Related
This is probably a stupid question, but just in case....
We have a 3rd party package with weird models like:
public partial class CountingDevice
{
public int countingDeviceNo { get; set; }
public string countingDeviceName { get; set; }
public string obis { get; set; }
public int integralPart { get; set; }
public bool integralPartFieldSpecified;
public int fractionalPart { get; set; }
public bool fractionalPartFieldSpecified;
public double value { get; set; }
public bool valueFieldSpecified;
public bool offPeakFlag { get; set; }
public bool offPeakFlagFieldSpecified;
public ExpectedMeterReading expectedMeterReading { get; set; }
// snipped for brevity
}
You'll notice that sometimes there are pairs of fields like integralPart and integralPartFieldSpecified.
Here is the problem: If I simply assign some value to integralPart but do not set integralPartFieldSpecified = true, the value of integralPart will be completely ignored causing the solution to fail.
So when mapping our own models to this madness, I need to litter the code with constructs like:
if (IntegralPart != null)
{
countingDevice.integralPartSpecified = true;
countingDevice.integralPart = (int)IntegralPart!;
}
Both in the interest of reducing lines of code and not stumbling over a minefield, I would like to do any one of the following:
A. Overload the = operator so it will automatically check for a property which is a boolean and has "Specified" concatenated to the current property's name. If such a property exists, it will be assigned true when the value is assigned; if not, then assignment will operate as normal. Ideally, it should be "smart" enough to assign "...Specified" to false if the value assigned is null/default/empty.
B. Create some customer operator which will do the same as A.
C. Create some method which I could invoke in a concise and preferably typesafe way to do the same.
Is this possible?
If so, how?
To make it clear: I need to build quite a few wrappers.
I don't want to repeat this logic for every field and worry about missing some fields which it applies to.
I want a generic way of assigning both fields at once if the "Specified" field exists and being able to do assignments in exactly the same way if it does not exist.
not stumbling over a minefield
Encapsulate the minefield.
If you don't control this 3rd party DTO then don't use it throughout your domain. Encapsulate or wrap the integration of this 3rd party tool within a black box that you control. Then throughout your domain use your models.
Within the integration component for this 3rd party system, simply map to/from your Domain Models and this 3rd party DTO. So this one extra line of code which sets a second field on the DTO only exists in that one place.
Another (expensive) solution would be to write a method that takes in an object, a property name, and the new property value. You can then use reflection to both set the property value for the specified property, as well as search for the bool field that you want to set (if it exists).
Note that you need to pass the correct type for the property. There's no compile-time checking that you're passing a double instead of a string for the value property, for example.
Below I've created an extension method on the object type to simplify calling the method in our main code (the method becomes a member of the object itself):
public static class Extensions
{
// Requires: using System.Reflection;
public static bool SetPropertyAndSpecified(this object obj,
string propertyName, object propertyValue)
{
// Argument validation left to user
// Check if 'obj' has specified 'propertyName'
// and set 'propertyValue' if it does
PropertyInfo prop = obj.GetType().GetProperty(propertyName,
BindingFlags.Public | BindingFlags.Instance);
if (prop != null && prop.CanWrite)
{
prop.SetValue(obj, propertyValue, null);
// Check for related "FieldSpecified" field
// and set it to 'true' if it exists
obj.GetType().GetField($"{propertyName}FieldSpecified",
BindingFlags.Public | BindingFlags.Instance)?.SetValue(obj, true);
return true;
}
return false;
}
}
After you add this class to your project, you can do something like:
static void Main(string[] args)
{
var counter = new CountingDevice();
// Note that 'valueFieldSpecified' and `integralPartFieldSpecified'
// are set to 'false' on 'counter'
// Call our method to set some properties
counter.SetPropertyAndSpecified(nameof(counter.integralPart), 42);
counter.SetPropertyAndSpecified(nameof(counter.value), 69d);
// Now 'valueFieldSpecified' and 'integralPartFieldSpecified'
// are set to 'true' on 'counter'
}
You cannot overload the = operator in C#.
You can just use custom properties and set the "FieldSpecified" fields in the setters e.g.
private int _integralPart;
public int integralPart
{
get { return _integralPart; }
set
{
_integralPart = value;
integralPartFieldSpecified = true;
}
}
public bool integralPartFieldSpecified;
Update
If you want a generic solution you can use a generic class for properties that you want to achieve the specified behaviour with e.g.
public class ValueWithSpecifiedCheck<T>
{
private T _fieldValue;
public T FieldValue
{
get
{
return _fieldValue;
}
set
{
_fieldValue = value;
FieldSpecified = true;
}
}
public bool FieldSpecified { get; set; }
}
public class Data
{
public ValueWithSpecifiedCheck<int> IntegralPart { get; set; }
}
Then the class/property would be used as following:
public static void Main()
{
var data = new Data();
data.IntegralPart = new ValueWithSpecifiedCheck<int>();
data.IntegralPart.FieldValue = 7;
Console.WriteLine(data.IntegralPart.FieldSpecified);// Prints true
}
If you implement a generic solution and add implicit conversion operators, it's quite convenient to use.
