TItemData> constraint.
where TEquipmentData : EquipmentData
{
//Data will have EquipmentData type here.
}
Create WeaponData
public WeaponData : EquipmentData
{
}
Create Weapon
public class Weapon : Equipment<WeaponData>
{
//Data will have WeaponData type here.
}
This works:
public class OverridePropertiesWithSameField
{
public void Test()
{
ChildItem ci = new ChildItem();
ChildItemData cid = new ChildItemData();
cid.ItemDataProp = "ItemDataProperty"; // Inherited
cid.ChildItemDataProp = "ChildItemDataProp"; // Specific
ci.ItemData = cid;
// You know you need ChildItemData type here.
var childItemData = ci.ItemData as ChildItemData;
string itemDataProp = childItemData.ItemDataProp;
string childItemDataProp = childItemData.ChildItemDataProp;
}
}
public class Item
{
protected ItemData data;
public virtual ItemData ItemData { get; set; }
}
public class ChildItem : Item
{
public override ItemData ItemData
{
get { return base.data; }
set { base.data = value; }
}
}
public class ItemData
{
public string ItemDataProp { get; set; }
}
public class ChildItemData : ItemData
{
public string ChildItemDataProp { get; set; }
}
You can use a generic type parameter, with a generic type constraint (where).
public class Item<DATA> where DATA : ItemData
{
public virtual DATA Data { get; set; }
}
Now your class can use a specific ItemData:
Item<ItemData> has property
public virtual ItemData Data { get; set; }
Item<EquipmentData> has property
public virtual EquipmentData Data { get; set; }
Item<ANOTHER> has property
public virtual ANOTHER Data { get; set; }
I have some classes with common properties, however, I cannot make them derive from a base type (LINQ-to-SQL limitations).
I would like to treat them as if they had a base type, but not by using Reflection (performance is critical).
For example:
public class User
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle
{
public int Id { get; set; }
public string Label { get; set; }
}
In this case I would be happy if I had the Id property available, regardless of the type I'm holding.
Is there any way in C# to to something similar to this:
public static int GetId<T>(T entity) where T // has an int property 'Id'
{
return entity.Id;
}
I guess I could have used dynamic, however, I'm looking for a way to restrict the code in compile time from using this method for an object that has no Id property.
You can use interfaces:
public interface IHasId
{
int Id { get; }
}
public class User : IHasId { ... }
public class Vehicle : IHasId { ... }
public static int GetId<T>(T entity) where T : IHasId
{
return entity.Id;
}
However, if you are not able to modify the classes to add the interface, you won't be able to do this. No compile-time checks will verify that a property exists on T. You'd have to use reflection - which is slow and obviously not ideal.
There is no way to guarantee a type has a given member without constraining to a common base type or interface. One way to work around this limitation is to use a lambda to access the value
public static int Use<T>(T value, Func<T, int> getIdFunc) {
int id = getIdFunc(value);
...
}
Use(new User(), u => u.Id);
Use(new Vehicle(), v => v.Id);
You can create an interface with the common properties and make your classes implement it:
public interface IEntity
{
int Id { get; set; }
}
public class User : IEntity
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle : IEntity
{
public int Id { get; set; }
public string Label { get; set; }
}
public static int GetId<T>(T entity) where T : IEntity
{
return entity.Id;
}
You could simplify GetId like this:
public static int GetId(IEntity entity)
{
return entity.Id;
}
The other answers mentioning the interface approach are certainly good, but I want to tailor the response to your situation involving Linq-to-SQL.
But first, to address the question title as asked
Can C# constraints be used without a base type?
Generally, the answer is no. Specifically, you can use struct, class, or new() as constraints, and those are not technically base types, and they do give some guidance on how the type can be used. That doesn't quite rise to the level of what you wish to do, which is to limit a method to types that have a certain property. For that, you will need to constrain to a specific interface or base class.
For your specific use case, you mention Linq-to-SQL. If you are working from models that are generated for you, then you should have options to modify those classes without modifying the generated model class files directly.
