Related
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;
}
}
I want to use CsvHelper.Configuration.ClassMap by dynamically assigned properties.
Usually you map a Property like this in a static manner: You have to assign each property and its 'text to display'.
using CsvHelper.Configuration;
public sealed class CleanSQLRowDescriptorMap : ClassMap<CleanSQLRowDescriptor>
{
public CleanSQLRowDescriptorMap()
{
Map(f => f.OriginalIndex).Name("Original Index");
Map(f => f.OriginalRow).Name("Original Row");
}
}
I want to do the following:
using CsvHelper.Configuration;
public sealed class CleanSQLRowDescriptorMap : ClassMap<CleanSQLRowDescriptor>
{
public CleanSQLRowDescriptorMap()
{
// Filter by attribute (implementation returns PropertyInfo List)
List<PropertyInfo> mappedProperties = CleanSQLRowDescriptor.Create().FilterPropertiesByAttribute();
// Dynamically assign each property and its assigned 'attribute value'
// At the moment I mapped the PropertyInfo.Name, but I actually need to use the Property as the static example above.
// Also need to figure out how to get the Attribute value (DisplayName in this example).
mappedProperties.ForEach(prop => Map(f => prop.Name).Name(prop.Name));
}
}
I currently have the following method used above:
[DisplayName("Original Index")]
public int OriginalIndex { get; set; }
[DisplayName("Original Row")]
public string OriginalRow { get; set; }
public string DonotWantToAssignThis { get; set; }
public List<PropertyInfo> FilterPropertiesByAttribute()
{
// This function already returns only the attributes that use
// [DisplayName] and other attributes defined for other properties,
// ignoring other properties that do not have any of these attributes.
return properties;
}
How can I use the PropertyInfo List of items to dynamically assign the ClassMap? I want to create a base class with these attributes as filters and all the classes implementing this base class would have the same capability, making it easier to 'maintain the mappings'.
I managed to figure it out, VS Code did not give me all the overloads for Map() function, so I missed overloads.
This one is used in all examples:
MemberMap<TClass, TMember> Map<TMember>(Expression<Func<TClass, TMember>> expression, bool useExistingMap = true);
I found this inside JoshClose/CSVHelper:
public MemberMap Map(Type classType, MemberInfo member, bool useExistingMap = true)
So instead of using 'Expression that requires the property name as TMember' which does not take the type I can now assign the MemberInfo directly.
The code below just shows a solution for a single attribute [DisplayName] by using its .DisplayName property value.
For additional Attributes like I have at the moment, I will need to handle the property value differently:
mappedProperties.ForEach(prop =>
{
Map(typeof(CleanSQLRowDescriptor), prop).Name(prop.GetCustomAttribute<DisplayNameAttribute>().DisplayName);
});
I have the current piece of code :
using System;
using System.ComponentModel.DataAnnotations;
using Boo.Lang;
class Program
{
public partial class Foo
{
public string SomeField { get; set; }
}
[MetadataType(typeof(FooMetadata))]
public partial class Foo
{
}
public class FooMetadata
{
[Required(ErrorMessage = "Some Field is required")]
public string SomeField { get; set; }
}
static void Main()
{
var sample = new Foo { SomeField = null };
var context = new ValidationContext(sample);
var results = new List<ValidationResult>();
var isValid = Validator.TryValidateObject(sample, context, results);
if (!isValid)
{
foreach (var validationResult in results)
{
Console.WriteLine(validationResult.ErrorMessage);
}
}
else
Console.WriteLine("sample is valid");
Console.ReadLine();
}
}
It says that sample is valid although it's not the case. Am I missing something ?
Validator.ValidateObject goes through all validation attributes and throws ValidationException for the first one that fails. I don't think one can get the list of all properties that failed this way.
The MSDN documentation is a bit confusing here, it says:
The ValidateObject method evaluates each ValidationAttribute attribute
that is associated with the object type. If validateAllProperties is
set to true, the method validates the property values of the object.
TryValidateObject method manual continues:
It also checks whether each property that is marked with
RequiredAttribute is provided.
I confirm that it always checks all the Required attributes, regardless of validateAllProperties. But if you have, for example, a Range validation attribute it will only check it when validateAllProperties is true (because this is when it validates the property value).
A bit confusing, but as you noticed, TryValidateObject shows all the failed properties.
I have a class with only string members like this :
public class MyClass
{
public string MyProp1 { get; set; }
public string MyProp2 { get; set; }
}
I create an instance :
Var myClass = new MyClass();
Later in the code, I’d like to know if all the member (MyProp1 and MyProp2) are not null or empty. I know I can use a if of course but there is much more properties than 2 in my real code.
Is there a way to do this ?