Here's a sample Optional<T> struct (I made it a readonly struct to ensure immutable mechanics):
public readonly struct Optional<T> where T : struct
{
public Optional(T value)
{
_value = value;
}
public static implicit operator T(Optional<T> opt) => opt.Value;
public static implicit operator Optional<T>(T opt) => new(opt);
public T Value => _value!.Value;
public bool Specified => _value is not null;
public override string ToString() => _value is null ? "<NONE>" : _value.ToString()!;
readonly T? _value;
}
You could use that to implement your CountingDevice class like so:
public partial class CountingDevice
{
public int countingDeviceNo { get; set; }
public string countingDeviceName { get; set; }
public string obis { get; set; }
public Optional<int> integralPart { get; set; }
public Optional<int> fractionalPart { get; set; }
public Optional<double> value { get; set; }
public Optional<bool> offPeakFlag { get; set; }
// snipped for brevity
}
Usage is quite natural because of the implicit conversions:
public static void Main()
{
var dev = new CountingDevice
{
integralPart = 10, // Can initialise with the underlying type.
value = 123.456
};
Console.WriteLine(dev.fractionalPart.Specified); // False
Console.WriteLine(dev.integralPart.Specified); // True
Console.WriteLine(dev.value); // 123.456
Console.WriteLine(dev.value.ToString()); // 123.456
Console.WriteLine(dev.fractionalPart.ToString()); // "<NONE>"
dev.fractionalPart = 42; // Can set the value using int.
Console.WriteLine(dev.fractionalPart.Specified); // True
Console.WriteLine(dev.fractionalPart); // 42
var optCopy = dev.offPeakFlag;
Console.WriteLine(optCopy.Specified); // False
dev.offPeakFlag = true;
Console.WriteLine(dev.offPeakFlag.Specified); // True
Console.WriteLine(optCopy.Specified); // Still False - not affected by the original.
Console.WriteLine(optCopy); // Throws an exception because its not specified.
}
You might also want to use optional reference types, but to do that you will need to declare a generic with the class constraint:
public readonly struct OptionalRef<T> where T : class
{
public OptionalRef(T value)
{
_value = value;
}
public static implicit operator T(OptionalRef<T> opt) => opt.Value;
public static implicit operator OptionalRef<T>(T opt) => new(opt);
public T Value => _value ?? throw new InvalidOperationException("Accessing an unspecified value.");
public bool Specified => _value is not null;
public override string ToString() => _value is null ? "<NONE>" : _value.ToString()!;
readonly T? _value;
}
Personally, I think that's a bit overkill. I'd just use nullable value types, int?, double? etc, but it depends on the expected usage.
C# doesn't allow overloading the = operator (unlike eg C++). However, your suggestion C should work. It's a bit of a hassle, too, since you'll have to write a bunch of methods, but you could write an extension method such as
public static class Extensions
{
public static void UpdateIntegralPart(this CountingDevice dev, double value)
{
dev.integralPart = value;
dev.integralPartSpecified = true;
}
}
Then you can call
countingDevice.UpdateIntegralPart(1234);
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;
}
}
Given the following code;
public class CustomControl {
private object _dataItem;
public object DataItem {
get { return _dataItem; }
set { _dataItem = value; }
}
public void Update(ref string t) {
t = "test";
}
}
public class Consume {
public void Example() {
CustomControl ctrl = new CustomControl();
ctrl.DataItem = anyObject.anyProperty;
string prop = anyObject.anyProperty;
ctrl.Update(ref prop);
anyObject.anyProperty = prop;
}
}
How can I change it so that the DataItem property is itself a reference, allowing you to pre-emptively set it to point to a variable thus allowing you to call Update() without any parameters.
So the Consume class would then look similar to;
public class Consume {
public void Example() {
CustomControl ctrl = new CustomControl();
ctrl.DataItem = anyObject.anyProperty;
ctrl.Update();
// anyObject.anyProperty has been updated to "test"
}
}
So the assigment of anyObject.anyProperty is then handled internally within CustomControl
You need to store the act of setting something to a string, so store an Action<string>:
public class CustomControl {
public Action<string> SetData { get; set; }
public void Update() {
// TODO nullity check
SetData("test");
}
}
Then Consume would look like:
public class Consume {
public void Example() {
CustomControl ctrl = new CustomControl();
// store {the act of setting this property of this object to a string}
ctrl.SetData = s => anyObject.anyProperty = s;
ctrl.Update();
}
}
The Update call will set anyObject.anyProperty to test. Note that you are storing specifically the act of setting this property of the particular anyObject you refer to in the assignment to SetData.
