# 11.1. List Syntax¶

Arrays are fine if you know ahead of time how long your sequence of items is. Then you create your array with that length, and you are all set.

If you want a variable sized container, you are likely to want a List. As with arrays, you might want a collection of any particular type. Unfortunately, you cannot use the simple notation of arrays to specify the type of element in a List. Array syntax is built into the language. Lists are handled in the library of types provided by C# from the .Net framework. There are all sorts of situations where you might want a general idea to have a version for each of many kinds of objects. There is not a new syntax for each one.

## 11.1.1. Generics¶

Instead .Net 4.0 introduced one new form of syntax that can apply to all sorts of classes, generics.

The type for a list of strings is

List<string>


The type for an int list is

List<int>


In general the new generic syntax allows a type (or several, comma separated) in angle brackets after a class name. Lists are an example that depends on just one included type. We will see more shortly.

There is a namespace for the generics for collections, including List: System.Collections.Generic.

We will use several generic library classes, though we will not write the definitions of new generic classes ourselves.

## 11.1.2. List Constructors and Methods¶

We can play with some List methods in csharp. Note that csharp informally displays the value of a List with a list of elements inside braces. This is not a legal way to assign values to lists.

The blocks below are all from one csharp session, with our comments breaking up the sequence.

With the no-parameter constructor, the List is empty to start:

csharp> List<string> words = new List<string>();
csharp> words;
{  }
csharp> words.Count;
0


You can add elements, and keep count with the Count property as the size changes:

csharp> words.Add("up");
csharp> words;
{ "up" }
csharp> words;
{ "up", "down" }
csharp> words;
{ "up", "down", "over" }
csharp> words.Count;
3


You can reference and change elements by index, like with arrays:

csharp> words[0];
"up"
csharp> words[2];
"over"
csharp> words[2] = "in";
csharp> words;
{ "up", "down", "in" }


You can use foreach like with arrays or other sequences:

csharp> foreach (string s in words) {
>    Console.WriteLine(s.ToUpper());
> }
UP
DOWN
ON


Note: Unfortunately C# is not user-friendly if you try to use Console.WriteLine to print a List object:

csharp> Console.WriteLine(words)
System.Collections.Generic.List1[System.Int32]


Next compare Remove, which finds the first matching element and removes it, and RemoveAt, which removes the element at a specified index. Remove returns whether the List has been changed:

csharp> words.Remove("down");
true
csharp> words;
{ "up", "in" }
csharp> words.Remove("around"); // no change
false
csharp> words;
{ "up", "in", "out", "on" }
csharp> words.RemoveAt(2); // "out" is at index 2
csharp> words;
{ "up", "in", "on" }


Removing does not leave a “hole” in the List: The list closes up, so the index decreases for the elements after the removed one:

csharp> words[2];
"on"
csharp> words.Count;
3


You can check for membership in a List with Contains:

csharp> words.Contains("in");
true
csharp> words.Contains("into");
false


You can also remove all elements at once:

csharp> words.Clear();
csharp> words.Count;
0


Here is a List containing int elements. Though more verbose than for an array, you can initialize a List with another collection, including an anonymous array, specified with an explicit sequence in braces:

csharp> List<int> nums = new List<int>(new[]{5, 3, 7, 4});
csharp> nums;
{ 5, 3, 7, 4 }


We have been using the explicit declaration syntax, but generic types tend to get long, so var is handy with them:

var stuff = new List<string>();


When initializing a generic object, you still need to remember both the angle braces around the type and the parentheses for the parameter list after that.

An aside on the Remove method: It both causes a side effect, changing the list, and it returns a value. If a function returns a value, we typically use the function call as an expression in a larger statement. This is not necessary, as described in Not using Return Values. In that section we discussed the mistake of not using return values. The Remove method illustrates that this is not always a mistake: If you just want the side effect, trying to remove an element, whether or not it is in the list, then there is no need to check for the return value. This complete C# statement is fine:

someList.Remove(element);


You should generally think carefully before defining a function that both has a side effect and a return value. Most functions that return a value do not have a side effect. If you see a function used in the normal way as an expression, it is easy to forget that it was also producing some side effect.

## 11.1.3. Interactive List Example¶

Lists are handy when you do not know how much data there will be. A simple example would be reading in lines from the user interactively:

/// Return a List of lines entered by the user in response
/// to the prompt.  Lines in the List will be nonempty, since an
/// empty line terminates the input.
`