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c#listattributes
marcuthh
marcuthh

Reputation: 596

C# - add attribute like '.Length' to custom class

I've playing around with a class that acts as a public interface for a private List<T> attribute. I noticed that the List<> class has an attribute Length that tells you how many elements it contains.

This is an attribute you cannot alter, and on the intellisense appears with an image of a spanner next to it. It is not a method as it does not require () after coding the name.

I've seen attributes of this type before, but never used them in my own classes. Does anybody have any idea how I can replicate Length in my custom class?

Thanks, Mark

Upvotes: 0

Views: 1025

Answers (6)

Barry O&#39;Kane
Barry O&#39;Kane

Reputation: 1187

Say for example you have a class like this:

public class MyClass
{
   private string _str;
   public MyClass()
   {
       _str = "Sample String";
   }

   public int Length
   {
       get
       {
           return _str.Length;
       }
   }
}

This is what's happening:

  • We're declaring a private field at the start of the class named _str.
  • In the constructor we're then assigning it a value of "Sample String".
  • After the constructor we're then declaring the public attribute Length of type int, and only giving it a get accessor. Like your example, this only allows the value to be read, and not set.
  • Within the get we then tell it to return the value of _str's length.

Using code similar to this you can implement a Length attribute for any custom class.

Upvotes: 1

Edmond Quinton
Edmond Quinton

Reputation: 1739

I must admit that your question is a bit vague. It sounds like you want know how to create a read only attribute / property. This can be achieved by creating a property wrapper for a private field member of your class as follow:

    class MyCustomClass
    {
        private int _length;

        public int Length
        {
            get { return _length; }
        }

    }

Upvotes: 1

user4189304
user4189304

Reputation: 

List<T> myList = new List<T>();

Now you can create your own implementation on your custom class. Something like:

   public int Length {get {return myList.Count; }}

Upvotes: 1

jason.kaisersmith
jason.kaisersmith

Reputation: 9610

This is actually not a method, but a property.

So you could have define in your class

private List<string> myList = new List<string>();

public int NumberOfElements
{
    get { return this.myList.Count; }
}

A normal property would be defined such as 

public bool ColumnNames { get; set; }

Upvotes: 1

Rion Williams
Rion Williams

Reputation: 76557

If you currently have a class that contains a List, then you can take advantage of the Count property already present on it by exposing a property that simply uses that :

public class YourExampleList<T>
{
     // Example of your inner list
     private List<T> _list { get; set; }
     // Use the Count property to expose a public "Length" equivalent
     public int Length { get { return _list.Count; } }   
}

Upvotes: 1

DrewJordan
DrewJordan

Reputation: 5314

It's a property with no setter. If you're wrapping a List<T> you can just use it's Count as your own:

public int Count {get {return _myPrivateList.Count; } }

If you're using C# 6, you can use this:

public int Count => _myPrivateList.Count;

Upvotes: 5

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