C# – initialize a C# attribute with an array or other variable number of arguments

attributesc

Is it possible to create an attribute that can be initialized with a variable number of arguments?

For example:

[MyCustomAttribute(new int[3,4,5])]  // this doesn't work
public MyClass ...

Best Answer

Attributes will take an array. Though if you control the attribute, you can also use params instead (which is nicer to consumers, IMO):

class MyCustomAttribute : Attribute {
    public int[] Values { get; set; }

    public MyCustomAttribute(params int[] values) {
       this.Values = values;
    }
}

[MyCustomAttribute(3, 4, 5)]
class MyClass { }

Your syntax for array creation just happens to be off:

class MyCustomAttribute : Attribute {
    public int[] Values { get; set; }

    public MyCustomAttribute(int[] values) {
        this.Values = values;
    }
}

[MyCustomAttribute(new int[] { 3, 4, 5 })]
class MyClass { }

Related Solutions

C# – String representation of an Enum

Try type-safe-enum pattern.

public sealed class AuthenticationMethod {

    private readonly String name;
    private readonly int value;

    public static readonly AuthenticationMethod FORMS = new AuthenticationMethod (1, "FORMS");
    public static readonly AuthenticationMethod WINDOWSAUTHENTICATION = new AuthenticationMethod (2, "WINDOWS");
    public static readonly AuthenticationMethod SINGLESIGNON = new AuthenticationMethod (3, "SSN");        

    private AuthenticationMethod(int value, String name){
        this.name = name;
        this.value = value;
    }

    public override String ToString(){
        return name;
    }

}

Update Explicit (or implicit) type conversion can be done by

adding static field with mapping
```
private static readonly Dictionary<string, AuthenticationMethod> instance = new Dictionary<string,AuthenticationMethod>();
```
- n.b. In order that the initialisation of the the "enum member" fields doesn't throw a NullReferenceException when calling the instance constructor, be sure to put the Dictionary field before the "enum member" fields in your class. This is because static field initialisers are called in declaration order, and before the static constructor, creating the weird and necessary but confusing situation that the instance constructor can be called before all static fields have been initialised, and before the static constructor is called.
filling this mapping in instance constructor
```
instance[name] = this;
```

and adding user-defined type conversion operator

public static explicit operator AuthenticationMethod(string str)
{
    AuthenticationMethod result;
    if (instance.TryGetValue(str, out result))
        return result;
    else
        throw new InvalidCastException();
}

C# – LINQ’s Distinct() on a particular property

What if I want to obtain a distinct list based on one or more properties?

Simple! You want to group them and pick a winner out of the group.

List<Person> distinctPeople = allPeople
  .GroupBy(p => p.PersonId)
  .Select(g => g.First())
  .ToList();

If you want to define groups on multiple properties, here's how:

List<Person> distinctPeople = allPeople
  .GroupBy(p => new {p.PersonId, p.FavoriteColor} )
  .Select(g => g.First())
  .ToList();

Note: Certain query providers are unable to resolve that each group must have at least one element, and that First is the appropriate method to call in that situation. If you find yourself working with such a query provider, FirstOrDefault may help get your query through the query provider.

Note2: Consider this answer for an EF Core (prior to EF Core 6) compatible approach. https://stackoverflow.com/a/66529949/8155

Best Answer

Related Solutions

C# – String representation of an Enum

C# – LINQ’s Distinct() on a particular property

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