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Can i expect a property to retain its assigned class?

Maybe a bit of a philosophical, but should I generally, in C#, expect a property with an interface type to retain its assigned class?
For example a class property like this:
public IBehavior Behavior { get; set; }
that gets assigned a implmented class
instance.Behavior = new ImplementedBehavior();
should i generally expect this cast to succeed?
Assert.IsNotNull(instance.Behavior as ImplementedBehavior);
Or is it lost, and i should have kept a reference to the instantiated ImplementedBehavior instead?
var ib = new ImplementedBehavior();
instance.Behavior = ib;
Assert.IsNotNull(ib as ImplementedBehavior);
Clarification:
Looking to use an external class, that when assigned a class similar to what is done above, changes the content of instance.Behavior internally after i set it, to return an instance of SomeOtherBehavior
Can I assume this is bad behavior and not in the spirit of C#?
GitHub issue related to the question:
https://github.com/aws/aws-cdk/issues/19013

Can I expect a property to retain its assigned class?
No.
It may not even be accepted in the first place, e.g.
public IBehavior Behavior {
get {...}
set { if (!value.HasCapabilityX) return; ... };
}
It may simply be changed by some other code that has access to the same object (instance):
Thread A:
instance.Behavior = new ImplementationA();
Thread B:
instance.Behavior = new ImplementationB();
There are object oriented design patterns that promote the idea of changing the object, e.g. the decorator pattern and proxy pattern, e.g.
public IBehavior Behavior {
get {...}
set { _behavior = new PermissionsDecorator(new LoggingDecorator(value)); };
}
Should I generally expect this cast to succeed?
No.
Or is it lost [...] ?
Maybe.
and I should have kept a reference to the instantiated ImplementedBehavior instead?
If you need something that ImplementedBehavior can do which IBehavior can't do, then yes, keep a reference.
Can I assume this is bad behavior and not in the spirit of C#?
This is not bad behavior. It is good practice to only expect the interface to fulfill the contract that was specified by the interface.

The property can be assigned any value that implements IBehavior:
public class BehaviorA : IBehavior { }
public class BehaviorB : IBehavior { }
...
instance.Behavior = new BehaviorA();
instance.Behavior = new BehaviorB();
So in general, you CANNOT expect a property's value to be of a particular type that derives from the property's declared type.

this
Assert.IsNotNull(instance.Behavior as ImplementedBehavior);
will work so long as you dont change instance.Behavior, it wont happen spontaneously. But if you also have
class Imp2 : IBehavior{
}
and then do
instance.Behavior = new Imp2();
then that assert will fail.

Let me give you a simple example using
public interface IType
{
string Text { get; set; }
int Number { get; set; }
}
public class MyClass
{
public IType MyProperty { get; set; }
}
public class MyFirstType : IType
{
public string Text { get; set; }
public int Number { get; set; }
public char Letter { get; set; }
}
public class MySecondType : IType
{
public string Text { get; set; }
public int Number { get; set; }
public decimal Price { get; set; }
}
And here is the usage with comments that explain what's going on:
var instance = new MyClass(); // creates a new object of type "MyClass" in memory
var myFirstType = new MyFirstType(); // creates a new object of type "MyFirstType" in memory
var mySecondType = new MySecondType(); // creates a new object of type "MySecondType" in memory
// setting property
instance.MyProperty = myFirstType; // MyProperty is now simply referencing "myFirstType" using the address to its position in memory
instance.MyProperty = mySecondType; // The memory address to "myFirstType" is now replaced by the memory address to "mySecondType"
// printing values of MyProperty
Console.WriteLine(instance.MyProperty.Text); // will work.
Console.WriteLine(instance.MyProperty.Number); // will work.
Console.WriteLine(instance.MyProperty.Letter); // compiler error. IType does not specify this "Letter" property.
Console.WriteLine(instance.MyProperty.Price); // compiler error. IType does not specify this "Price" property. However, it is in memory on the object, but the compiler will not infer the type of the property unless the property is explicitly casted.
// casting
var convertedPropertyA = (MyFirstType)instance.MyProperty; // throws an exception since the property is currently set to an object of type "MySecondType".
var convertedPropertyB = (MySecondType)instance.MyProperty; // works because the property is set to an object of type "MySecondType".
// modifying the values of the property
instance.MyProperty.Text = "new text"; // this works
instance.MyProperty.Number = 123; // this works
((MySecondType)instance.MyProperty).Price = 14.99; // this works
Console.WriteLine(instance.MyProperty.Text); // prints "new text"
Console.WriteLine(instance.MyProperty.Number); // prints "123"
Console.WriteLine(((MySecondType)instance.MyProperty).Price); // prints "14.99"

Instance { get; } = new <class>() - Is this a C#7/8/9 feature? [duplicate]

How do you give a C# auto-property an initial value?
I either use the constructor, or revert to the old syntax.
Using the Constructor:
class Person
{
public Person()
{
Name = "Initial Name";
}
public string Name { get; set; }
}
Using normal property syntax (with an initial value)
private string name = "Initial Name";
public string Name
{
get
{
return name;
}
set
{
name = value;
}
}
Is there a better way?

