How do you define a getter and setter for complex data types such as a dictionary?
public Dictionary<string, string> Users
{
get
{
return m_Users;
}
set
{
m_Users = value;
}
}
This returns the entire dictionary? Can you write the setter to look and see if a specific key-value pair exists and then if it doesn't, add it. Else update the current key value pair? For the get, can you return a specific key-value pair instead of the whole dictionary?
Use an indexer property (MSDN):
public class YourClass
{
private readonly IDictionary<string, string> _yourDictionary = new Dictionary<string, string>();
public string this[string key]
{
// returns value if exists
get { return _yourDictionary[key]; }
// updates if exists, adds if doesn't exist
set { _yourDictionary[key] = value; }
}
}
Then use like:
var test = new YourClass();
test["Item1"] = "Value1";
It is not possible to do it in a way that would involve only properties. You theoretically could write a setter, but for a getter, you would need to specify a key that you want to retrieve. That is impossible since properties do not accept parameters. Natural way to accomplish what you want would be to use methods:
private Dictionary<string, string> users = new Dictionary<string, string>();
public void Set(string key, string value)
{
if (users.ContainsKey(key))
{
users[key] = value;
}
else
{
users.Add(key, value);
}
}
public string Get(string key)
{
string result = null;
if (users.ContainsKey(key))
{
result = users[key];
}
return result;
}
Alternatively, as others have already said, you could use indexers, but I've always found them a little cumbersome. But I guess it's just a matter of personal preference.
And just for the sake of completeness, this is how a setter could look like, although it's highly unusual and counter-intuitive to have such a property:
public KeyValuePair<string, string> Users
{
set
{
Set(value.Key, value.Value);
}
}
Internally, it uses the Set method from my previous snippet.
It looks like you want an "named indexer". Here's (my) one way to accomplish that using C#.
My approach exposes a property that returns an object (with a default indexer) which will perform the indexing into the appropriate field given the lambdas to do it.
There are reasons you may or not want to use this method, but I'll leave that to you. :)
You won't be able to do that with a property. You'll need to use methods for that, or add an indexer to your class. The get method can't accept a parameter (the key).
Another option, if you want someone to be able to easily add/remove keys to the dictionary but prevent them from setting an entirely new one would be to make the property a read-only property that returns a dictionary created in the constructor. It would be less flexible then adding get/set methods, but in common, simple cases it can do just fine.
It is possible to do so with the setter but highly unrecommended, and is completely impossible with the getter as it takes no parameter to determine what to get.
For the setter you would have to pass a Dictionary<string, string> with a single pair but it goes against what you would expect the getter/setter to usually do and completely stops you setting the entire Dictionary.
A much better way is to use a pair of methods which you can name Get and Set if you so desire.
Dictionary<string, string> param = new Dictionary<string, string>();
public void SetYourParameter(string parametrName, string paramValue)
{
param[parametrName] = paramValue;
}
public string GetYourParameter(string parametrName)
{
// ContainKey ---> It returns value if the key was found
if( param.ContainsKey(parametrName))
return param[parametrName];
else
return null;
}
Related
In C# I'm confused about properties. Specifically when you set get and set as just { get; set;}. What does it mean when you do that? For example I have class property:
public Dictionary<string, string> clientDict { get; set; }
I've seen properties where they actually define get and set. I also understand that removing set makes it read only. But what happens in this case? Does it just use the default accessors of a normal dict?
this is called "Automatic Properties". (when you just write the set and the get method without any code inside them)
the target of the Automatic Properties is the simplifying the coding process, and make it faster.
when you write a property like the previous one.
public Dictionary clientDict { get; set; }
the compiler translates it to the following
private Dictionary<string, string> _clientDic;
public Dictionary<string, string> clientDic
{
get { return _clientDic; }
set { _clientDic = value; }
}
and when you write a property like the following one
public int X {get;}
the compiler translates it to the following
private int _x;
public int X{
get { return _x; }
}
You may be confusing the accessors to a class member with the accessors to a collection. As a commenter noted, the MSDN page on Auto-Implemented Properties explains that this:
public Dictionary<string, string> clientDict { get; set; }
Is equivalent to this:
private Dictionary<string, string> _clientDict;
public Dictionary<string, string> clientDict
{
get { return _clientDict; }
set(Dictionary<string, string> value)
{
_clientDict = value;
}
}
Those accessors just return a reference to the collection. They aren't passing anything through (as implied by your question "Does it just use the default accessors of a normal dict?"), and they are unrelated to the Dictionary<T> class's [] and .Add() methods.
