I have code as below
public class LocalDB
{
public static int e_SessionID;
public static string e_EventName;
public static string e_TimeCreate;
}
in other class:
public static LocalDB newEvent ;
public void something()
{
newEvent.e_SessionID=123;
}
but it is can not pass value.
Problem : You are trying to access the static feilds using instance reference variable newEvent as below:
newEvent.e_SessionID=123;
//^^^Here
Solution : You need to use classname to access the static fields
newEvent.e_SessionID=123;
//^^^Replace with classname LocalDB here
Replace this:
newEvent.e_SessionID = 123;
With this:
LocalDB.e_SessionID = 123;
Why don't you set them up as properties? Have a read of this SO post why prefer properties to public variables
"Fields are considered implementation details of classes and exposing them publicly breaks encapsulation."
public class LocalDB
{
public int SessionID { get; set; }
}
Static methods and variables can only invoke using the class name
and you are trying to call using the class object.
if you want to set the value of e_SessionID set the value using class name as follows
LocalDB.e_SessionID=123;
try to use property instead:
public class LocalDB
{
public int e_SessionID { get; set; }
public string e_EventName { get; set; }
public string e_TimeCreate { get; set; }
}
Prefer instance data to static data.
Static data is effectively global state. Do you have only one event in the lifetime of your program? What if you need to support multithreading? This is object-oriented programming; use objects.
Encapsulate data.
Avoid making fields public. Prefer properties, as others have stated. Note that this allows assigning the creation time at construction (and only then).
Use appropriate types.
If you are storing a date/time value, normally you would use the DateTime class.
Favor immutability.
If you know the values of properties at construction time, set them then and don't allow them to be changed.
Think about names.
Descriptive names matter, especially when you're doing maintenance after six months. I didn't change the name of LocalDB in my example, as I don't know your domain or use case. However, this class looks more like an event than a database to me. Would Event be a better name?
The following example uses C# 6 syntax; earlier versions would need to add private setters and move the initialization to the constructor.
public class LocalDB
{
public LocalDB(int sessionID, string eventName)
{
SessionID = sessionID;
EventName = eventName;
}
public int SessionID { get; }
public string EventName { get; }
public DateTime TimeCreate { get; } = DateTime.UtcNow;
}
public class Other
{
public void DoSomething()
{
NewEvent = new LocalDB(1, "Other Event");
}
public LocalDB NewEvent { get; private set; }
}
A flaw in this example is that the NewEvent property of an Other instance will be null on creation. Avoid nulls where possible. Perhaps this should be a collection of events; not knowing your use case I can't say.
Related
Every time I talk to experienced programmers, they talk about having global variables being a bad practice because of debugging or security exploits. I have a simple List of strings I want to load from a a textfile and access across different methods in my form. Before, I would simply initialize said variable at the top, inside of my form class and use it across methods. I always try to reduce that practice when I can and only initialize those variables when I really need them. Is it a bad practice to do this or do more experienced programmers do this too? Is there a standard design pattern method of doing this so you don't have to use "global variables" at the top of your form?
As you're talking about C# and it's a fully-object-oriented programming language, there's no way to declare global variables.
In an OOP language like C#, a bad practice can be simulating global variables using static classes:
public static class Global
{
public static string Value1 { get; set; }
public static int Value2 { get; set; }
}
...to later get or set these values from other classes. Definitely, this a bad practice because state should be held by specific and meaningful objects.
Usually, in a perfect/ideal OOP solution, you should pass such values from class to class using constructors:
public class X
{
public int Value1 { get; set; }
public void DoStuff()
{
Y y = new Y(this);
y.DoChildStuff();
}
}
public class Y
{
public class Y(X parent)
{
Parent = parent;
}
public X Parent { get; }
public void DoChildStuff()
{
// Do some stuff with Parent
}
}
Or also, you might pass states providing arguments to some method:
public class Y
{
public void DoChildStuff(X parent)
{
// Do some stuff with "parent"
}
}
Since you're passing states with reference types, if any of the methods in the chain decide to change Parent.Value1 with another value, all objects holding a reference to the same X object will get the new X.Value1.
