This seems like a simple question, but for some reason I can't find the answer anywhere. Basically, I'd like to be able to implement a constructor that takes NamedParameters.
By named parameters, I do not mean parameters with default values (optional parameters) such as:
public SomeMethod(){
string newBar = Foo(bar2 : "customBar2");
}
public string Foo(string bar1 = "bar1", bar2 = "bar2" ){
//...
}
A good example of what I'm trying to achieve is the AuthorizeAttribute from the System.Web.Mvc assembly. Which you can use the following way:
[Authorize(Roles = "Administrators", Users = "ThatCoolGuy")]
public ActionResult Admin(){
}
The constructor's signature in intellisense looks like the following example and I believe (please confirm) that those NamedParameters are mapping to class properties.
AuthorizeAttribute.AuthorizeAttribute(NamedParameters...)
Initiliaze new instance of the System.Web.Mvc.AuthorizeAttribute class
Named parameters:
Order int
Users string
Roles string
Please note:
The syntax of defining the parameter name when calling a method has nothing to do with optional parameters:
You can use Foo(bar1 : "customBar1"); even if Foo is declared like this: void Foo(string bar1)
To answer the question:
My guess is that this is syntactic sugar similar to the object initializers introduced in Visual Studio 2010 and therefore you can't use this for your own classes.
The behaviour you are talking about is specific for attributes and cannot be reused in "normal" classes constructors.
You don't need to "implement" anything.
The parameters can be used in the manner you describe just by existing as parameters on the constructor.
You do need to be using C# 3.5 or above, when they were introduced.
Your example will compile and run on C# 4.0 / Visual Studio 2010 without modification.
See Named and Optional Arguments (C# Programming Guide) on MSDN.
In regards to properties on the object, that do not have a corresponding constructor arguments, the exact syntax is specific to attributes and can't be replicated, though, with object initializers you can get close.
You can use the builder/constructor info pattern together with property initializers.
class PersonInfo
{
public string Name { get; set; }
public int? Age { get; set; }
public Color? FavoriteColor { get; set; }
public Person BuildPerson()
{
return new Person(this);
}
}
class Person
{
public Person(PersonInfo info)
{
// use info and handle optional/nullable parameters to initialize person
}
...
}
var p = new Person(new PersonInfo { Name = "Peter", Age = 15 });
// yet better
var p = new PersonInfo { Name = "Peter", Age = 15 }.BuildPerson();
I however don't understand, why you don't just use named parameters and provide null for indicating optional parameters.
class Person
{
public Person(string name = null, int? age = null, Color? favoriteColor = null) { /* ... */ }
}
var p = new Person(name: "Peter", age: 15);
Named parameters are NOT specific to attributes. It's a language syntax that can be used everywhere. It's fine to use properties for initialisers but you don't always want to have internals set as set properties.
Just instantiate you class using:
TheClass c = new Theclass(param3:firstValue, param1:secondValue, param2:secondValue);
With regards to this part of the question:
"I however don't understand, why you don't just use named parameters and provide null for indicating optional parameters."
The reason named parameters are nice is you don't need to provide extraneous values in parentheses, just what you want to specify, because if it's optional you shouldn't even need to put null. Furthermore, if you specify null, you are overriding any default value for that parameter which makes it optional. Being optional implies there's a default value meaning nothing passed in.
Property initialisation at instance time is purely there for convenience. Since C there has been the ability to initialise values at construction time on types. Which is handy if those values can't be specified in the constructor. I personally feel that the convenience of them has spoiled people and it get a little too liberal and make everything public get AND set. Just depends on the design and security of properties you need.
I doubt that's possible. This is something specific for attributes.
I think the closest option is to use an object initializer:
class Foo {
public string Name {get;set;}
public int Data {get;set;}
}
var foo = new Foo {Name = "MyName", Data = 12};
try to use this signature
[AttributeUsage(AttributeTargets.Class)]
before the name of your class
Please refer to MSDN specification for full description:
https://msdn.microsoft.com/en-us/library/aa664614(v=vs.71).aspx
"Each non-static public read-write field and property for an attribute class defines a named parameter for the attribute class".