When you access the Dictionary through the property:
var foo = clientDict["SomeKey"];
That will first return the result of the clientDict* property's access, namely, a reference to _clientDict, and will then index into that dictionary, returning the resulting value (assuming the key exists) and assigning it to foo.
Please comment or edit the question if something further is confusing you about auto-properties.
* By the way, it's taking everything I've got not to write ClientDict as the name of the property, since the C# convention is to capitalize property names just like method names :)
public Dictionary<string, string> clientDict { get; set; }
Is equivalent to defining get and set manually. The compiler handles all of it for you behind the scenes. So your above would become:
private Dictionary<string, string> _clientDict;
public Dictionary<string, string> clientDict;
{
get { return _clientDict; }
set { _clientDict = value; }
}
Simplify it. What would it mean if it were int instead of Dictionary?
public int ClientInt {get;set;}
There are no accessors for int, so you wouldn't find yourself asking that question.
In C# I'm confused about properties. Specifically when you set get and
set as just { get; set;}. What does it mean when you do that?
It means you're defining an Automatic Property, and behind the scenes, the compilers creates a private, anonymous backing field that can only be accessed through the property's get and set accessors.
So, your next question might be...
What's a backing field?
A backing field is a field which a property references to store and retrieve values from.
You've seen what an automatic property looks like, here's what a property with a backing field looks like...
private string name; // Backing field
public string Name // Property
{
get { return this.name; }
set { this.name = value }
}
I've seen properties where they actually define get and set. I also
understand that removing set makes it read only.
Yes it makes the Property read-only, but be careful, as C# also contains a keyword called readonly which is set on Fields. E.g. private readonly string name;..
Just as properties can be read only, they can also be write only, if you remove the Get accessor, and only leave the Set accessor, clients will only be able to write to it.
But what happens in this case? Does it just use the default accessors of a normal dict?
The dictionary example you've given above will have a backing field... Clients will be able to write to it and read from it.
We could change things up if we wanted however...
Imagine that we're providing the scores of a live football game, giving write access to clients would be a nightmare... so we could restrict that and only expose a get accessor, whilst keeping the set accessor private.
public int TeamFoo { get; private set; }
public int TeamBar { get; private set; }
Or, we could do it with an explicitly defined backing field...
private int teamFoo;
public int TeamFoo { get { return teamFoo; } }
Honestly,I know I can miss the set clause to avoid user setting my property.
Like below code:
private Dictionary<int, string> _configurations = new Dictionary<TState, StateConfiguration>();
public Dictionary<int, string> Configurations { get { return _configurations; } }
So, we can't run below code at outer code:
XXX.Configurations = new Dictionary<int, string>();
It prompts :
Property or indexer 'XXX.Configurations' cannot be assigned to -- it is read only
But we can change the value like below code:
XXX.Configurations.Add(1,"1000")
Because dictionary is a reference type.
So how can I avoid this. Let user access the reference property with no changing.
You could try returning the dictionary as read-only:
public IDictionary<int, string> Configurations
{
get { return new ReadOnlyDictionary(_configurations); }
}
It sounds like you want to expose a ReadOnlyDictionary. You could create a new one each time the property is accessed, or you could have two fields - one for the writable dictionary (which you never expose) and one for the read-only wrapper.
I have a class definition in which a readonly member is defined.
private readonly Dictionary<string, string> map = new Dictionary<string, string>();
Now in order to test my design, I want to access this member outside its class definition. I was thinking of providing a get method but unable to write an error free syntax.
Is it possible to assign value to a member(using new) and still able to define its get method?
PS: I am new to C# language.
EDIT:
I have not written the code, its just a statement I have copied from an already written module. I have made some design changes in the module and want to test it with minimal changes possible in the code, so for that I was looking to get the readonly access of this member outside the class.
You can define a read-only property for permitting public access to your field:
private readonly Dictionary<string, string> map =
new Dictionary<string, string>();
public Dictionary<string, string> Map
{
get { return map; }
}
Note that this will only prevent external classes from changing the instance reference assigned to map, not from changing the content of the dictionary itself.
One could argue that you shouldn't write tests for private members of a class. Tests should only use the public interface and don't rely on the internals of the class, since you should be able to refactor internals of the class without breaking any tests.
If you add a public getter 'only for testing' there's no guarantee that someone will start using somewhere in the project.