You probably have something like
// code generated by tool
// Customer.cs
public partial class Customer // : EntityBaseClasses, interfaces, etc
{
public int ID
{
get { /* implementation */ }
set { /* implementation */ }
}
}
And other similar files for things such as Accounts or Orders or things of that nature. If you are writing code that wishes to take advantage of the commonly available ID property, you can take utilize the partial in the partial class to define a second class file to introduce a common interface type to these models.
public interface IIdentifiableEntity
{
int ID { get; }
}
And the beauty here is that using it is easy, because the implementation already exists in your generated models. You just have to declare it, and you can declare it in another file.
public partial class Customer : IIdentifiableEntity { }
public partial class Account : IIdentifiableEntity { }
// etc.
This approach has proven valuable for me when using a repository pattern, and wishing to define a general GetById method without having to repeat the same boilerplate in repository after repository. I can constrain the method/class to the interface, and get GetById for "free."
Either you need to make both classes implement an interface with the properties you need, and use that in the generic constraint, or you write separate methods for each type. That's the only way you'll get compile-time safety.
I'm trying to get my head around a polymorphism/inheritance situation in C#.
What I have right now is these classes:
Lease (the base class containing the general data)
PrivateLease (inheriting from the Lease class)
BusinessLease (inheriting from the Lease class)
What I want to achieve is this:
Lease lease = new PrivateLease();
This works at the moment, but I am not able to access the properties on the PrivateLease object when doing this. At least not without casting the Lease object to a PrivateLease object first.
I'd like the Lease object to be the general object of either a PrivateLease or BusinessLease object which holds all the data for one of the objects. Then when inserting/updating/deleting to the database I'm going to ask which type it is first to dertermine which tables to insert the data into.
I've got a strange feeling that the above is not the right approach to solve this problem. Does anyone have any hints on this? :-) I've searched on google and read in my programming books and everyone suggests this approach of having a base class and then inherit from it to the other classes.
Any help/hint is greatly appreciated!
Thanks in advance.
EDIT
Should've elaborated a bit on this from the beginning, I'm sorry for that!
The above mentioned classes are merely just holding data from the UI of my ASP.NET solution to perform CRUD operations against the database via a Data Access Layer. So bascially these classes only contains a bunch of properties to hold data. I.e:
public class Lease
{
public int Id { get; set; }
public bool IsActive { get; set; }
public string TypeOfRental { get; set; }
public string RentalPeriod { get; set; }
public DateTime TakeoverDate { get; set; }
}
public class PrivateLease : Lease
{
public string Floor { get; set; }
public string Side { get; set; }
public int FloorSize { get; set; }
public int NumberOfRooms { get; set; }
}
etc..
The PrivateLease and BusinessLease classes are different because of the different leaseing-variables that exists in the real world :-)
Basically I could just go with the two separate PrivateLease and BusinessLease objects, but since the model dictates that an Address object can hold one or more Leases, this is not an option.
To me it seems like I'm going to go through a major casting hell both on the ASP.NET frontend and on the DAL? :-/
Don't decide (choose a logic) on the layer of consumer, but let to decide by the classes themselves:
// or you ILease interface if a parent class will not contain any shared logic
abstract class Lease
{
public abstract void Do();
// example of shared logic
protected void Save(Lease l) { }
}
class PrivateLease : Lease
{
public override void Do() { // private logic here }
}
class BusinessLease : Lease
{
public override void Do() { // business logic here }
}
Usage:
Lease l = ...
l.Do(); // execute the logic
You may want to create a factory for objects creation:
static class LeaseFactory<T> where T : Lease, new() // constraint to require default constructor existence
{
public static Leas Create()
{
return new T();
}
}
You're right in the basic approach of having a base class.