Visual C# 2010 introduces named and optional arguments. Named argument able you to specify an argument for a particular parameter by associating the argument with the parameter's name rather than with the parameter's position in the parameter list.Named arguments free you from the need to remember or to look up the order of parameters in the parameter lists of called methods.
static void Main(string[] args)
{
mapingFunction(snum2: "www.stackoverflow.com", num1: 1);
}
public static void mapingFunction(int num1, string snum2)
{
Console.WriteLine(num1 + " and " + snum2);
}
here you can see that argument are passed with our their order
What you probably want to do is implement public properties in your attribute:
public class AuditFilterAttribute : ActionFilterAttribute
{
public string Message { get; set; }
public AuditFilterAttribute() { }
}
They can be accessed through Named Parameters where you apply it:
[AuditFilter(Message = "Deleting user")]
public ActionResult DeleteUser(int userId)
Hope that helps...
Related
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.
I have a class called Package, in this class I have around 10 attributes, let's call them 1,2,3,4.. etc to 10. The types of these attributes are strings, ints and some DateTimes. When making a new object of Package sometimes I just need attribute 1, sometimes I need 5, 6 and 9, and sometimes I need 3 and 10 etc.
So just two examples: new Package("bla", "bla bla",100) or new Package(2983)
I've read:
An interface looks like a class, but has no implementation. The only
thing it contains are declarations of events, indexers, methods and/or
properties. The reason interfaces only provide declarations is because
they are inherited by classes and structs, which must provide an
implementation for each interface member declared.
Since there are no other methods in the class and just a constructor and attributes, is it better to use like 20 constructors or should I make an interface for this situation?
EDIT:
I should've probably mentioned that I also have some enums to 'kind of' determine what kind of Package it is.
An interface doesn't help you in any way here.
If you want to force that specific variables are filled in together, like 1, 2 and 3 should always be filled together but in another case just 4 is enough, you could use separate constructors, or static methods with helpful names that create the objects (like CreateFromId, CreateFromNameAndAge).
If you don't care at all, you can simply make a parameterless constructor (or a constructor with optional fields) and set the fields required with object initializers:
var x = new Class() { Field1 = 1, Field2 = "2" };
Maybe this is a sign you are doing too much in a single object, but without actual information about your class design, we can't tell that much.
Inheritance seems to be a decent solution here too, if the packages have distinct uses (like ProductPackage, PersonPackage, etc.). The shared properties reside in the base class, and all specific properties can reside in the deriving classes.
Constructors provide guidelines as to how can an object be created. Assuming that by using an interface you mean specify the properties which need to exist, you are not giving any guidelines as to how properties need to be initialized.
Having multiple constructors should be better since you are providing means in which users can instantiate your objects. This will allow you to initialize your other parameters accordingly.
You could still use an interface if you require to stipulate what fields need to exist.
Besides above points, consider using Builder pattern - https://en.wikipedia.org/wiki/Builder_pattern
Here is the example:
class Package
{
public string Name { get; set; }
public string Description { get; set; }
public string Prop1 { get; set; }
public string Prop2 { get; set; }
public static PackageBuilder Create()
{
return new PackageBuilder(new Package());
}
}
class PackageBuilder
{
private readonly Package _package;
public PackageBuilder(Package package)
{
_package = package;
}
public PackageBuilder WithName(string name)
{
_package.Name = name;
return this;
}
public PackageBuilder WithDescription(string description)
{
_package.Description = description;
return this;
}
public PackageBuilder Prop1(string prop)
{
_package.Prop1 = prop;
return this;
}
public PackageBuilder Prop2(string prop)
{
_package.Prop2 = prop;
return this;
}
public static implicit operator Package(PackageBuilder pb)
{
return pb._package;
}
}
class Client
{
Package BuildPackage()
{
var package =
Package.Create().WithName("My Package").WithDescription("Description").Prop1("foo").Prop2("bar");
return package;
}
}
C# 4 introduced a feature called named arguments which is especially useful in scenarios like
int RegisterUser(string nameFirst, string nameLast, string nameMiddle, string email)
Is there a way to force using named arguments? Maybe some attribute to apply to a method or a compiler switch I'm not aware of? I guess it can be done with code inspector tools but just want to know if there is other way.
p.s.
For those interested why one may need it and why not just use a class/struct to utilize object initializers there are scenarios when it's impossible. Like calls to libraries not in your control or weird code conventions you have to obey.
