I have requirement in a custom class where I want to make one of my properties required.
How can I make the following property required?
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
If you mean "the user must specify a value", then force it via the constructor:
public YourType(string documentType) {
DocumentType = documentType; // TODO validation; can it be null? blank?
}
public string DocumentType {get;private set;}
Now you can't create an instance without specifying the document type, and it can't be removed after that time. You could also allow the set but validate:
public YourType(string documentType) {
DocumentType = documentType;
}
private string documentType;
public string DocumentType {
get { return documentType; }
set {
// TODO: validate
documentType = value;
}
}
.NET 7 or newer
Syntax
public class MyClass
{
public required string Name { get; init; }
}
new MyClass(); // illegal
new MyClass { Name = "Me" }; // works fine
Remarks
The required properties must declare a setter (either init or set).
Access modifiers on properties or setters cannot be less visible than their containing type, as they would make impossible to initialize the class in some cases.
public class MyClass
{
internal required string Name { get; set; } // illegal
}
Documentation
Official documentation here
Feature demo here
.NET 6 or older
See this answer
If you mean you want it always to have been given a value by the client code, then your best bet is to require it as a parameter in the constructor:
class SomeClass
{
private string _documentType;
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
public SomeClass(string documentType)
{
DocumentType = documentType;
}
}
You can do your validation – if you need it – either in the property's set accessor body or in the constructor.
With the release of .NET 7 and C# 11 in November 2022 you can now use the required modifier this way:
public class Person
{
public Person() { }
[SetsRequiredMembers]
public Person(string firstName) => FirstName = firstName;
public required string FirstName { get; init; }
public int Age { get; set; }
}
And when you don't have the required properties it will throw an error when you try to initialize an object.
For more information refer to:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11#required-members
https://learn.microsoft.com/en-us/dotnet/csharp/properties#init-only
Add a required attribute to the property
Required(ErrorMessage = "DocumentTypeis required.")]
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
For custom attribute detail Click Here
I used an other solution, not exactly what you want, but worked for me fine because I declare the object first and based on specific situation I have different values. I didnt want to use the constructor because I then had to use dummy data.
My solution was to create Private Sets on the class (public get) and you can only set the values on the object by methods. For example:
public void SetObject(string mandatory, string mandatory2, string optional = "", string optional2 = "")
This one liner works in C# 9:
public record Document(string DocumentType);
new Document(); // compiler error
new Document("csv"); // correct way to construct with required parameter
This explains how it works. In the above code, Document is the name of the class or "record". That first line of code actually defines an entire class. In addition to this solution essentially making a required DocumentType property (required by an auto implemented constructor), because it uses records, there are additional implications. So this may not always be an appropriate solution, and the C# 11 required keyword will still come in handy at times. Just using record types doesn't automatically make properties required. The above code is a special syntax way of using records that essentially has this effect as well as making the property init only and causes a deconstructor to be automatically implemented.
A better example would be using an int property instead of a string since a string could still be empty. Unfortunately I don't know of any good way to do extra validation within the record to make sure the string is not empty or an int is in range, etc. You would have to go deeper down the TOP (type driven development) rabbit hole, which may not be a bad thing. You could create your own type that doesn't allow empty strings or integers outside your accepted range. Unfortunately such an approach would lead to runtime discovery of invalid input instead of compile time. There might be a better way using static analysis and metadata, but I've been away from C# for too long to know anything about that.
Related
How to let an Attribute in one property know the existence of another property?
Lets say i have this class, and like this, many others:
public class MyClass
{
[CheckDirty] //a custom attribute (that it is empty for now)
public int A { get; set; }
public int B { get; set; }
public string Description { get; set; }
public string Info { get; set; }
}
Somewhere in our program, if we want to see if an object changed values on any CheckDirty property, for example lets say it is diferent from DB, MyPropertyUtils.GetPropertiesIfDirty() does this, giving us an array of changed propertys, on any property with that attribute:
PropertyInfo[] MyPropertyUtils.GetPropertiesIfDirty(SomeBaseObject ObjectFromDB, SomeBaseObject NewValues);
Perfect.
So, lets say A changed and in this case Info holds some information we need(in another class might be any other property). If we want 'A' we just do property.GetValue(NewValues, null);
But we dont want 'A's value, we want 'A' or CheckDirty to tell us where to read some data we want. How can i tell my attribute CheckDirty where to get the values from?