If you really want to expose the dictionary and use .NET 4.5, use ReadOnlyDictionary class to make sure that the caller won't change anything.
public IDictionary<string, string> Map
{
get { return new ReadOnlyDictionary<string, string>(map); }
}
Just create a simple getter
public Dictionary<string, string> Mapping
{
get { return map; }
}
You mean something like this?
readonly Dictionary<string, string> _map = new Dictionary<string, string>();
public Dictionary<string, string> Map
{
get { return _map; }
}
Why are you making this variable readonly?
If you're trying to give access to the values contained in the dictionary you could create a method that exposes the dictionary without allowing it to be modified:
public string GetMapValue(string key)
{
return _map[key];
}
You could write a public property function to return the private readonly map, e.g:
public Dictionary<string, string> Map { { get { return map; } } }
However, the Dictionary is still mutable. If you want a read-only Dictionary, see this SO question.
Let's say I have the following class:
public class MyClass {
public string FirstAttribute {
get {
return Attributes["FirstAttribute"];
}
set {
Attributes["FirstAttribute"] = value;
}
}
public string SecondAttribute {
get {
return Attributes["SecondAttribute"];
}
set {
Attributes["SecondAttribute"] = value;
}
}
public Dictionary<string, string> Attributes;
public MyClass(Dictionary<string,string> att) {
Attributes = att;
}
}
and I wanted to be able to obtain a pointer to the value that is stored in the Dictionary, so that I can get and set the values (yes unsafe) directly without having to wait for the Dictionary to search for the element by key every time.
Is there a way to do that in c#?
No, I don't believe so. You could store a mutable wrapper in the dictionary though:
public class Wrapper<T>
{
public T Value { get; set; }
}
Then create the dictionary (which should be private, by the way - public fields are a really bad idea other than for constants) as a Dictionary<string, Wrapper<string>>. You can then keep fields for the Wrapper<string> objects associated with "FirstAttribute" and "SecondAttribute".
Frankly I don't think this would be a particularly good idea - I'd just stick with the dictionary lookup - but it's an option. (Assuming nothing's going to change which wrapper is associated with the keys.)
Another option is simply to use fields for the two specific attributes - when you're asked to set a new value, set it in the dictionary and set a field. When you're asked for the current value, just return the value from the field. Of course, that's assuming that you're in control of the dictionary (i.e. so it can't change outside your class).
No, no, no, no.
strings are immutable for a reason. A string might be shared between multiple references: modifying it directly with unsafe code might lead to weird behavior.
A way to go around this would be to write a wrapper class that olds your value, and keep a reference to an instance of that wrapper. The getters and setters would then modify the wrapper's property.
If the values in the dictionary were mutable, you could grab the reference to them and mutate them. But strings are immutable. So the dictionary has its own copy of a reference to your string, and you can't alter the string it is pointing to. If you try to replace it with another string, you need to update the dictionary with the new reference. So basically there's no way to improve on what you have in this situation.
Like we do Session.Add("LoginUserId", 123);
and then we can access Session["LoginUserId"], like an Array, how do we implement it?
You need an indexer:
public Thing this[string index]
{
get
{
// get the item for that index.
return YourGetItemMethod(index)
}
set
{
// set the item for this index. value will be of type Thing.
YourAddItemMethod(index, value)
}
}
This will let you use your class objects like an array:
MyClass cl = new MyClass();
cl["hello"] = anotherObject;
// etc.
There's also a tutorial available if you need more help.
Addendum:
You mention that you wanted this to be available on a static class. That get's a little more complicated, because you can't use a static indexer. If you want to use an indexer, you'd need to access it off of a static Field or some such sorcery as in this answer.
You should use indexers
See the link:
http://msdn.microsoft.com/en-us/library/2549tw02.aspx
Sounds like all you need is a generic dictionary.
var session = new Dictionary<string, object>();
//set value
session.Add("key", value);
//get value
var value = session["key"] as string;
If you want to make this static, just make it a static member in another class.
public static class SharedStorage
{
private static Dictionary<string, object> _data = new Dictionary<string,object>();
public static Dictionary<string, object> Data { get { return _data; } }
}
Then you can access it as such, without having to initialize it:
SharedStorage.Data.Add("someKey", "someValue");
string someValue = (string) SharedStorage.Data["someKey"];
If you want to be more adventurous and are using .NET 4 you can also use an Expando Object, like the ViewBag member available to controllers in ASP.NET MVC 3:
dynamic expando = new ExpandoObject();
expando.UserId = 5;
var userId = (int) expando.UserId;
With the way you usually use the Session variable all you really need is a generic Dictionary collection like this one. You don't really need to write a class. But if you need to add extra functionality and/or semantics you could certainly wrap the collection with a class and just include and indexer.
For other collections check out the Collections.Generic namespace.