It's possible to force the callers to always use named args. I wouldn't do this in most circumstances because it's rather ugly, but it depends on how badly safe method usage is needed.
Here is the solution:
int RegisterUser(
#if DEBUG
int _ = 0,
#endif
string nameFirst = null,
string nameLast = null,
string nameMiddle = null,
string email = null) { /*...*/ }
The first parameter is a dummy that shouldn't be used (and is compiled away in Release for efficiency). However, it ensures that all following parameters have to be named.
Valid usage is any combination of the named parameters:
RegisterUser();
RegisterUser(nameFirst: "Joe");
RegisterUser(nameFirst: "Joe", nameLast: "Smith");
RegisterUser(email: "joe.smith#example.com");
When attempting to use positional parameters, the code won't compile.
No, not in the C# language. It will always accept positional parameters if all the parameters are supplied.
You could build a custom FxCop rule or an StyleCop rule to enforce this - as pointed out in the comments, it is likely a StyleCop rule you would be interested in (thanks to Kris).
Sorry for a shameless plug!
I implemented a Roslyn analyzer to enforce using named arguments for a method.
So if you install the RequireNamedArgs analyzer and add a special comment before a method that should be invoked with named arguments:
//[RequireNamedArgs]
int RegisterUser(string nameFirst, string nameLast, string nameMiddle, string email)
The analyzer will emit an error if a caller attempts to use positional arguments instead of named.
Take a look at it in action:
If you decide to give it a go -- do so at your own risk :)
I've also sought a way to force named arguments. Optional parameters can be dangerous, especially if you have multiple parameters of the same type. Overloads are almost always a safer solution, but there are times when you have a method that can take many combination of arguments, so creating 20 overloads to account for ever possibility is overkill.
In extreme situations where it is of the utmost importance that arguments be named at all times, I will create an argument class with no defined constructor. In your case, you could do this:
public class UserRegistrationArguments
{
public string nameFirst { get; set; }
public string nameLast { get; set; }
public string nameMiddle { get; set; }
public string email { get; set; }
}
Call it like this:
RegisterUser(new UserRegistrationArguments { nameFirst = "Bob", nameLast = "Slob" });
You could also simplify it like this:
public class UserRegistrationArguments
{
public string nameMiddle { get; set; }
public string email { get; set; }
}
int RegisterUser(string nameFirst, string nameLast, UserRegistrationArguments args = null)
...and do this:
RegisterUser("Bob", "Slob", new UserRegistrationArguments { nameMiddle = "Teh" });
This way, you only have one optional parameter and that's for your optional parameters.
Edit: Maybe I didn't read the OP correctly. You're not using optional arguments? If not then this answer probably doesn't help you.
I'm using another method. In my setup I have 1 parameter which I always expect, then come a bunch of optional strings which I really want to be sure the user chose actively. So my first string in this list is a "trap" value, which if set, throws an error. Like this:
public HtmlString Toolbar(DynamicEntity target = null, string dontRelyOnParameterOrder = Constants.RandomProtectionParameter, string actions = null, string contentType = null, object prefill = null)
{
if (!Enabled) return null;
protectAgainstMissingParameterNames(dontRelyOnParameterOrder);
var toolbar = new ItemToolbar(target, actions, contentType, prefill);
return new HtmlString(toolbar.Toolbar);
}
private void protectAgainstMissingParameterNames(string criticalParameter)
{
if(criticalParameter != Constants.RandomProtectionParameter)
throw new Exception("when using the toolbar command, please use named parameters - otherwise you are relying on the parameter order staying the same.");
}
Hope you like it :)
Today, I was cleaning up some of my code with FXCop and it complained about a Attribute class I had with this violation.
CA1019: Define accessor for attribute argument.
On this page, http://msdn.microsoft.com/en-us/library/ms182136.aspx there is more information, but I still do not get the reason for this as it seems to me more verbose and less relevant.
It gives two codes samples.
using System;
namespace DesignLibrary
{
// Violates rule: DefineAccessorsForAttributeArguments.