I was thinking in giving an expression to CheckDirty but an Attribute's argument "must be a constant expression, typeof expression or array creation expression of an attribute parameter type"(thats what VS says).
So I decided, "ok, lets give it a string with the property's name", and so my try failed:
(this is all the code we need to work on, the rest was just to give some kind of context example)
public class CheckDirty : Attribute
{
public String targetPropertyName;
public CheckDirty(String targetPropertyName)
{
this.targetPropertyName = targetPropertyName;
}
}
public class MyClass
{
//Code fails on this line
[CheckDirty(BoundPropertyNames.Info)]
public int Id { get; set; }
public string Info { get; set; }
public static class BoundPropertyNames
{
public static readonly string Info = ((MemberExpression)
((Expression<Func<MyClass, string>>)
(m => m.Info)
).Body
).Member.Name;
}
}
This is the error i get:
An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type
We do NOT want to pass the name of the proprety as a String saing [CheckDirty("Info")] because that way if anyone in the future changes the class, and in concrete the property's name, no error would get thrown in compile time by it, only occuring the error in run time, when an "edit" to that field would occur. Or maybe it would just not do anything because it could not find the property.
Any idea how to not use the strongly typed string as a property name?
You may use something like this, first declare an interface that will be implemented by every class that need dirty checking:
interface IDirtyCheckPropertiesProvider {
string GetPropertyName(string dirtyProperty);
}
then implement it like that
class DataEntity : IDirtyCheckPropertiesProvider {
[CheckDirty]
public int Id { get; set; }
public string Info { get; set; }
string GetPropertyName(string dirtyProperty) {
if (GetPropertyNameFromExpression(x => Id) == dirtyProperty)
return GetPropertyNameFromExpression(x => Info);
return null;
}
}
In class that will be responsible for handling dirty checks you must use this interface to get target property names.
There is a bit too much boilerplate that may be removed further by using Reflection API.
On the other hand using string for property names looks like more simple solution. If you use tool like Resharper - using string is a viable option - Resharper will automatically refactor string when you change property name.
Also for a long time string'ed property names were used in implementation of WPF INotifyPropertyChanged.
As comments suggested nameof is the best option in VS2015.
I have created two properties with same name but the data type is different.
I am getting this error:
The type 'Fields' already contains a definition for 'Subject'
Is it possible to overcome this issue?
public String Subject
{
get { return this.subject; }
set { this.subject = value; }
}
public AppSettings Subject
{
get {
return this.subjectObj; }
set {
this.subjectObj = value; }
}
No, it's not possible. You would not be able to tell these properties apart when using them. How would you know which property is set in this case?
someInstance.Subject = null;
The official documentation on class members states:
The name of a constant, field, property, event, or type must differ
from the names of all other members declared in the same class.
Possible solution:
Your naming suggests that you may want to create another class Subject with at least two properties. (Without more context, I can't say if this is appropriate.)
public class Subject
{
public string Name { get; set; }
public AppSettings Settings { get; set; }
}
This new class could be used in the original class:
public OriginalClass
{
public Subject subject { get; set; }
}
No, you can't overcome that. Imagine you're doing this line:
object myValue = MyClassInstance.Subject;
How would it know how to tell them apart? It's better to give them a more appropriate name:
public String Subject
{
get { return this.subject; }
set { this.subject = value; }
}
public AppSettings Settings
{
get { return this.subjectObj; }
set { this.subjectObj = value; }
}
No you cannot create that. The C# 4 spec, section 10.2:and the standards does not allow you do that
The names of constants, fields, properties, events, or types must
differ from the names of all other members declared in the same class.
On a side note you can refer this thread: Is there a way to use a property with same name but is of different type in derived class?
Most of the answers seem to put forth the reasoning that polymorphism is not allowed in Properties due to something like:
How would you resolve this:
MyObject.MyProperty = null;
Well that's essentially the same as:
MyObject.set_Property(null);
Where set_Property is a method like:
void set_Property(String name) { ... }
void set_Property(AppSettings settings) { ... }
This is legal in C#
So MyObject.MyProperty = null; can be resolved as:
MyObject.set_MyPropert(null);
The compiler will report an Error in the line where the call is made, since it could use both overloads.
But MyObject.set_MyProperty(string.Empty);
is legal all together.
