Suppose I have:
public class Bob
{
public int Value { get; set; }
}
I want to pass the Value member as an out parameter like
Int32.TryParse("123", out bob.Value);
but I get a compilation error, "'out' argument is not classified as a variable." Is there any way to achieve this, or am I going to have to extract a variable, à la:
int value;
Int32.TryParse("123", out value);
bob.Value = value;
You'd have to explicitly use a field and "normal" property instead of an auto-implemented property:
public class Bob
{
private int value;
public int Value
{
get { return value; }
set { this.value = value; }
}
}
Then you can pass the field as an out parameter:
Int32.TryParse("123", out bob.value);
But of course, that will only work within the same class, as the field is private (and should be!).
Properties just don't let you do this. Even in VB where you can pass a property by reference or use it as an out parameter, there's basically an extra temporary variable.
If you didn't care about the return value of TryParse, you could always write your own helper method:
static int ParseOrDefault(string text)
{
int tmp;
int.TryParse(text, out tmp);
return tmp;
}
Then use:
bob.Value = Int32Helper.ParseOrDefault("123");
That way you can use a single temporary variable even if you need to do this in multiple places.
You can achieve that, but not with a property.
public class Bob {
public int Value { get; set; } // This is a property
public int AnotherValue; // This is a field
}
You cannot use out on Value, but you can on AnotherValue.
This will work
Int32.TryParse("123", out bob.AnotherValue);
But, common guidelines tells you not to make a class field public. So you should use the temporary variable approach.
Related
Assume I had this property:
public int Money
{
get;
set{
Money = value;
}
}
This won't compile, saying that the get accessor must have a body because it is not marked abstract, extern or partial. If I add a body to it and return the Money property like so:
public int Money
{
get{
return Money;
}
set{
Money = value;
}
}
.. I'll have an infinite loop on my hands and my program will throw a stack overflow exception.
So my question ultimately boils down to: Is there a way I can keep the get/set accessors, return the current value in get without creating an infinite loop, and still have a body for the set accessor?
Either use:
public int Money { get; set; }
or if you really need to have a body for accessors, you need to use a backing field:
private int _money;
public int Money
{
get { return _money; }
set { _money = value; }
}
However, the latter is only used if you need to perform some additional logic (e.g. raise an event) when getter or setter is used.
Also, the latter is more or less what the compiler generates for you automatically and it can ensure that the backing field is used consistently.
If you provide only one body, it becomes hard to define how should it behave: after all you don't have access to the generated backing field in your code so the whole idea doesn't make sense.
If you declare a body for one of the accessors, you're not dealing with an auto-implemented property anymore and need to implement the other accessor as well.
This is an auto-implemented property:
public int Foo { get; set; }
Which will generate a backing field when compiled. It will represent something like this in IL:
private int BackingField_Foo;
public int Foo
{
get { return BackingField_Foo; }
set { BackingField_Foo = value; }
}
In the case of an auto-implemented property, the compiler generates the field, so it knows where to read and write the value - but you can't access it.
Now if you implement one of the accessors yourself, to write to a self-defined field yourself, it's not an auto-implemented property anymore, so you'll have to implement the other accessor as well (as long as you declare it; you can create a regular read- or write-only property just fine).
Your current code throws StackOverflowExceptions because the accessors access themselves, ad infinitum.
See also Correct use of C# properties.
You can do the one of the following:
// 1. a public variable
public int Money;
// 2. an auto-implemented, or implicit property (very much like the above)
public int Money { get; set; }
// 3. An explicit property declaration with a private variable
private int _money;
public int Money {
get {return _money;}
set {
_money = value;
// possibly do something else here
}
}
Lets have a simple class with an int property
public class SimpleClass {
public int myInt { get; set; }// for having a property and not "public int myInt;", see Jon Skeet remark
}
I instanciate it twice assigning myInt or not
assignedObject = new SimpleClass() { myInt=0};
notAssignedObject = new SimpleClass();
Now by reflection, I query the value of myInt in each case using
Object value;
value=assignedObject.GetType().GetProperties().Where(o=>o.Name.Equals("myInt")).First().GetValue(assignedObject,null)
value=notAssignedObject.GetType().GetProperties().Where(o=>o.Name.Equals("myInt")).First().GetValue(notAssignedObject,null)
I am getting twice 0 for myInt, but I need to be able to differenciate them. How?
Unless you have code to specifically remember the difference between a property which has been initialized with its default value, and one which hasn't been set at all, you can't tell the difference. I have two suggestions, however:
You could make it an int? property, and then check whether the value is null. Of course, it's possible for it to be set to null explicitly, unless you prohibit that, but it may be good enough.
You could keep a separate bool value to know whether or not it's been set explicitly, and just set it in your property setter, along with the value.
