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Properties with Parameters (and Making Them the Default)

Most developers aren't aware that you can write properties that accept parameters, just like a method does. Of course, it isn't often that you need a property that accepts a parameter. One way I recognize that I could use a property rather than a method is when my method name begins with the word "Get." Here, for example, is a method that (I assume) returns the specified part of a name:

Public Class NameManager
  Public Function GetNamePart(NamePosition As Integer) As String
    '...code
  End Function

If I rewrite the method as a property (and rename it), my class code looks like this:

Public Class NameManager
  Public ReadOnly Property NamePart(NamePosition As Integer) As String
    Get
      '...code
    End Get
  End Property

A developer who wants to get the customer's middle name would write code like this:

Dim nm As New NameManager("Peter Hunter Vogel")
Dim middleName As String
middleName = nm.NamePart(2)

You can go one step further and make this property the default property for the class, in which case a developer doesn't even have to use the property name. Here's the relevant code in Visual Basic (which adds the modifier Default to the property's signature):

Public Class NameManager
  Default Public ReadOnly Property NamePart(NamePosition As Integer) As String
    Get
      '...code
    End Get
  End Property

Here it is in C# (which renames the property to the keyword "this"):

public class NameManager
  public string this(int NamePosition) 
    get {
      '...code
   }

Regardless of how the class is written, the code to retrieve the middle name now looks like this:

middleName = nm(2)

Posted by Peter Vogel on 10/29/2015 at 2:20 PM


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