In C# 5 and earlier, to give auto implemented properties an initial value, you have to do it in a constructor.
Since C# 6.0, you can specify initial value in-line. The syntax is:
public int X { get; set; } = x; // C# 6 or higher
DefaultValueAttribute is intended to be used by the VS designer (or any other consumer) to specify a default value, not an initial value. (Even if in designed object, initial value is the default value).
At compile time DefaultValueAttribute will not impact the generated IL and it will not be read to initialize the property to that value (see DefaultValue attribute is not working with my Auto Property).
Example of attributes that impact the IL are ThreadStaticAttribute, CallerMemberNameAttribute, ...

Edited on 1/2/15
C# 6 :
With C# 6 you can initialize auto-properties directly (finally!), there are now other answers that describe that.
C# 5 and below:
Though the intended use of the attribute is not to actually set the values of the properties, you can use reflection to always set them anyway...
public class DefaultValuesTest
{
public DefaultValuesTest()
{
foreach (PropertyDescriptor property in TypeDescriptor.GetProperties(this))
{
DefaultValueAttribute myAttribute = (DefaultValueAttribute)property.Attributes[typeof(DefaultValueAttribute)];
if (myAttribute != null)
{
property.SetValue(this, myAttribute.Value);
}
}
}
public void DoTest()
{
var db = DefaultValueBool;
var ds = DefaultValueString;
var di = DefaultValueInt;
}
[System.ComponentModel.DefaultValue(true)]
public bool DefaultValueBool { get; set; }
[System.ComponentModel.DefaultValue("Good")]
public string DefaultValueString { get; set; }
[System.ComponentModel.DefaultValue(27)]
public int DefaultValueInt { get; set; }
}

When you inline an initial value for a variable it will be done implicitly in the constructor anyway.
I would argue that this syntax was best practice in C# up to 5:
class Person
{
public Person()
{
//do anything before variable assignment
//assign initial values
Name = "Default Name";
//do anything after variable assignment
}
public string Name { get; set; }
}
As this gives you clear control of the order values are assigned.
As of C#6 there is a new way:
public string Name { get; set; } = "Default Name";

Sometimes I use this, if I don't want it to be actually set and persisted in my db:
class Person
{
private string _name;
public string Name
{
get
{
return string.IsNullOrEmpty(_name) ? "Default Name" : _name;
}
set { _name = value; }
}
}
Obviously if it's not a string then I might make the object nullable ( double?, int? ) and check if it's null, return a default, or return the value it's set to.
Then I can make a check in my repository to see if it's my default and not persist, or make a backdoor check in to see the true status of the backing value, before saving.

In C# 6.0 this is a breeze!
You can do it in the Class declaration itself, in the property declaration statements.
public class Coordinate
{
public int X { get; set; } = 34; // get or set auto-property with initializer
public int Y { get; } = 89; // read-only auto-property with initializer
public int Z { get; } // read-only auto-property with no initializer
// so it has to be initialized from constructor
public Coordinate() // .ctor()
{
Z = 42;
}
}

Starting with C# 6.0, We can assign default value to auto-implemented properties.
public string Name { get; set; } = "Some Name";
We can also create read-only auto implemented property like:
public string Name { get; } = "Some Name";
See: C# 6: First reactions , Initializers for automatically implemented properties - By Jon Skeet

In Version of C# (6.0) & greater, you can do :
For Readonly properties
public int ReadOnlyProp => 2;
For both Writable & Readable properties
public string PropTest { get; set; } = "test";
In current Version of C# (7.0), you can do : (The snippet rather displays how you can use expression bodied get/set accessors to make is more compact when using with backing fields)
private string label = "Default Value";
// Expression-bodied get / set accessors.
public string Label
{
get => label;
set => this.label = value;
}

In C# 9.0 was added support of init keyword - very useful and extremly sophisticated way for declaration read-only auto-properties:
Declare:
class Person
{
public string Name { get; init; } = "Anonymous user";
}
~Enjoy~ Use:
// 1. Person with default name
var anonymous = new Person();
Console.WriteLine($"Hello, {anonymous.Name}!");
// > Hello, Anonymous user!
// 2. Person with assigned value
var me = new Person { Name = "#codez0mb1e"};
Console.WriteLine($"Hello, {me.Name}!");
// > Hello, #codez0mb1e!
// 3. Attempt to re-assignment Name
me.Name = "My fake";
// > Compilation error: Init-only property can only be assigned in an object initializer