The reason why C# disallows polymorphism on Properties is the same as why it disallows different return types of functions with the same name.
Illigeal:
String get_Property() { return ...; }
AppSettings get_Property() { return ...; }
Now if you do var value = get_Property(); the compiler can not decide which overload to use.
So in principle set only properties could be overloaded. The problem arises when you introduce a getter.
I guess for convenience and consistency reasons the standard disallows polymorphism on properties entirely.
Also because in the language context Properties are not really considered as methods.
I'd like to use the following C#6 code
var joe = new Self();
Console.WriteLine(joe);
... and get the following output:
joe
The following attempt
class Self {
public string Name { get; set; } = nameof(this);
public override string ToString() {
return Name;
}
}
fails as nameof cannot be applied to this. Is it there a workaround for this problem?
EDIT. The scenario I'm working with assures that no two references point to the same Self object.
No, nameof is designed to refer to the compile-time name of the member you're referring to. If you want an object to have a Name property as part of its state, that is independent of how you get to the Name property - as Frédéric Hamidi says, there could be multiple variables (or none) referring to the same object. Basically you need to differentiate between an object and a variable which happens to refer to that object.
However, if you have a constructor to specify the name, you could then use a couple of tricks to make it easier to get the right name:
class Self
{
public string Name { get; }
public Self([CallerMemberName] string name = null)
{
this.Name = name;
}
}
Then:
class Foo
{
private Self me = new Self(); // Equivalent to new Self("me")
public void SomeMethod()
{
// Can't use the default here, as it would be "SomeMethod".
// But we can use nameof...
var joe = new Self(nameof(joe));
}
}
Maybe you can use the following method:
class Self
{
public override string ToString()
{
return this.GetType().Name;
}
}
You can simply use nameof on the variable itself:
Console.WriteLine(nameof(joe));
Here's a working example using the current Roslyn version
The idea for nameof is to make things type safe for specifying program elements during runtime but with compile time type safety checking.
One should atomize what one wants to display. For example in my error messages I include the pertinent information of the class name and the method as such and its checked, so if I change any of the names they are caught as a compile time error:
class Operation
{
public void Execute()
{
try { ... }
catch (Exception ex)
{
Console.Writeline($"{nameof(Operation)}.{nameof(Execute)} has encountered exception:{Environment.NewLine}{Environment.NewLine}{ex.Message}" );
}
}
}
Output
Operation.Excecute has exception:
...
With that said you should override ToString() and report the class name as such
public override string ToString() { return nameof(Self); }
I usually create an internal constant for it when dealing with long class names:
private const string SomeConst = nameof(Self);
Then you can use that in your code:
Console.WriteLine(SomeConst);
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Difference between Property and Field in C# .NET 3.5+
What's the difference between using
public string Username { get; set; }
and using
public string Username;
I have always been using the first one, but wanted to understand if there is any difference between the two, and scenarios when one should prefer over the other.
public string Username { get; set; }
is a Property.
while
public string Username;
is a Public variable.
For more comparison,
Reflection works differently on variables vs. properties, so if you rely on reflection, it's easier to use all properties.
You can't databind against a variable.
Changing a variable to a property is a breaking change.
Other link
Properties vs. Public Variables
One thing you can do with properties that you can't do with fields is limit visibility for either setter or getter:
public string MyProperty { get; private set; }
Something I use quite a lot.
And something (more powerful) you can't do with fields is define them inside an interface. Suppose you want an interface that requires implementing classes to have a certain property:
public interface MyInterface
{
string MyProperty { get; }
}
Note that you do not need to have a setter here. It is entirely up to implementing classes to determine how they should set MyProperty.
Here is a very small example of one way you could use a string property over simply using a string.
Say you have a private variable called:
private string _name;
Now lets say you wanted to make that string read only? In other words, you can't change the value. You could use the following:
public string Name
{
get { return _name; }
}
It can allow you to control access to that value. Alternatively, you can have it so that that variable can only be write only doing the following:
public string Name
{
set { _name = value; }
}
Now if you put it together, it will allow you to set to value or simply get the value. See the following:
public string Name
{
get { return _name; }
set { _name = value; }
}
You may be wondering what the point of that is since it looks like you can do the same thing with a regular string, well of course but this controls direct access to the _name variable from outside classes that aren't derived from said class.
