Indexers

Indexers

An indexer lets you access an object like an array: list[3], matrix[1, 2], board["e4"]. An indexer is declared as a property named this with parameters in square brackets:

public double this[int power]
{
    get => power < coefficients.Length ? coefficients[power] : 0;
    set => coefficients[power] = value;
}

Features of indexers:

• the get and set accessors work as in properties; a read-only indexer has only get;
• there can be several parameters (this[int row, int col]) of any type, including strings (this[string square]);
• indexers can be overloaded by the number and types of parameters;
• an indexer should check bounds and throw ArgumentOutOfRangeException or IndexOutOfRangeException for invalid indexes.

The access m[1, 2] = 5 calls the set accessor with the parameters row = 1, col = 2, and value = 5 (Fig. 12.4). In the debugger, you can evaluate indexer values in the Watch window (Fig. 12.5).

Figure 12.4. An indexer for a two-dimensional matrix

Figure 12.5. Evaluating indexers in the Watch window

Indexes from the end and ranges

Indexes from the end ^1 and ranges 1..3 (Topic 4) also work with your own types. If a type has a Length or Count property and an indexer with an int parameter, the compiler converts the expression seq[^1] into seq[seq.Length - 1]. Ranges also require a Slice(int start, int length) method:

Readings r = new();
Console.WriteLine(r[^1]);                     // 14,2
Console.WriteLine(string.Join("; ", r[1..3])); // 13,1; 12,9

class Readings
{
    private readonly double[] values = [12.5, 13.1, 12.9, 14.2];

    public int Length => values.Length;
    public double this[int index] => values[index];
    public double[] Slice(int start, int length) =>
        values[start..(start + length)];
}