Previous Page
Next Page

What Is an Indexer?

An indexer is a smart array in exactly the same way that a property is a smart field. The syntax that you use for an indexer is exactly the same as the syntax you use for an array. Let's work through an example. First, we'll examine a problem and see a weak solution that doesn't use indexers. Then we'll work through the same problem and look at a better solution that does use indexers. The problem concerns integers, or more precisely, the int type.

An Example That Doesn't Use Indexers

You normally use an int to hold an integer value. Internally an int stores its value as a sequence of 32 bits, where each bit can be either 0 or 1. Most of the time you don't care about this internal binary representation; you just use an int type as a bucket to hold an integer value. However, sometimes programmers use the int type for other purposes; some programs manipulate the individual bits within an int. (If you are an old C programmer you should feel at home with what follows!) In other words, occasionally a program might use an int because it holds 32 bits and not because it can represent an integer.

Some older programs might use int types to try to save memory. A single int holds 32 bits, each of which can be 1 or 0. In some cases, programmers assigned 1 to indicate a value of true and 0 to indicate false, and then employed an int as a set of Boolean values.

For example, the following expression uses the << and & bit manipulation operators to find out whether the bit at index 6 of the int called bits is set to 0 or to 1:

(bits & (1 << 6)) != 0

If the bit at index 6 is 0, this expression evaluates to false; if the bit at index 6 is 1, this expression evaluates to true. This is a fairly complicated expression, but it's trivial in comparison to the following expression that sets the bit at index 6 to 0:

bits &= ~(1 << 6)

It's also trivial compared with this expression that sets the bit at index 6 to 1:

bits |= (1 << 6)

The trouble with these examples is that although they work, it's not clear why or how they work. They're complicated and the solution is a very low-level one. It fails to create an abstraction of the problem it solves.

The Same Example Using Indexers

Let's pull back from the previous low-level solution for a moment and stop to remind ourselves what the problem is. We'd like to use an int not as an int but as an array of 32 bits. Therefore, the best way to solve this problem is to use an int as if it were an array of 32 bits! In other words, if bits is an int, what we'd like to be able to write to access the bit at index 6 is:


And, for example, set the bit at index 6 to true, we'd like to be able to write:

bits[6] = true

Unfortunately, you can't use the square bracket notation on an int. It only works on an array or on a type that behaves like an array; that is, on a type that declares an indexer. So the solution to the problem is to create a new type that acts like, feels like, and is used like an array of bool variables but is implemented by using an int. Let's call this new type IntBits. IntBits will contain an int value (initialized in its constructor), but the idea is that we'll use IntBits as an array of bool variables.

Because IntBits is small and lightweight, it makes sense to create it as a struct rather than as a class.
struct IntBits
    public IntBits(int initialBitValue)
        bits = initialBitValue;

    // indexer to be written here

    private int bits;

To define the indexer, you use a notation that is a cross between a property an an array. The indexer for the IntBits struct looks like this:

struct IntBits
    public bool this [ int index ]
            return (bits & (1 << index)) != 0; 
            if (value)  // Turn the bit on if value is true, otherwise turn it off
                bits |=  (1 << index);
                bits &= ~(1 << index);

Notice the following:

After the indexer has been declared, we can use a variable of type IntBits instead of an int and apply the square bracket notation as desired:

int adapted = 63;
IntBits bits = new IntBits(adapted);
bool peek = bits[6];  // retrieve bool at index 6
bits[0] = true;       // set the bit at index 0 to true
bits[31] = false;     // set the bit at index 31 to false

This syntax is certainly much easier to understand. It directly and succinctly captures the essence of the problem.

Indexers and properties are similar in that both use get and set accessors. An indexer is like a property with multiple values. However, although you're allowed to declare static properties, static indexers are illegal.
Understanding Indexer Accessors

When you read an indexer, the compiler automatically translates your array-like code into a call to the get accessor of that indexer. For example, consider the following example:

bool peek = bits[6];

This statement is converted into a call to the get accessor for bits, and the value of the index argument is set to 6.

Similarly, if you write to an indexer, the compiler automatically translates your array-like code into a call to the set accessor of that indexer, setting the index argument to the specified value. For example, consider the following statement:

bits[6] = true;

This statement is converted into a call to the set accessor for bits where the value of index is 6. As with ordinary properties, the value you are writing to the indexer (in this case, true) is made available inside the set accessor by using the value keyword. The type of value is the same as the type of indexer itself (in this case, bool).

It's also possible to use an indexer in a combined read/write context. In this case, the get and set accessors are used. For example, consider the following statement:

bits[6] ^= true;

This is automatically translated into:

bits[6] = bits[6] ^ true;

This code works because the indexer declares both a get and a set accessor.

You're also allowed to declare an indexer that contains only a get accessor (a read-only indexer), or a set accessor (a write-only accessor).
Comparing Indexers and Arrays

When you use an indexer, the syntax is deliberately very array-like. However, there are some important differences between indexers and arrays:

Previous Page
Next Page