In addition to the answer already accepted, for the scenario when you want to define a default property as a function of other properties you can use expression body notation on C#6.0 (and higher) for even more elegant and concise constructs like:
public class Person{
public string FullName => $"{First} {Last}"; // expression body notation
public string First { get; set; } = "First";
public string Last { get; set; } = "Last";
}
You can use the above in the following fashion
var p = new Person();
p.FullName; // First Last
p.First = "Jon";
p.Last = "Snow";
p.FullName; // Jon Snow
In order to be able to use the above "=>" notation, the property must be read only, and you do not use the get accessor keyword.
Details on MSDN

In C# 6 and above you can simply use the syntax:
public object Foo { get; set; } = bar;
Note that to have a readonly property simply omit the set, as so:
public object Foo { get; } = bar;
You can also assign readonly auto-properties from the constructor.
Prior to this I responded as below.
I'd avoid adding a default to the constructor; leave that for dynamic assignments and avoid having two points at which the variable is assigned (i.e. the type default and in the constructor). Typically I'd simply write a normal property in such cases.
One other option is to do what ASP.Net does and define defaults via an attribute:
http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx

My solution is to use a custom attribute that provides default value property initialization by constant or using property type initializer.
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class InstanceAttribute : Attribute
{
public bool IsConstructorCall { get; private set; }
public object[] Values { get; private set; }
public InstanceAttribute() : this(true) { }
public InstanceAttribute(object value) : this(false, value) { }
public InstanceAttribute(bool isConstructorCall, params object[] values)
{
IsConstructorCall = isConstructorCall;
Values = values ?? new object[0];
}
}
To use this attribute it's necessary to inherit a class from special base class-initializer or use a static helper method:
public abstract class DefaultValueInitializer
{
protected DefaultValueInitializer()
{
InitializeDefaultValues(this);
}
public static void InitializeDefaultValues(object obj)
{
var props = from prop in obj.GetType().GetProperties()
let attrs = prop.GetCustomAttributes(typeof(InstanceAttribute), false)
where attrs.Any()
select new { Property = prop, Attr = ((InstanceAttribute)attrs.First()) };
foreach (var pair in props)
{
object value = !pair.Attr.IsConstructorCall && pair.Attr.Values.Length > 0
? pair.Attr.Values[0]
: Activator.CreateInstance(pair.Property.PropertyType, pair.Attr.Values);
pair.Property.SetValue(obj, value, null);
}
}
}
Usage example:
public class Simple : DefaultValueInitializer
{
[Instance("StringValue")]
public string StringValue { get; set; }
[Instance]
public List<string> Items { get; set; }
[Instance(true, 3,4)]
public Point Point { get; set; }
}
public static void Main(string[] args)
{
var obj = new Simple
{
Items = {"Item1"}
};
Console.WriteLine(obj.Items[0]);
Console.WriteLine(obj.Point);
Console.WriteLine(obj.StringValue);
}
Output:
Item1
(X=3,Y=4)
StringValue

little complete sample:
using System.ComponentModel;
private bool bShowGroup ;
[Description("Show the group table"), Category("Sea"),DefaultValue(true)]
public bool ShowGroup
{
get { return bShowGroup; }
set { bShowGroup = value; }
}

You can simple put like this
public sealed class Employee
{
public int Id { get; set; } = 101;
}

In the constructor. The constructor's purpose is to initialized it's data members.

private string name;
public string Name
{
get
{
if(name == null)
{
name = "Default Name";
}
return name;
}
set
{
name = value;
}
}

Have you tried using the DefaultValueAttribute or ShouldSerialize and Reset methods in conjunction with the constructor? I feel like one of these two methods is necessary if you're making a class that might show up on the designer surface or in a property grid.

Use the constructor because "When the constructor is finished, Construction should be finished". properties are like states your classes hold, if you had to initialize a default state, you would do that in your constructor.

To clarify, yes, you need to set default values in the constructor for class derived objects. You will need to ensure the constructor exists with the proper access modifier for construction where used. If the object is not instantiated, e.g. it has no constructor (e.g. static methods) then the default value can be set by the field. The reasoning here is that the object itself will be created only once and you do not instantiate it.
#Darren Kopp - good answer, clean, and correct. And to reiterate, you CAN write constructors for Abstract methods. You just need to access them from the base class when writing the constructor:
Constructor at Base Class:
public BaseClassAbstract()
{
this.PropertyName = "Default Name";
}
Constructor at Derived / Concrete / Sub-Class:
public SubClass() : base() { }
The point here is that the instance variable drawn from the base class may bury your base field name. Setting the current instantiated object value using "this." will allow you to correctly form your object with respect to the current instance and required permission levels (access modifiers) where you are instantiating it.

public Class ClassName{
public int PropName{get;set;}
public ClassName{
PropName=0; //Default Value
}
}