Sample code for the second option:
public class SimpleClass
{
private int value;
private bool valueSet;
public int Value
{
get { return value; }
set
{
this.value = value;
this.valueSet = true;
}
}
public bool ValueSet { get { return valueSet; } }
}
If I want a read-only property, I write it like:
public int MyProperty { get { //Code goes here } }
However, the Microsoft example (and a few other examples I've seen) are written like:
public int MyProperty { get; private set; }
Is there any difference between these two, and should I start writing properties like this?
As you can see in your second sample, you can leave out the implementation for a property. .NET will then automatically create a local variable for the property and implement simple getting and setting.
public int MyProperty { get; private set; }
is actually equivalent to
private int _myProperty;
public int MyProperty {
get { return _myProperty; }
private set { _myProperty = value; }
}
Writing
public int MyProperty { get; }
does not work at all, as automatic properties need to implement a getter and a setter, while
public int MyProperty { get; private set; }
leaves you with a property that may return any int, but can only be changed within the current class.
public int MyProperty { get { ... } }
creates a read-only property.
Question is: what do you need? If you already have a member variable that's used within your class and you only want to return the current value using a property, you're perfectly fine with
public int MyProperty { get { return ...; }}
However, if you want a read-only property, which you need to set within your code (but not from other classes) without explicitly declaring a member variable, you have to go with the private set approach.
With private setter you can only assign property value inside of instance when property is without setter you can't set its value anywhere.
If you don't use an explicit member assignment in the property, you'll have to declare a private set at least in order to be able to set a value to this property. Otherwise, you'll get a warning at compile-time saying that your property cannot be assigned.
If you use an explicit member, you'll be able to assign a value to this member directly, without needing to add a private set:
private int member ;
public int MyProperty {
get { return member; }
}
// ...
member = 2;
int anotherVariable = MyProperty; // anotherVariable == 2
public int MyProperty
{
get
{
// Your own logic, like lazy loading
return _myProperty ?? (_myProperty = GetMyProperty());
}
}
A property with only a getter is very useful if you need your own logic behind the access of that property, in particular when you need the property to be lazy loaded.
public int MyProperty { get; private set; }
A property with a private setter is useful if you need the property not te be changed from the outside, but still maintained from within the class.
In both cases, you can have a backing data field for the actual value, but in the former, you'll have to maintain that yourself, and in the latter, it is maintained for you by the generated code.
There is a difference when you access the object with reflection.
public class Foo
{
public string Bar { get; private set; }
}
// .....
internal static void Main()
{
Foo foo = new Foo();
foo.GetType().GetProperty("Bar").SetValue(foo, "private?", null);
Console.WriteLine(foo.Bar);
}
I am learning C#, and am learning about making fields private to the class, and using Getters and Setters to expose Methods instead of field values.
Are the get; set; in Method 1 and Method 2 equivalent? e.g. is one a shorthand of the other?
class Student
{
// Instance fields
private string name;
private int mark;
// Method 1
public string Name { get; set; }
// Method 2
public int Mark
{
get { return mark; }
set { mark = value; }
}
}
Finally, would Method 2 be used when you want to for example perform a calculation before getting or setting a value? e.g. converting value to a percentage or perform validation? e.g.
class Student
{
// Instance fields
private string name;
private double mark;
private int maxMark = 50;
// Method 1
public string Name { get; set; }
// Method 2
public double Mark
{
get { return mark; }
set { if ( mark <= maxMark ) mark = value / maxMark * 100; }
}
}
Yes, the Method2 is the way to go when you have a custom getter and setter function. By default when you use Method1, there will be a default private property handled internally. Please refer this URL for more details.
Sample:
string _name;
public string Name
{
get => _name;
set => _name = value;
}
Yes, Method 1 is a shortcut to Method 2. I suggest using Method 1 by default. When you need more functionality, use Method 2. You can also specify different access modifiers for get and set.
Can somebody help me understand the get & set?
Why are they needed? I can just make a public variable.
Warning: I am assuming you already know about object-oriented programming.
What are properties?
Properties are language elements that allow you to avoid the repetitive getXYZ() accessors and setXYZ() mutators techniques found in other languages, like Java.
Why do they exist?
They aim to solve the following problems:
Saying get and set in the beginning of every access or mutation of a value is annoying and distracting.
In Java, you often say:
class person
{
private int _age;
public void setAge(int value) { /*check value first, then set _age*/ }
public int getAge() { return this._age; }
}
and then consistently say:
if (person.getAge() > blah || person.getAge() < 10)
{
person.setAge(5);
}
After a while, the get and set become rather annoying.
Providing direct access to the actual variable breaks encapsulation, so that's not an option.
How are they used?
They are used just like variables. You read/write to them just like variables.
How are they created?
They are created as methods. You define a pair of methods that:
Return the current value of the property. Oftentimes, this is nothing more than something like the following:
class Person
{
private int _age; //Declare the backing field
public int Age
{
get { return this._age; }
set { ... }
}
}
Set the value of the property:
class Person
{
public int Age
{
get { ... }
set
{
if (value < 0) //'value' is what the user provided
{ throw new ArgumentOutOfRangeException(); } //Check validity
this._age = value;
}
}
}
Other notes:
Auto-implemented Properties
C# 3.0 introduced auto-implemented properties:
public int Age { get; set; }
This is equivalent to:
private int _age; //The name is auto-generated
public int Age { get { return this._age; } set { this._age = value; } }
Why does it exist?