Some fellows might argue that we usually build configuration objects which own a lot of properties accessed by other arbitrary objects, right? BTW, configuration is a concept per se, isn't it? And we usually categorize configuration values using composition:
public class ApplicationConfiguration
{
public DatabaseConfiguration Database { get; } = new DatabaseConfiguration();
public StorageConfiguration Storage { get; } = new StorageConfiguration();
}
public class DatabaseConfiguration
{
public string ConnectionString { get; set; }
}
public class StorageConfiguration
{
public string TemporalFileDirectoryPath { get; set; }
public string BinaryDirectoryPath { get; set; }
}
So later we inject the application configuration wherever we need it:
// Please note that it's a VERY hypothetical example, don't take
// it as an actual advise on how to implement a data mapper!!
public class DataMapper
{
public DataMapper(ApplicationConfiguration appConfig)
{
AppConfig = appConfig;
}
public ApplicationConfiguration AppConfig { get; }
private IDbConnection Connection { get; }
public void Connect()
{
// We access the configured connection string
// from the application configuration object
Connection = new SqlConnection(AppConfig.Database.ConnectionString);
Connection.Open();
}
}
In summary, and since I love comparing real-world and programming use cases, imagine that you never clean your room and you would use a single box to store every tool you might need some day. One day you need a screwdriver from the whole box, and you know that's inside it... But you need to throw everything in the box to the ground and work out the mess prior to find the priceless screwdriver to complete some home task.
Or imagine that you've bought a toolbox to store your tools in order, and once you need a screwdriver, you know that's in the toolbox and in the section where you store your screwdrivers.
You know that the second approach is the most mind-friendly. That is, when you develop software, you need to design mind-friendly architectures rather than a big mess of unrelated data and behaviors working together.
I want to call non-static variable from another class. If I make it static, it affects my other code.
I've two classes Harvest_Client and Harvest_Project.
In my Harvest_Project class I've
public int _client_id{ get; set;}
I'just want to do in Harvest_Client class is,
public int _id = Harvest_Project._client_id;
How should I do this?
Firstly, you should rename the property (it's not a variable) to conform with .NET naming conventions, e.g.
public int ClientId { get; set; }
Next, you need an instance of HarvestProject (post-renaming). Don't just create a new one - you need the right instance, the one whose client ID you're interested in. We can't tell you which one it is - but if you don't already have an instance of HarvestProject to hand, you should work out how you're expecting to specify which client ID you want.
Think of it this way: if I were to ask you "How old is a person?" you'd naturally want to know which person I was talking about. It's exactly the same here.
maybe passing the reference to the class will do what you need:
public class Harvest_Project
{
public string Name { get; set; }
public int clientId { get; set; }
}
public class Harvest_Client
{
private Harvest_Project MyInstance;
private int myid;
public int MyId
{
get
{
return myid;
}
private set
{
MyId = value;
}
}
public Harvest_Client(Harvest_Project cls)
{
MyInstance = cls;
MyId = cls.clientId;//since class reference present no
//need for the property.
//its just here to show if in your
//project you really just need ID
//in this example its redundant
}
}
Depending on what you are trying to do you can make a List of Harvest_Project objects or better a dictionary,if there are various types of "projects" they could be all placed in a dictionary cataloged accordingly to the specified key.
One moment I would like to recommend to take into consideration the following declaration of property public int ClientId {get; private set;"} It allow you to prohibited setting this variable from third party code, if you actually need this. Otherwise you will have ordinary global variable
If I want to set my property in a Class private, so it should be only possible to use and set this property in this class, what is the better way? This
public string Name { private get; private set }
or
private string Name { get; set }
hmmm and there is also
private string Name { private get; private set }
It's worth noting that originally, C# wouldn't let you set different accesses on a getter or setter, so the only possible choices were:
public string Name { get; set; }
protected internal string Name { get; set; }
internal string Name { get; set; }
protected string Name { get; set; }
private string Name { get; set; }
(For that matter, you couldn't have automatic properties and always had to do the writing to and from a backing field yourself, but we'll ignore that just because we'll have shorter examples that way).
