You’ve done great with lists so far! It’s time to take a look at the bigger picture.

Remember the one line we mentioned at the beginning of this lesson?

using System.Collections.Generic;

The list class is in a group of classes called generic collections. They don’t exist in the default set of System classes, so we need to make a reference to them with this line.

Generic collections are data structures that are defined with a generic type. Each class is defined generally without a specific type in mind. When we make an actual instance, we define the specific type:

List<string> citiesList = new List<string>(); List<Object> objects = new List<Object>();

Imagine it like a set of general instructions: in a toy store, we can tell the employees how to add and remove items from a shelf without specifying the type of toy. In the same way, we can use Add() and Remove() without knowing a lists’s data type.

For this reason, the formal class name of lists is List<T>. That T is a type parameter: it represents some type that we can specify later. The general instructions, however are neatly contained in the generic List<T> class.

Let’s see why this is useful by imagining the other, more difficult ways we could create “generic” collections:

  • Use type-specific classes, like StringList, IntList, etc. — We would have to make a list class for EVERY type, defining the same properties and methods for each list class.
  • Use a list containing Object types, List<Object> — Using Object means we can’t use any of the unique functionality of each type and it takes a lot of computing power to convert references to and from the Object type.

As you continue coding, you’ll see for yourself how useful generic collections are!



Add a reference to the System.Collections.Generic namespace.


Declare three empty lists:

  • one should hold bool types
  • one should hold Random types
  • one should hold IServiceProvider types

That’s right, interfaces work here too!

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