This is old now, and my position has changed. I'm leaving the original answer for posterity only.
Personally, I don't see the point of making it a property at all if you're not going to do anything at all beyond the auto-property. Just leave it as a field. The encapsulation benefit for these item are just red herrings, because there's nothing behind them to encapsulate. If you ever need to change the underlying implementation you're still free to refactor them as properties without breaking any dependent code.
Hmm... maybe this will be the subject of it's own question later

class Person
{
/// Gets/sets a value indicating whether auto
/// save of review layer is enabled or not
[System.ComponentModel.DefaultValue(true)]
public bool AutoSaveReviewLayer { get; set; }
}

I know this is an old question, but it came up when I was looking for how to have a default value that gets inherited with the option to override, I came up with
//base class
public class Car
{
public virtual string FuelUnits
{
get { return "gasoline in gallons"; }
protected set { }
}
}
//derived
public class Tesla : Car
{
public override string FuelUnits => "ampere hour";
}

I think this would do it for ya givng SomeFlag a default of false.
private bool _SomeFlagSet = false;
public bool SomeFlag
{
get
{
if (!_SomeFlagSet)
SomeFlag = false;
return SomeFlag;
}
set
{
if (!_SomeFlagSet)
_SomeFlagSet = true;
SomeFlag = value;
}
}

Can I change this extension method to remove the "magic string"? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Finding the Variable Name passed to a Function in C#
The class below contains the field city.
I need to dynamically determine the field's name as it is typed in the class declaration
i.e. I need to get the string "city" from an instance of the object city.
I have tried to do this by examining its Type in DoSomething() but can't find it when examining the contents of the Type in the debugger.
Is it possible?
public class Person
{
public string city = "New York";
public Person()
{
}
public void DoSomething()
{
Type t = city.GetType();
string field_name = t.SomeUnkownFunction();
//would return the string "city" if it existed!
}
}
Some people in their answers below have asked me why I want to do this.
Here's why.
In my real world situation, there is a custom attribute above city.
[MyCustomAttribute("param1", "param2", etc)]
public string city = "New York";
I need this attribute in other code.
To get the attribute, I use reflection.
And in the reflection code I need to type the string "city"
MyCustomAttribute attr;
Type t = typeof(Person);
foreach (FieldInfo field in t.GetFields())
{
if (field.Name == "city")
{
//do stuff when we find the field that has the attribute we need
}
}
Now this isn't type safe.
If I changed the variable "city" to "workCity" in my field declaration in Person this line would fail unless I knew to update the string
if (field.Name == "workCity")
//I have to make this change in another file for this to still work, yuk!
{
}
So I am trying to find some way to pass the string to this code without physically typing it.
Yes, I could declare it as a string constant in Person (or something like that) but that would still be typing it twice.
Phew! That was tough to explain!!
Thanks
Thanks to all who answered this * a lot*. It sent me on a new path to better understand lambda expressions. And it created a new question.

Maybe you need this. Works fine.
I found this here.
static void Main(string[] args)
{
var domain = "matrix";
Check(() => domain);
Console.ReadLine();
}
static void Check<T>(Expression<Func<T>> expr)
{
var body = ((MemberExpression)expr.Body);
Console.WriteLine("Name is: {0}", body.Member.Name);
Console.WriteLine("Value is: {0}", ((FieldInfo)body.Member)
.GetValue(((ConstantExpression)body.Expression).Value));
}
Output will be:
Name is: 'domain'
Value is: 'matrix'

I know this is old question, but I was trying to achieve the same and google sent me here. After many hours I finally found a way. I hope somebody else will find this useful.
There are actually more ways to accomplish this:
static void Main(string[] args)
{
GetName(new { var1 });
GetName2(() => var1);
GetName3(() => var1);
}
static string GetName<T>(T item) where T : class
{
return typeof(T).GetProperties()[0].Name;
}
static string GetName2<T>(Expression<Func<T>> expr)
{
return ((MemberExpression)expr.Body).Member.Name;
}
static string GetName3<T>(Func<T> expr)
{
return expr.Target.GetType().Module.ResolveField(BitConverter.ToInt32(expr.Method.GetMethodBody().GetILAsByteArray(), 2)).Name;
}
The first is fastest. The last 2 are approx 20 times slower than the 1st one.
http://abdullin.com/journal/2008/12/13/how-to-find-out-variable-or-parameter-name-in-c.html

city in this case is an instance of type string. When you call .GetType() you return the actual string type, which has no knowledge at all of your particular city instance.
I'm having a hard time seeing why you can't just type "city" in the code as a string literal here, if that's what you need. Perhaps it would help if you shared what you want to use this value for and in what circumstances you will call your DoSomething() function.
At the moment, my best guess is that what you really want to do is reflect the entire Person class to get a list of the fields in that class:
public void DoSomething()
{
MemberInfo[] members = this.GetType().GetMembers();
// now you can do whatever you want with each of the members,
// including checking their .Name properties.
}
Okay, based on your edit I have some more for you.
You can find the name of fields that are decorated with your attribute at run-time like this:
Type t = typeof(Person);
foreach (MemberInfo member in t.GetMembers()
.Where(m =>
m.GetCustomAttributes(typeof(MyCustomAttribute)).Any() ) )
{
// "member" is a MemberInfo object for a Peson member that is
// decorated with your attribute
}
You can also use binding flags in the first GetMembers() call to limit it to just fields, if you want.

