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Making Generic Classes Useful with Constraints

Many developers are aware that they can write a method that works with many different kinds of objects without using the Object data type: Just write a generic class or method. For example, this method will work with any class, provided that the developer specifies the class when calling the method:

Public Sub MyMethod(Of T)()
  Dim im As T
  '...using the variable im
End Sub

To use this method with a Customer class, a developer would write code like this:

 MyMethod(of Customer)

The problem is that you can't do much with the variable declared as T. You can't instantiate the class with code like this, for example:

Public Sub MyMethod(Of T)()
  Dim im As T
  im = New T
  '...using the variable im
End Sub

That is, unless you promise to .NET that the class will have a constructor (in Visual Basic, a New method) that accepts no parameters. A version of the method that would let you instantiate the class would look like this:

Public Sub MyMethod(Of T As New)()
  Dim im As T
  im = New T
  '...using the variable im
End Sub

Of course, you do have to specify a Class that has a parameterless constructor when you call the method -- you've constrained the number of classes that will work with this method.

You can specify other constraints on the class: that the class must inherit from some other class and/or implement a specific interface, for example. If you do, you'll be able to use methods or properties defined in those classes or interfaces in your method. You can even combine multiple constraints by enclosing them in curly braces ({}).

The following example requires that the class used in the method inherit from Customer, implement the ICreditRating interface, and have a parameterless constructor. That doesn't leave a lot of classes that can be used with this method, but the trade-off is that you can do more with the class in the method:

Public Sub MyMethod(Of T As {New, Customer, ICreditRating})()
  Dim im As T
  im = New T
  im.FirstName = "Peter"
End Sub

Posted by Peter Vogel on 05/27/2014 at 11:14 AM


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