It is often useful to have different accesses for the two, most often a more restrictive setter than getter, and so the likes of
public string Name { get; private set; }
was introduced.
Now, by extension of that, it would seem logical enough to allow:
public string Name { private get; private set; }
private string Name { private get; private set; }
However, what are these two expressing?
The second isn't too bad, it's just needlessly repetitious. Still though, it's quite likely that some confused thinking got us there (most likely an incomplete refactoring). Good code is as much about expressing what you are doing as making a computer do something (if anything, more so), better to have it express clearly.
Hence if you end up with the likes of { private get; private set; } then it'd likely be worth looking at again and thinking about what you really want to say here. Hurrah for it being a compiler error.
The first case is even worse. It says "this property is public, except for the setter that is private, and the getter that is private". That's not an exception, "it's this thing, except for all the time" is no real expression of anything. Doubly hurrah the compiler for not letting us do it.
Have you tried compiling your examples? Only the middle one will translate.
If you want to specify extra accessibility level keyword, you can only do it on one of the accessors (getter/setter), and that level of the accessor must be more restrictive than the accessibility of the entire property.
Here you see the rules: Restricting Accessor Accessibility
public string Name { get; private set; }
This is what I think you are wanting to do.
There is no point trying to make the get private when the property is public unless you only want your class to see it. In that situation you should use:
private string Name { get; set; }
Update: On second read, you definitely want the second example.
The better way depends on what you want:
public string Name { private get; private set } The property is public but noone can read or wrote to it, except class itself. That is completely useless, so use just
private string Name { get; set }.
In general if you view the property like a couple of methods (which actually is)
private string get_Name() { }
private string set_Name(value) { }
The reason of having a possibility to apply that identifiers to a property get/set becomes evident, I hope.
For a private member, you don't need to define accessors.
Just do this:
private string _name;
It seems like you want
private string name;
public string Name
{
get { return name; }
set { name = value; }
}
this would allow you to access private string Name.
surely enough,
That is the definition of private classifier in OOP: just allow access within methods and scope of the class that owns such private member. Thus if your aim is to disable anyother means of access to a particular member of a class, claiming it as:
private <Type_name> <member_identifier> ;
is the simplest and enough to make it such.
Say I have this simple class
public class MyEntity
{
public DateTime DateUpdated { get; private set; }
public string Author { get; private set; }
public string Comment { get; private set; }
public void AddComment(string comment, string author)
{
Author = author;
Comment = comment;
DateUpdated = DateTime.Now;
}
}
I have made the setters private to encapsulate the class and added the AddComment method to add some behaviour to my class. This works perfectly fine when creating a new object but when I want to load the Entity from the db the DateUpdated is of course set to the current date which I would like to avoid.
Is there any patterns I could use to avoid making the DateUpdated setter public as that does seem to break my nice encapsulation and messing up the clean interface of the class? The class is of course just an example of a more generic problem.
The closest I have got to now without making more public constructors is creating a private constructor which I access through a public static method.
Use a constructor that takes parameters matching the fields of the object.
This will allow you to populate the objects on startup and keep them immutable.
public MyEntity(DateTime dateUpdated, string author, string comment)
{
DateUpdated = dateUpdated;
Author = author;
Comment = comment;
}
Look into the Memento pattern for re-hydrating your object. Use the constructor only for creating a new instance.