It helps you avoiding breaking changes in client executables.
Let's say you're lazy and don't want to type the whole thing, and decide to expose a variable publicly. You then create an executable that reads from or writes to that field. Then you change your mind and decide that you in fact needed a property, so you change it to one.
What happens?
The depending executable breaks, because the code is no longer valid.
Auto-implemented properties help you avoid that, without extra redundancy in your initial code.
Indexers
Indexers extend the property syntax to let you index objects (surprise!), just like arrays.
For C++ users: This is similar to overloading operator [].
Example:
private int[] _elements;
public int this[int index] //Indexed property
{
get { return this._elements[index]; }
set
{
//Do any checks on the index and value
this._elements[index] = value;
}
}
You then use them like obj[5] = 10;, which is equivalent to calling the set method of obj's indexer.
In fact, System.Collections.Generic.List<T> is indexed:
var list = new List<int>();
list.Add(10);
list[0] = 5; //You're indexing list, as though it were an array!
Isn't that neat? :)
Anything else?
There are many more features to properties, not all of which are available in C#:
Parametrized properties, of which indexers are a special kind
Getter/setter access modifiers (in C#)
Multiple getters or setters (not in C#)
Et cetera
They are called Accessors
The accessor of a property contains the executable statements associated with getting (reading or computing) or setting (writing) the property. The accessor declarations can contain a get accessor, a set accessor, or both.
The body of the get accessor resembles that of a method. It must return a value of the property type.
http://msdn.microsoft.com/en-us/library/w86s7x04.aspx
private string m_Name; // the name field
public string Name // the Name property
{
get
{
return m_Name;
}
}
The set accessor resembles a method whose return type is void. It uses an implicit parameter called value, whose type is the type of the property.
private m_Name;
public string Name {
get {
return m_Name;
}
set {
m_Name = value;
}
}
Then in the incarnation of C# 3, you can do this much easier through auto-properties
public string Name {get; set; } // read and write
public string Name {get; } // read only
public string Name { get; private set; } //read and parent write
http://msdn.microsoft.com/en-us/library/bb384054.aspx
Properties act as accessors to the internal state of an object, hiding the implementation of that state.
So, for example, you may have a first name property in a class
public class Example
{
private string firstName;
public string FirstName
{
get {return this.firstName;}
}
}
So anyone using the class doesn't need to know how first name is stored, they just know they can get a string representation of it. By adding a set you also add a mutator, something which changes an objects internal state
public class Example
{
private string firstName;
public string FirstName
{
get {return this.firstName;}
set {set this.firstName = value;}
}
}
Again you're still isolating how the first name is stored internally (encapsulation), but users can change it by passing in a string.
Simply put, get and set accessors are the functions called on a Property; that is, when you retrieve the value or when you set it. It forces a type of behavior on the way values are retrieved or set.
For example, you may want to have a mechanism to get/set passwords. Generally speaking, you'll only want to compare the hash of a password instead of storing things plaintext, so you'd have the getter variable retrieve the stored hash, and the setter would take the provided input and hash it for storage.
Here's what I mean:
public class User {
//Usery properties here, and...
private string _password;
public string Password {
get {
return _password;
}
set {
_password = SomeHashingFunction(value);
}
}
}
value is the variable provided to the setter from what has been given in the variable assignment. e.g.: someuser.Password = "blah";
Get and set are used in properties. They can each be public, protected, or private. Similar to accessor and mutator methods, they allow some computation when code tries to access/mutate the property. Of course, as long as you define one of get/set, the other is optional.
Example without properties:
private int test;
public int getTest() {
// some computation on test here, maybe?
return test;
}
private void setTest(int test) {
// some error/range checking, maybe?
this.test = test;
}
With properties:
private int test;
public int Test {
get {
// some computation on test here, maybe?
return test;
}
private set {
// some error/range checking, maybe?
test = value; // value is a keyword here
}
}
get{} and set{} are accessors that offer up the ability to easily read and write to private fields. Working with a simple example:
public class Foo()
{
//Field
private int _bar;
//Property
public int Bar
{
get { return _bar; }
set { _bar = value; }
//value is an implicit parameter to the set acccessor.
//When you perform an assignment to the property, the value you
//assign is the value in "value"
}
}
In this case, Bar is a public property that has a getter and a setter that allows access to the private field _bar that would otherwise be inaccessible beyond class Foo.
Now in a class that has an instace of Foo, you can do this:
public class IHasAFoo()
{
private Foo _myFoo = new Foo();
public void SomeMethod()
{
_myFoo.Bar = 42;
}
}
So the public accessor allows you to set the value of the private field back in Foo.
Hope that helps!