You mentioned "i.e. I need to get the string "city" from an instance of the object city."
Are you looking to get the field name from the value of the field.
For example:If there are 2 Person object one with city "New York" and the other with city "London", are you looking for the function to return "city". Is this what you mean by dynamic?
With your current design you will always need to compare the name of the field from the FieldInfo against a string.
What if you instead decouple this so that you hold the identifier to use for comparison purposes during reflection as part of the attribute.
Something like this:
public enum ReflectionFields
{
CITY = 0,
STATE,
ZIP,
COUNTRY
}
[AttributeUsage(AttributeTargets.Field,AllowMultiple=false)]
public class CustomFieldAttr : Attribute
{
public ReflectionFields Field { get; private set; }
public string MiscInfo { get; private set; }
public CustomFieldAttr(ReflectionFields field, string miscInfo)
{
Field = field;
MiscInfo = miscInfo;
}
}
public class Person
{
[CustomFieldAttr(ReflectionFields.CITY, "This is the primary city")]
public string _city = "New York";
public Person()
{
}
public Person(string city)
{
_city = city;
}
}
public static class AttributeReader<T> where T:class
{
public static void Read(T t)
{
//get all fields which have the "CustomFieldAttribute applied to it"
var fields = t.GetType().GetFields().Where(f => f.GetCustomAttributes(typeof(CustomFieldAttr), true).Length == 1);
foreach (var field in fields)
{
var attr = field.GetCustomAttributes(typeof(CustomFieldAttr), true).First() as CustomFieldAttr;
if (attr.Field == ReflectionFields.CITY)
{
//You have the field and you know its the City,do whatever processing you need.
Console.WriteLine(field.Name);
}
}
}
}
public class Program
{
public static void Main(string[] args)
{
PPerson p1 = new PPerson("NewYork");
PPerson p2 = new PPerson("London");
AttributeReader<PPerson>.Read(p1);
AttributeReader<PPerson>.Read(p2);
}
}
You can now freely rename _city field of Person to something else and your calling code will still work since the code using reflection is trying to identify the field using the ReflectionFields enum value set as part of initialization of the attribute set on the field.

Yes its possible !!!
Try this out...
public string DoSomething(object city)
{
return city.GetType().GetProperty("Name",typeof(string)).GetValue(city,null);
}

Two things here.
Number one, as someone above pointed out, you're getting the Type for string, not for Person. So typeof(Person).GetMembers() will get you the list of members.
Number two, and more importantly, it looks like you're misunderstanding the purpose of attributes. In general attributes are used to mark a member for specific processing or to add additional information. Here you're using the name to indicate what processing you want, and the attribute to specify parameters, which is mixing of metaphors, or something.
Abhijeet's answer is more appropriate, you mark the field as a city field, then do what you like with it. Where I disagree is that I would use different attribute classes, rather than an enumeration.
Something like:
public class MyAttribute : Attribute
{
}
[AttributeUsage(AttributeTargets.Field)]
public class MyCityAttribute : MyAttribute
{
}
[AttributeUsage(AttributeTargets.Field]
public class MyNameAttribute: MyAttribute
{
}
public class Person
{
[MyCity]
public string city = "New York";
[MyCity]
public string workCity = "Chicago";
[MyName]
public string fullName = "John Doe";
public Person()
{
}
public void DoSomething()
{
Type t = typeof(Person);
FieldInfo[] fields = t.GetFields(BindingFlags.Instance | BindingFlags.Public);
foreach (var field in fields)
{
MyAttribute[] attributes = field.GetCustomAttributes(typeof(MyAttribute));
if (attributes.Count > 0)
{
if (attributes[0] is MyCityAttribute)
{
//Dosomething for city
break;
}
if (attributes[0] is MyNameAttribute)
{
//Dosomething for names
break;
}
}
}
}
}
This would allow you to use different parameters for MyCity vs MyName that would make more sense in the context of processing each.
I think with your 'yuk' comment above, you hit the nail on the head. That you would have to change a string constant if you rename your variable is an indicator that you're doing something wrong.

t.GetField("city", BindingFlags.Public | BindingFlags.Instance);
or you can call GetFields() to get all fields

You need to call get type on the class Person. The iterate the fields of the class as in the answer below

This is not possible (I think it actually is but involes several hacks and using lambdas). If you want to store attributes about a Person and be able to get the name of the attribute easily, I suggest using a Dictionary<TKey, TValue> from the System.Collections.Generic namespace.
And you can always make public properties that wrap the dictionary.
public class Person
{
Dictionary<string, string> attributes = new Dictionary<string, string();
public string City
{
get { return attributes["city"]; }
set { attributes["city"] = value; }
}
public Person()
{
City = "New York";
}
}
And you can get a list of all attributes with attributes.Keys.