You can overload the AddComment method like so:
public class MyEntity
{
public DateTime DateUpdated { get; private set; }
public string Author { get; private set; }
public string Comment { get; private set; }
public void AddComment(string comment, string author)
{
Author = author;
Comment = comment;
DateUpdated = DateTime.Now;
}
public void AddComment(string comment, string author, DateTime dateUpdated)
{
Author = author;
Comment = comment;
DateUpdated = dateUpdated;
}
}
If you are using an ORM such as NHibernate to implement the repository, then it will assign values to properties based on data from the database even if the properties are private set. In other words, it bypasses the AddComment method and injects data directly. This makes sense because when reconstituting an entity, behavior doesn't repeat, only the data needs to be copied. NHibernate does require the entity to contain a protected parameter-less constructor. If using your own ORM implementation, then you can employ the constructor pattern as suggested by Oded because in that case your entity can truly remain persistence ignorant.
If the repository responsible for creating these objects lives in the same assembly, you should check out the internal access modifier. If this fits your project's needs, you can implement it in one of two ways...
Change your setters from private to internal. The creator would then just set the values of the properties after instantiation.
Add an internal constructor that accepts a value for all the properties and sets them up.
Either way, you can still allow change through public methods as you demonstrated in your example.
I have a couple of ideas but I wanted to see what the SO community would suggest.
I have an abstract class with an abstract Calculate method on it. I have 2 implementations of it that calculate differently. This screams Strategy pattern to me however one of the implementations requires that a selected_type variable be set because it is used inside the Calculate method. I want to follow the OCP so my Calculate method shouldn't take in the dependencies.
This class is retrieved from the DB via NHibernate and the selected_type variable won't be set until after the object has been created. I'm trying to avoid an if statement to set the selected_type only if it is of a specific implementation. What would be the best way?
Here is a code example:
public abstract class TagType
{
public virtual long id { get; protected set; }
public virtual string description { get; protected set; }
protected TagType(){}
protected TagType(string description)
{
this.description = description;
}
public abstract decimal Calculate();
}
public class TagTypeImpl1
{
public virtual int tag_months { get; protected set; }
protected TagType() { }
protected TagType(string description, int tag_months): base(description)
{
this.tag_months = tag_months;
}
public override decimal Calculate()
{
return (12*tag_months);
}
}
public class TagTypeImpl2
{
public virtual int tag_months { get; protected set; }
public virtual TagType selected_tag_type { get; protected set; }
protected TagType() { }
protected TagType(string description, int tag_months, TagType selected_tag_type): base(description)
{
this.tag_months = tag_months;
this.selected_tag_type = selected_tag_type;
}
public override decimal Calculate()
{
return selected_tag_type.Calculate() + (12*tag_months);
}
}
public class ConsumerController
{
private readonly IRepository<TagType> repository;
public ConsumerController(IRepository<TagType> repository)
{
this.repository = repository;
}
public ActionResult Index(long id)
{
var tag_type = repository.get(id);
//If using TagTypeImpl2 then the selected_tag_type variable needs to be set
//I want to avoid if(tag_type.GetType() == typeof(TagTypeImpl2)) set selected_tag_type
var result = tag_type.Calculate();
return Json(new {result});
}
}
I might be trying to do too much with this class adn maybe the persisted entity class is the wrong place to have the Calculate method but it seemed the best place since it knows the most about how to do the calculation.
Would it make sense to create a virtual (overridable) function "Initialize" that one should call on all tag_type objects loaded from repository so they can do what was skipped by the default constructor that the parameterized constructor would have done?
Or can you change the default constructor to initialize selected_type to either the correct value or some value that will instruct the calculate method to correct it before using it?
It's not the responsibility of your class to decide what the strategies need, it's the responsibility of the strategy. The whole idea is that you can call whatever strategy you're using the same way, all the time.
Simply make all strategies implement the same interface -including selected_type-, but one of them ignores the selected_type, the other uses it. It's up to the strategy itself to decide this.
Alternatively, your implementations of strategy can have more properties than are defined in the interface. If you can initialize the strategies from outside of your class, and it's not a problem for the initializing class to know more about the specific implementation you might be able to set the properties for only the specific strategy that needs it. The former solution is cleaner though (always using the same interface).