What you need to do is to put any common properties in the base class. Then if you have different business rules, those can be implemented with virtual functions, being called polymorphically.
abstract class Lease
{
public int MonthlyCost {get;set;}
public string CustomerName {get;set;}
// Declare that all Leases have to have an IncreaseCost method.
public abstract void IncreaseCost();
}
class PrivateLease : Lease
{
// Private leases are incremented by an absolute number (10).
public override void IncreaseCost()
{
MonthlyCost += 10;
}
}
class BusinessLease : Lease
{
// Business leases are incremented by 10%.
public override void IncreaseCost()
{
MonthlyCost *= 1.10;
}
}
// Somewhere in your code...
Lease lease = new PrivateLease();
// This call is polymorphic. It will use the actual type of the lease object.
lease.IncreaseCost();
In the modern OOD you can use interfaces, for this situation.
Edit:
In my opinion, to avoid casting, you can have multiple interfaces for multiple purposes. then PrivateLease and BusinessLease can implement the appropriate ones.
interface IWrite
{
string Data { get; set; }
void Write();
}
interface IRead
{
string Data { get; set; }
void Read();
}
public class Lease
{
//..
}
public class PrivateLease : Lease, IWrite, IRead
{
// other implementations
public string Data { get; set; }
public void Read()
{
//..
}
public void Write()
{
//..
}
}
public class BusinessLease : Lease, IRead
{
// other implementations
public string Data { get; set; }
public void Read()
{
//..
}
}
In Lease class add virtual method called DBUpdate and override it in both the derived classes.
Let's say some Utility class has LeaseDBOperation Method looks like this :
public static void LeaseDBOperation (Lease anylease)
{
anyleaase.DBUpdate();
}
you can call this method as :
var pl = new PrivateLease();
..set all the properties of **pl**
//call this for db operations :
Utility.LeaseDBOperation(pl)
Here in LeaseDBOperation method , if based on the type send , DBUpdate method of required class will be called.
Lease l = (Lease)sth;
if (l is PrivateLease)
{
PrivateLease p = (PrivateLease)l;
//do private logic here
}
else if (l if BussinessLease)
{
BussinessLease b = (BunessinessLease)l;
//do bussiness logic here
}
I have a scenario where I have a bunch of jobs that I am scheduling to run at various times, the jobs themselves are being handled generically already which is great. And I have an abstract BaseJob class that they all inherit from that I use for common things (like the jobPK, startTime, exception logging, reporting, etc). But beyond that the jobs are very different, they have different properties and data associated with them that is entriely specific to them (I call these proprties JobDetails). So for example:
JobDetails for Job1
-customerId int
-cost double
-sku string
-userDefinedProperties SomeCustomObjectType
JobDetails for Job2
-name string
-executionDate DateTime
-otherProperties SomeOtherCustomObjectType
In the base class I would like to be able to store a reference to these JobDetails in as generic a fashion as possible (so in other words I don't want to just store it as object) to minimize the overhead for boxing/unboxing. Then I want to have the BaseJob class handle a lot of the common functionality that is needed for the app, so for example, if a job fails, I want to save its JobDetails to the database so that it can be restarted, I also want to log any errors that may have occured to a given job. For this I need to be able to extract those JobDetails and make use of them.
It seems like I need to make use of .NET generics and have a class of generic properties that I can stuff anything into and not have to worry about typing. What's the best way to handle this and make it efficient and flexible?
I hope that is clear, thanks for the help
You can make the JobDetails implement an interface and let have BaseJob have an abstract reference to it. Then in the actual jobs you implement the abstract JobDetails with the implementation you want. Then let the JobDetails interface define the methods BaseJob needs to work with. This is a slight variation on the Template Method design pattern. It would look something like this:
public interface IJobDetails {
void DoSomeWork();
}
public abstract BaseJob {
protected abstract IJobDetails JobDetails { get; set; }
public ExecuteJob {
//execute the job
JobDetails.DoSomeWork();
}
}
public Job1 : BaseJob {
public Job1() {
JobDetails = new ConcreteJobDetails();
}
protected override IJobDetails JobDetails { get; set; }
}
How about something like...
public abstract class JobBase<TDetails>
{
private TDetails details;
protected TDetails Details
{
get
{
if (details == null)
{
this.details = this.LoadDetails();
}
return this.details;
}
}
protected virtual TDetails LoadDetails()
{
// Some kind of deserialization of TDetails from your DB storage.