Have a look at this post as it looks similar to what you're trying to do:
Finding the variable name passed to a function
(especially Konrad Rudolph's answer) Another approach could be to just add "city" as one of the parameters in the attribute and fish that out later.

You are already looping through the collection of FieldInfo objects. Look for your attribute on those and when you find the FieldInfo that contains your attribute, you have the one you want. Then call .Name on it.
system.reflection.fieldinfo.attributes

Default values without default constructor

How do the data members of the type get their default values when the default constructor is omitted and if they're given default values anyways what is the use of the default constructor in the first place ?
class Program
{
static void Main(string[] args)
{
Person person = new Person("SomeName");
Console.WriteLine(person.Age);
Console.WriteLine(person.FamileName == null);
}
}
class Person
{
public string Name { get; set; }
public int Age { get; set; }
public string FamileName { get; set; }
public Person(string name)
{
Name = name;
}
}
Edit:
This is from a book I've been reading and it's what got me thinking that the default constructor is responsible for giving the data members their default values but it seems that I got it wrong and it's only a way for me to modify these values if I needed to, please correct me if I'm wrong.
Every C# class is provided with a “freebie” default constructor that you can redefine if need be. By definition,
a default constructor never takes arguments. After allocating the new object into memory, the default
constructor ensures that all field data of the class is set to an appropriate default value

Object fields are always initialized with default values if you won't specify differently using constructor or field initialization (same goes for properties).
Default value usualy corresponds to all bits set to zero (null for reference types, 0 for numeric types).
You can set other value using constructor:
public Person()
{
Name = "Sebastian";
}
or initializer:
public string Name { get; set; } = "Sebastian";

In C# you can assign default value.
class Person
{
public string Name { get; set; } ="Someone";
public int Age { get; set; }
public string FamileName { get; set; }
}
If you want to assign the values in the initialization of object you can do this:
class Program
{
static void Main(string[] args)
{
Person person = new Person { Name= "SomeOne"; Age=16; };
Console.WriteLine(person.Age);
Console.WriteLine(person.FamileName == null);
}
}
If you only need to assign values or default value, you do not need the constructor. You can do it in the above mention methods.
I normally use constructors when I need to do some additional tasks e.g. database connection, setting up other object and etc.
Link: https://msdn.microsoft.com/en-us/library/bb397680.aspx

The members in your class will be initialized with their default values. For reference types, this is null and for value types, it's the default value. There's nothing special about being members in an object - it's the same values they would have if they were in the middle of a function:
public void Foo()
{
string name; // null
int age; // 0
bool isChild; // false
}
As for the default constructor, it's a nice place to make sure your data types are safe. For example, you might want to initialize any reference typed properites to a non-null value:
class Widget
{
public string Name { get; set; }
public Widget()
{
Name = string.Empty; // this way someone can call Widget.Name safely
}
}
On a related note, you should also see: How do you give a C# Auto-Property a default value?

What is the best way to give a C# auto-property an initial value?

How do you give a C# auto-property an initial value?
I either use the constructor, or revert to the old syntax.
Using the Constructor:
class Person
{
public Person()
{
Name = "Initial Name";
}
public string Name { get; set; }
}
Using normal property syntax (with an initial value)
private string name = "Initial Name";
public string Name
{
get
{
return name;
}
set
{
name = value;
}
}
Is there a better way?

In C# 5 and earlier, to give auto implemented properties an initial value, you have to do it in a constructor.
Since C# 6.0, you can specify initial value in-line. The syntax is:
public int X { get; set; } = x; // C# 6 or higher
DefaultValueAttribute is intended to be used by the VS designer (or any other consumer) to specify a default value, not an initial value. (Even if in designed object, initial value is the default value).
At compile time DefaultValueAttribute will not impact the generated IL and it will not be read to initialize the property to that value (see DefaultValue attribute is not working with my Auto Property).
Example of attributes that impact the IL are ThreadStaticAttribute, CallerMemberNameAttribute, ...