Now what if you wanted to control how that value is set? What if you want to do some calculation or perhaps you wanted to add a prefix or suffix to that value? You do the following:
public string Name
{
get
{
return _name;
}
set
{
if (value.ToLower() == "bilbo")
_name = "Bilbo Baggins";
}
}
Now, if you set the Name property of the class to bilbo, the value of _name will be set to Bilbo Baggins as opposed to if you set the property to Amy, the _name variable will contain simply, amy.
You can do this to guarantee that whatever value that the property is set to is automatically upper or lowercase, or perhaps you can do some validation on the value or something of that sort.
I hope this explains the uses of properties and how they can be useful without making it too complicated.
Properties provide you with more flexibility, especially in .NET. C# shows bias toward properties, so keep that in mind. However, as a general rule, use accessors/mutators when getting or setting needs "processing" or an accompanying action. Use fields for holding values. E.g.,
public class Name
{
public string First;
public string Last;
public string Full{ get { return this.First + " " + this.Last; } }
}
I learned c# recently, so when I learned to write properties, I was taught to do it like this:
public string Name { get; set; }
Auto properties are great! But now I'm trying to do something a little more complicated, so I need to write a custom pair of accessors.
private string _Name;
public string Name {
get { return _Name; }
set { _Name = value }
}
I know the compiler makes a private instance variable down in it's murky depths when one uses autos, but I'm spoiled and don't want that private variable sitting around looking pointless.
Is there a way to use custom accessors without a private variable?
Properties don't need backing variables (fields) at all. While they can be used for encapsulating simple fields you can also use them to access other data.
public Decimal GrandTotal { get { return FreightTotal + TaxTotal + LineTotal; } }
or
public string SomeStatus { get { return SomeMethodCall(); } }
If the goal is to simply encapsulate some field with a property you would need some sort of backing field if you are not using automatic properties.
The answer is No, you cannot do that.
It is because of recursion. (See line numbers 9 and 7):
Line 1 : public string Name
Line 2 : {
Line 3 : get
Line 4 : {
Line 5 : return FirstName + " " + LastName;
Line 6 : }
Line 7 : set
Line 8 : {
Line 9 : Name = value; // <-- Goes back to Line 7
Line 10 : }
Line 11 : }
No, I'm afraid not. The compiler is smart enough to make this happen for you on auto-generated properties, but with standard properties I imagine the logic behind something like that would end up getting in the way and doing more harm than good.
For example, what if I create a property like this...
public int SomeValue
{
get
{
return 0;
}
}
Would the compiler (with the feature you're looking for) create a backing private variable? Why? It doesn't need one.
Additionally, if the private value isn't created until compilation time, what are you going to reference in your code:
public string Name {
get { return _Name; }
set { _Name = value }
}
What is _Name? What if you have another value somewhere else called _Name? Then what would the compiler call the backing value for this property? What if I need two backing values? Would the compiler be smart enough for that?
public string Name
{
get
{
return string.Format("{0} {1}", _FirstName, _LastName);
}
set
{
// some parsing magic
}
}
It's been asked before, but I imagine the answer is going to continue to be "no" for the foreseeable future.
An auto-property is syntactic shorthand for simple direct member access. (And I imagine one of its driving forces was simply to try to get people to stop creating public values directly.) Properties can grow in complexity well beyond that very easily and I personally wouldn't want the compiler trying to figure out what I can easily just tell it to do.
I know this is an old question, but there is at least one other option here. I'm doing something similar to the below for my own app.
This might not exactly be for your use case, but it shows that a custom getter and setter can be used without a private instance variable. In this case, the getter and setter are shortcut or helper methods to access the Name property of the User for the Account.
We can let the value be set by doing Account.AccountUser.Name = "John Doe";, but sometimes that seems a bit clunky and it works against the idea of separation of concerns. Do we want someone using the Account class to know there's a User imbedded in it? If for some reason we don't, we now have a way to still update the User.Name even if we make AccountUser private.
In this case, AccountUser is public, but it doesn't have to be. When it's private, a Json or XML conversion utility (such as Newtonsoft) should ignore the AccountUser and show just the Name as if the Account were a flat model, instead of having multiple levels.
public class User
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Account
{
public int Id { get; set; }
public User AccountUser { get; set; }
public string Name
{
get
{
return AccountUser.Name;
}
set
{
AccountUser.Name = value;
}
}
}