[AttributeUsage(AttributeTargets.All)]
public sealed class BadCustomAttribute :Attribute
{
string data;
// Missing the property that corresponds to
// the someStringData parameter.
public BadCustomAttribute(string someStringData)
{
data = someStringData;
}
}
// Satisfies rule: Attributes should have accessors for all arguments.
[AttributeUsage(AttributeTargets.All)]
public sealed class GoodCustomAttribute :Attribute
{
string data;
public GoodCustomAttribute(string someStringData)
{
data = someStringData;
}
//The constructor parameter and property
//name are the same except for case.
public string SomeStringData
{
get
{
return data;
}
}
}
}
I don't understand why the SomeStringData property is required. Isn't the someStringData a parameter? Why does it need to have its own property if it is already stored in another property?
Actually, mine is a little different as it looks like this.
[AttributeUsage(AttributeTargets.Property)]
public sealed class ExampleAttribute : Attribute
{
public ExampleAttribute(string attributeValue)
{
this.Path = attributeValue;
}
public string Name
{
get;
set;
}
// Add to add this to stop the CA1019 moaning but I find it useless and stupid?
public string AttributeValue
{
get
{
return this.Name;
}
}
}
Rather than a private field, I have used a public autoproperty, I had to add the last part to make the warning stop but I don't see the point and it also adds another public field to this class, which is redundant, and seems less clean.
That said, I assume that this warning is raised for a reason so what good reason I am missing here?
Thanks in advance.
FxCop is complaining because your existing property doesn't match the parameter name.
Therefore, it doesn't realize that the parameter actually is exposed.
You should rename the property or parameter to match (except for case), or suppress the warning.
FxCop rule CA1019 is just enforcing the .Net Framework coding guidelines for Attributes.
Use named arguments (read/write properties) for optional parameters. Provide a read/write property with the same name as each named argument, but change the case to differentiate between them.
Documentation Link: http://msdn.microsoft.com/en-us/library/2ab31zeh(v=vs.71).aspx
The reason behind the FxCop warning is that every piece of data you pass into the attribute's constructor should be made publicly available to access when the attribute instance is being retrieved by Reflection.
Let's say you have this:
[BadCustom("My String Data")]
public class DecoratedClass
{
}
How will you get "My String Data" back from that attribute instance when you read it using:
BadCustomAttribute attr = typeof(DecoratedClass)
.GetCustomAttributes(typeof(BadCustomAttribute), false)
.Single() as BadCustomAttribute;
Now you have the instance of your attribute, but no way to read the string passed into the constructor because you didn't at least declare a read-only property for it.
the idea is that you should write just:
[AttributeUsage(AttributeTargets.Property)]
public sealed class ExampleAttribute : Attribute
{
public ExampleAttribute(string attributeValue)
{
this.AttributeValue = attributeValue;
}
public string AttributeValue
{
get;
set;
}
}
This violation will also be thrown when the parameter name matches the property name, but the data types are different.
Does NewExpression.Members inform the LINQ runtime how to map a type's constructor parameters to its properties? And if so, is there an attribute to set the mapping? I'm imagining something like this:
public class Customer
{
public Customer(int id, string name)
{
Id = id;
Name = name;
}
[CtorParam("id")]
public int Id { get; set; }
[CtorParam("name")]
public string Name { get; set; }
}
But none of the MSDN docs really inform you how exactly Members is initialized.
My limited understanding is that you don't usually need to pass the member information; the arguments are taken (by position) from the arguments parameter. The member info is (I suspect) intended to help some internal APIs when dealing with things like anonymous-types, which look (in C#) like they are initialized by member (like an object-initializer), but which are actually initialized by constructor. This means things like LINQ-to-SQL will see a constcutor use, and then (in the next part of the query) access to obj.Name - it needs a way to understand that this means "the 3rd parameter to the constructor (which never actually gets called). In particular for things like groupings.
So this is fine:
var param = Expression.Parameter(typeof(string), "name");
var body = Expression.New(typeof(Customer).GetConstructor(new[] {typeof(int), typeof(string)}),
Expression.Constant(1), param);
var func = Expression.Lambda<Func<string, Customer>>(body, param).Compile();
var cust = func("abc");
If you do need them, I would expect them to be positional relative to the "arguments" expressions - so you would pass in (in an array) the member for id and name. Note that there is also a separate expression for intialzer-style binding.