Thanks,
Using a dictionary based store for your properties is probably the easiest way of doing this:
public class MyClass
{
private IDictionary<String, String> _store;
public MyClass()
{
_store = new Dictionary<String, String>();
}
public string MyProp1 {
get { return GetOrDefault("MyProp1"); }
set { _store["MyProp1"] = value; }
}
public string MyProp2 {
get { return GetOrDefault("MyProp2"); }
set { _store["MyProp2"] = value; }
}
public Boolean HasData()
{
return _store.Any(x => !String.IsNullOrWhiteSpace(x.Value));
}
public Boolean IsEmpty()
{
return _store.All(x => String.IsNullOrWhiteSpace(x.Value));
}
private String GetOrDefault(String propertyName)
{
if (_store.ContainsKey(propertyName))
{
return _store[propertyName];
}
return String.Empty;
}
}
Another method for doing this would be to compare it with a default instance:
public class MyClass
{
public string MyProp1 { get; set; }
public string MyProp2 { get; set; }
public static readonly MyClass Empty = new MyClass();
public Boolean HasData()
{
return !Empty.Equals(this);
}
public Boolean IsEmpty()
{
return Empty.Equals(this);
}
}
You can try to use the reflect to check the properties. You should need confirm that all the properties are public, and the type is string. Here is the code.
public static bool IsNullOrEmpty(MyClass prop)
{
bool result = true;
PropertyInfo[] ps = prop.GetType().GetProperties();
foreach (PropertyInfo pi in ps)
{
string value = pi.GetValue(prop, null).ToString();
if (string.IsNullOrEmpty(value))
{
result = false;
break;
}
}
return result;
}
To check if your class contains 'any' properties which are null:
System.Reflection.PropertyInfo[] properties = myClass.GetType().GetProperties
(BindingFlags.Public | BindingFlags.Instance);
bool hasNullProperty = properties.Any(y => y.GetValue(x, null) == null);
You can always initialize your class like
public class MyClass
{
public MyClass() {
this.MyProp1 = this.MyProp2 = String.Empty;
}
public string MyProp1 { get; set; }
public string MyProp2 { get; set; }
}
and, unless your programmatically assign a null value to it, the new MyClass() will always have String.Empty in their 2 properties...
from comment:
What I do in those cases is call a helper, for example: string name = myHelper.CheckNode(xmlNode); and in that helper I check if it's null, any other check, I can easily tweek the helper method and it will be available to all elements, and you can extend it to support not only strings but all other data types as well
So, imagine that you are reading nodes from your XML, you write them like:
string name = myHelper.CheckNode(node);
in your helper, you could have something like:
XmlNodeList datasourceNodes = rootNode.SelectNodes("dataSources/dataSource");
foreach (XmlNode datasourceNode in datasourceNodes)
{
DataSource dataSource = new DataSource();
dataSource.Name = myHelper.CheckAttr(datasourceNode.Attributes["name"]);
dataSource.ODBC = myHelper.CheckNode(datasourceNode.SelectSingleNode("odbc"));
// or a variant (Extension Method)
dataSource.UID = datasourceNode.CheckNode("user");
dataSource.PWD = datasourceNode.CheckAttr("password");
ds.Add(dataSource);
}
your helper then could have a method like:
public static string CheckAttr(XmlAttribute attr)
{
return attr == null ? "" : attr.Value.Trim();
}
public static string CheckNode(XmlNode node)
{
return node == null ? "" : node.InnerText.Trim();
}
or for the variant (Extension Method)
public static string CheckAttr(this XmlNode, string attrName)
{
return attrName[attrName] == null ? "" : attrName[attrName].Value.Trim();
}
public static string CheckNode(this XmlNode, string nodeName)
{
return node.SelectSingleNode(nodeName) == null ?
"" :
node.SelectSingleNode(nodeName).InnerText.Trim();
}
If there are many properties in the class, one way of handling this is storing them in a collection, such as an array or a dictionary, instead of declaring each property as a separate member of the class.
Then you can access data in the dictionary by key, which is as easy as accessing a property of a class. And the advantage is that you can loop over the dictionary and check all the properties in a loop.
I would suggest creating a function in your class where you check String.IsNullOrEmpty(MyProp1) etc. for all your properties. This way you at least have gathered all the ckecking functionality in a single place. And you only have this place to modify whenever you add new properties.
To check if all the elements are set you could add a IsEmpty() method to your class that would check the internal properties. Then you wouldn't have to duplicate the if statements everywhere trough your code.
In your IsEmpty() method you can use a regular if statement to check all the fields or you can use reflection to automaticaly retrieve all string properties and check their values. The performance of reflection will be worse then a normal if check but if that's not a problem you can reuse the reflection code in all your entities to check their values.
By using Attribute Base programming you can achieve this. In this approach you will need to place attribute over the class member, and validation is can be done. You can also use Microsoft Enterprise Library for this.
Probably the best way would be to :
Restructure your properties in the form of a Dictionary of strings. Loop through the dictionary to test the strings using string.IsNullOrEmpty(). You could replace the N getter/setters by a single Indexer property which sets and retrieves the strings directly from the dictionary based on a key
You can use try the following style. I haven't tried it before but you might see if it helps
If (String.IsNullOrEmpty(string1 && string2 && string3))
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;
}
}