To expand on the lambda: we want to create a value of type Action<string>, that is, a thing which takes a string and returns no result. Such a thing is going to be executable code. Prior to C# 3, to create a 'value' that was executable code, we would have had to do something like:
ctrl.SetData = delegate(string s) { someObject.SomeProperty = s; };
With this syntax it's more obvious that we're creating a method - it has a { } delimited body, it has statements in it, and it's clear there is a string parameter that is used by the body.
One thing achieved by lambda expressions in C# 3 is the ability to condense this down; loosely, the whole of
// not compilable code
delegate(parameters) { body; }
can be replaced with
// not compilable code
(parameters) => body;
and in the case where there's only one parameter
// not compilable code
parameter => body;
which is what we have here: the expression assigned to ctrl.SetData is a piece of behaviour that accepts a string (s) and sets anyObject.anyProperty to that string. The real power is in the way the C# compiler can work out the types to it know we're creating an Action<string>.
At first I didn't understand what you're trying to do. What you're looking for is the Adapter or Facade pattern. That is, you have an object with a particular interface, but you need to adapt it to a different interface or provide a simpler interface.
One way to implement these patterns is to use composition and delegate the new interface to methods on the existing interface.
public interface IUpdatable<U>
{
void Update( U newValue );
}
public abstract class CustomControl<T,U> : IUpdatable<U>
where T : Control
{
private T Control { get; set; }
protected CustomControl( T control )
{
this.Control = control;
}
public abstract void Update( U newValue );
}
public class TextBoxFacade : CustomControl<TextBox,string>, IUpdatable<string>
{
public TextBoxFacade( TextBox textbox ) : base(textbox) { }
public override void Update( string newValue )
{
this.Control.Value = newValue;
}
}
I have an object that I pass around a lot.
I need to add a piece of data to it and cannot modify the base class
So I have
static OriginalThing GetNewThing()
{
return new OriginalThing();
}
Now i want to add my piece of data
class EnhancedThing : OriginalThing
{
string name;
static EnhancedThing GetNewThing(string name)
{
EnhancedThing ething = new OriginalThing(); <---doesnt work even if i cast it
ething.Name = name;
}
}
How do I do this?
You can't assign an OriginalThing to a NewThing because it simply is not a NewThing. The other way around works fine because a NewThing is capable of everything an OriginalThing is, but the reverse is not true.
Just create an instance of EnhancedThing, assign the name, and return it. You can treat the EnhancedThing as if it were an OriginalThing because it is an OriginalThing.
class EnhancedThing : OriginalThing
{
public string Name { get; private set; }
static EnhancedThing GetNewThing(string name)
{
EnhanedThing thing = new EnhancedThing();
thing.Name = name;
return thing;
}
}
// ...
OriginalThing foo = EnhancedThing.GetNewThing( "someName" );
Also realize that doesn't buy you much as name is currently a private member variable (in your example), and you won't be able to access any additional functionality of NewThing objects unless you treat them as NewThings (as opposed to OriginalThings, but you can pass them around as OriginalThings if needed)
You need to do this:
EnhancedThing ething = new EnhancedThing();
This is because an OriginalThing is not an EnhancedThing, but an EnhancedThing is an OriginalThing.
One thing you could do is have a constructor for EnhancedThing that takes an OriginalThing and copies over the members that apply.
class EnhancedThing : OriginalThing
{
public EnhancedThing()
{
// default constructor
}
public EnhancedThing( OriginalThing src )
{
// copy over the significant items
}
}
If the other class is sealed you can use encapsulation in your new class, and then modify/extend the API. You can then define a implicit cast from one object to the other and use these types interchangeably without casting.
Whether this is suitable in your case comes down to what you intend to do, and what you are trying to achieve but it's a valuable technique. It's more useful for hiding some/all members of the original and redefining an API.
// no need to cast
EnhancedThing thing = new OriginalThing();
var result = thing.NewMethod();
// again no need to cast, treat as original when passing around
OriginalThing original = thing;
public class EnhancedThing
{
private readonly OriginalThing originalThing;
public static implicit operator OriginalThing(EnhancedThing enhancedThing)
{
return enhancedThing.originalThing;
}
public static implicit operator EnhancedThing(OriginalThing originalThing)
{
return new EnhancedThing(originalThing);
}
private EnhancedThing(OriginalThing originalThing)
{
this.originalThing = originalThing;
}
public string OriginalMethod()
{
return originalThing.OriginalMethod();
}
public string NewMethod()
{
var value = originalThing.OriginalMethod();
// extra processing ...
return value;
}
}
This technique is used extensively in Sitecore to provide different data type Models from a common base implementation. One caveat if you intend to add a new data field, it will be lost on upcast.
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;
}
}