}
}
public class ExampleJob : JobBase<ExampleJob.ExampleJobDetails>
{
public class ExampleJobDetails
{
public string ExampleProperty { get; set; }
public int AnotherProperty { get; set; }
}
}
You'd either want to have tables for each type used as TDetails or one big Key/Value based table for all of them. There are pros/cons to both. If you are super paranoid about boxing, there's no reason why TDetails can't be constrained to be a struct, too.
Edit: Got it backwards, you want to save the details on a failure. How about...
public abstract class JobBase<TDetails>
{
protected TDetails Details { get; private set; }
public JobBase()
{
this.Details = this.CreateDetails();
}
protected abstract TDetails CreateDetails();
protected void SaveDetails()
{
// Generic save to database.
}
}
public class ExampleJob : JobBase<ExampleJob.ExampleJobDetails>
{
public class ExampleJobDetails
{
public string ExampleProperty { get; set; }
public int AnotherProperty { get; set; }
}
protected override ExampleJobDetails CreateDetails()
{
return new ExampleJobDetails() { ExampleProperty = "Hi.", AnotherProperty = 1 };
}
}
I have a class like the below, I want to override the set value of "School,Country..etc.." property when some one sets a value , i don't want to change the student class but i need to do it in the base class and use it as a generic method
public class Student : BaseClass
{
public String School { get; set; }
public String Country{ get; set; }
}
ie:
When some one sets
Student.School="Harvard",
I need to store it as
Student.School="Harvard my custom value";
Note:
Basically calling OnPropertyChanged in base class rather than the main class.
If you want to do it with aspects, then try Postsharp
Basically you cannot override a non-virtual property. You can hide it by other property with the same name in the derived class, but this won't give you the desired effect if some other code accesses your object by the reference to the base class.
public class Student : BaseClass
{
private string _school
public string School
{
get { return _school; }
set
{
if(value == "Harvard")
value = "Harvard custom";
_school = value;
}
}
public String Country{ get; set; }
}
is that what you mean?
If the School property is in the BaseClass then you can either use the new keyword, or if you control the BaseClass, then you can add the virtual keyword to the School property there, and override it in the Student class.
This is just not doable by solely modifying BaseClass. Think about it this way: If it were possible to "annotate" automatic properties that easily, then we wouldn't need all those <rant>useless tons of</rant> manual property implementations for data model classes that implement INotifyPropertyChanged (same for DependencyProperties).
You need to provide hooks in your subclasses that your base class can use. Implementing PropertyChanged, which you already mentioned, is one possible solution, another one would be a simple method call:
public class Student : BaseClass
{
private string _school;
public String School
{
get { return _school; }
set {
_school = value;
DoMoreChanges(ref _school); // DoMoreChanges is defined in BaseClass
}
}
public String Country{ get; set; }
}
If you have lots of subclasses that need this, you can either use Visual Studio Code Snippets to create the code or T4 templates.
Since your base class does not have those properties you will not be able to modify them from within the base class using standard OOD patterns or principles.
Now if you move the properties to your base class either as normal properties or virtual properties you can modify what you do in the set block of the properties to do extra work.
However if you cannot move these to the base class, and you cannot modify the Student class, as you seem to imply in you question, then you could encapsulate the student class within a new class like StudentProxy or something and then have it expose similar properties that will then call into the real student class how you want.
For example:
public class StudentProxy
{
private Student _student;
public StudentProxy(Student student)
{
this._student = student;
}
public String School
{
get { return _student.School; }
set
{
_student.School = value + " my custom value";
}
}
public String Country
{
get { return _student.Country; }
set
{
_student.Country = value + " my custom value";
}
}
}