Edited on 1/2/15
C# 6 :
With C# 6 you can initialize auto-properties directly (finally!), there are now other answers that describe that.
C# 5 and below:
Though the intended use of the attribute is not to actually set the values of the properties, you can use reflection to always set them anyway...
public class DefaultValuesTest
{
public DefaultValuesTest()
{
foreach (PropertyDescriptor property in TypeDescriptor.GetProperties(this))
{
DefaultValueAttribute myAttribute = (DefaultValueAttribute)property.Attributes[typeof(DefaultValueAttribute)];
if (myAttribute != null)
{
property.SetValue(this, myAttribute.Value);
}
}
}
public void DoTest()
{
var db = DefaultValueBool;
var ds = DefaultValueString;
var di = DefaultValueInt;
}
[System.ComponentModel.DefaultValue(true)]
public bool DefaultValueBool { get; set; }
[System.ComponentModel.DefaultValue("Good")]
public string DefaultValueString { get; set; }
[System.ComponentModel.DefaultValue(27)]
public int DefaultValueInt { get; set; }
}

When you inline an initial value for a variable it will be done implicitly in the constructor anyway.
I would argue that this syntax was best practice in C# up to 5:
class Person
{
public Person()
{
//do anything before variable assignment
//assign initial values
Name = "Default Name";
//do anything after variable assignment
}
public string Name { get; set; }
}
As this gives you clear control of the order values are assigned.
As of C#6 there is a new way:
public string Name { get; set; } = "Default Name";

Sometimes I use this, if I don't want it to be actually set and persisted in my db:
class Person
{
private string _name;
public string Name
{
get
{
return string.IsNullOrEmpty(_name) ? "Default Name" : _name;
}
set { _name = value; }
}
}
Obviously if it's not a string then I might make the object nullable ( double?, int? ) and check if it's null, return a default, or return the value it's set to.
Then I can make a check in my repository to see if it's my default and not persist, or make a backdoor check in to see the true status of the backing value, before saving.

In C# 6.0 this is a breeze!
You can do it in the Class declaration itself, in the property declaration statements.
public class Coordinate
{
public int X { get; set; } = 34; // get or set auto-property with initializer
public int Y { get; } = 89; // read-only auto-property with initializer
public int Z { get; } // read-only auto-property with no initializer
// so it has to be initialized from constructor
public Coordinate() // .ctor()
{
Z = 42;
}
}

Starting with C# 6.0, We can assign default value to auto-implemented properties.
public string Name { get; set; } = "Some Name";
We can also create read-only auto implemented property like:
public string Name { get; } = "Some Name";
See: C# 6: First reactions , Initializers for automatically implemented properties - By Jon Skeet

In Version of C# (6.0) & greater, you can do :
For Readonly properties
public int ReadOnlyProp => 2;
For both Writable & Readable properties
public string PropTest { get; set; } = "test";
In current Version of C# (7.0), you can do : (The snippet rather displays how you can use expression bodied get/set accessors to make is more compact when using with backing fields)
private string label = "Default Value";
// Expression-bodied get / set accessors.
public string Label
{
get => label;
set => this.label = value;
}

In C# 9.0 was added support of init keyword - very useful and extremly sophisticated way for declaration read-only auto-properties:
Declare:
class Person
{
public string Name { get; init; } = "Anonymous user";
}
~Enjoy~ Use:
// 1. Person with default name
var anonymous = new Person();
Console.WriteLine($"Hello, {anonymous.Name}!");
// > Hello, Anonymous user!
// 2. Person with assigned value
var me = new Person { Name = "#codez0mb1e"};
Console.WriteLine($"Hello, {me.Name}!");
// > Hello, #codez0mb1e!
// 3. Attempt to re-assignment Name
me.Name = "My fake";
// > Compilation error: Init-only property can only be assigned in an object initializer

In addition to the answer already accepted, for the scenario when you want to define a default property as a function of other properties you can use expression body notation on C#6.0 (and higher) for even more elegant and concise constructs like:
public class Person{
public string FullName => $"{First} {Last}"; // expression body notation
public string First { get; set; } = "First";
public string Last { get; set; } = "Last";
}
You can use the above in the following fashion
var p = new Person();
p.FullName; // First Last
p.First = "Jon";
p.Last = "Snow";
p.FullName; // Jon Snow
In order to be able to use the above "=>" notation, the property must be read only, and you do not use the get accessor keyword.
Details on MSDN

In C# 6 and above you can simply use the syntax:
public object Foo { get; set; } = bar;
Note that to have a readonly property simply omit the set, as so:
public object Foo { get; } = bar;
You can also assign readonly auto-properties from the constructor.
Prior to this I responded as below.
I'd avoid adding a default to the constructor; leave that for dynamic assignments and avoid having two points at which the variable is assigned (i.e. the type default and in the constructor). Typically I'd simply write a normal property in such cases.
One other option is to do what ASP.Net does and define defaults via an attribute:
http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx

My solution is to use a custom attribute that provides default value property initialization by constant or using property type initializer.
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class InstanceAttribute : Attribute
{
public bool IsConstructorCall { get; private set; }
public object[] Values { get; private set; }
public InstanceAttribute() : this(true) { }
public InstanceAttribute(object value) : this(false, value) { }
public InstanceAttribute(bool isConstructorCall, params object[] values)
{
IsConstructorCall = isConstructorCall;
Values = values ?? new object[0];
}
}
To use this attribute it's necessary to inherit a class from special base class-initializer or use a static helper method:
public abstract class DefaultValueInitializer
{
protected DefaultValueInitializer()
{
InitializeDefaultValues(this);
}
public static void InitializeDefaultValues(object obj)
{
var props = from prop in obj.GetType().GetProperties()
let attrs = prop.GetCustomAttributes(typeof(InstanceAttribute), false)
where attrs.Any()
select new { Property = prop, Attr = ((InstanceAttribute)attrs.First()) };
foreach (var pair in props)
{
object value = !pair.Attr.IsConstructorCall && pair.Attr.Values.Length > 0
? pair.Attr.Values[0]
: Activator.CreateInstance(pair.Property.PropertyType, pair.Attr.Values);
pair.Property.SetValue(obj, value, null);
}
}
}
Usage example:
public class Simple : DefaultValueInitializer
{
[Instance("StringValue")]
public string StringValue { get; set; }
[Instance]
public List<string> Items { get; set; }
[Instance(true, 3,4)]
public Point Point { get; set; }
}
public static void Main(string[] args)
{
var obj = new Simple
{
Items = {"Item1"}
};
Console.WriteLine(obj.Items[0]);
Console.WriteLine(obj.Point);
Console.WriteLine(obj.StringValue);
}
Output:
Item1
(X=3,Y=4)
StringValue

little complete sample:
using System.ComponentModel;
private bool bShowGroup ;
[Description("Show the group table"), Category("Sea"),DefaultValue(true)]
public bool ShowGroup
{
get { return bShowGroup; }
set { bShowGroup = value; }
}

You can simple put like this
public sealed class Employee
{
public int Id { get; set; } = 101;
}

In the constructor. The constructor's purpose is to initialized it's data members.

private string name;
public string Name
{
get
{
if(name == null)
{
name = "Default Name";
}
return name;
}
set
{
name = value;
}
}

Have you tried using the DefaultValueAttribute or ShouldSerialize and Reset methods in conjunction with the constructor? I feel like one of these two methods is necessary if you're making a class that might show up on the designer surface or in a property grid.

Use the constructor because "When the constructor is finished, Construction should be finished". properties are like states your classes hold, if you had to initialize a default state, you would do that in your constructor.

To clarify, yes, you need to set default values in the constructor for class derived objects. You will need to ensure the constructor exists with the proper access modifier for construction where used. If the object is not instantiated, e.g. it has no constructor (e.g. static methods) then the default value can be set by the field. The reasoning here is that the object itself will be created only once and you do not instantiate it.
#Darren Kopp - good answer, clean, and correct. And to reiterate, you CAN write constructors for Abstract methods. You just need to access them from the base class when writing the constructor:
Constructor at Base Class:
public BaseClassAbstract()
{
this.PropertyName = "Default Name";
}
Constructor at Derived / Concrete / Sub-Class:
public SubClass() : base() { }
The point here is that the instance variable drawn from the base class may bury your base field name. Setting the current instantiated object value using "this." will allow you to correctly form your object with respect to the current instance and required permission levels (access modifiers) where you are instantiating it.

public Class ClassName{
public int PropName{get;set;}
public ClassName{
PropName=0; //Default Value
}
}

This is old now, and my position has changed. I'm leaving the original answer for posterity only.
Personally, I don't see the point of making it a property at all if you're not going to do anything at all beyond the auto-property. Just leave it as a field. The encapsulation benefit for these item are just red herrings, because there's nothing behind them to encapsulate. If you ever need to change the underlying implementation you're still free to refactor them as properties without breaking any dependent code.
Hmm... maybe this will be the subject of it's own question later

class Person
{
/// Gets/sets a value indicating whether auto
/// save of review layer is enabled or not
[System.ComponentModel.DefaultValue(true)]
public bool AutoSaveReviewLayer { get; set; }
}

I know this is an old question, but it came up when I was looking for how to have a default value that gets inherited with the option to override, I came up with
//base class
public class Car
{
public virtual string FuelUnits
{
get { return "gasoline in gallons"; }
protected set { }
}
}
//derived
public class Tesla : Car
{
public override string FuelUnits => "ampere hour";
}

I think this would do it for ya givng SomeFlag a default of false.
private bool _SomeFlagSet = false;
public bool SomeFlag
{
get
{
if (!_SomeFlagSet)
SomeFlag = false;
return SomeFlag;
}
set
{
if (!_SomeFlagSet)
_SomeFlagSet = true;
SomeFlag = value;
}
}