C# Indexers — NullReference Ambush in String-Indexed Stores

using System;

namespace io.thecodeforge
{
    /// <summary>
    /// Stores hourly temperature readings for a single day.
    /// Demonstrates the most fundamental indexer pattern.
    /// </summary>
    public class TemperatureLog
    {
        // Internal storage — callers never touch this directly
        private readonly double[] _hourlyReadings = new double[24];

        // --- THE INDEXER ---
        // 'this[int hour]' is the syntax. 'hour' is the index parameter.
        // It behaves exactly like a property but accepts an argument.
        public double this[int hour]
        {
            get
            {
                // Validate before blindly returning data
                if (hour < 0 || hour > 23)
                    throw new ArgumentOutOfRangeException(nameof(hour),
                        "Hour must be between 0 and 23.");

                return _hourlyReadings[hour];
            }
            set
            {
                if (hour < 0 || hour > 23)
                    throw new ArgumentOutOfRangeException(nameof(hour),
                        "Hour must be between 0 and 23.");

                // 'value' is the implicit keyword, just like in a regular property setter
                _hourlyReadings[hour] = value;
            }
        }
    }

    class Program
    {
        static void Main()
        {
            var todayLog = new TemperatureLog();

            // SET via indexer — reads just like array assignment
            todayLog[9]  = 18.5;   // 9 AM reading
            todayLog[12] = 22.1;   // noon reading
            todayLog[18] = 19.8;   // 6 PM reading

            // GET via indexer
            Console.WriteLine($"9 AM  : {todayLog[9]} °C");
            Console.WriteLine($"Noon  : {todayLog[12]} °C");
            Console.WriteLine($"6 PM  : {todayLog[18]} °C");

            // This will throw ArgumentOutOfRangeException — hour 25 is invalid
            try
            {
                double bad = todayLog[25];
            }
            catch (ArgumentOutOfRangeException ex)
            {
                Console.WriteLine($"Error: {ex.Message}");
            }
        }
    }
}

using System;
using System.Collections.Generic;

namespace io.thecodeforge
{
    /// <summary>
    /// A simple key-value settings store with a string indexer.
    /// Models the same access pattern as IConfiguration in ASP.NET Core.
    /// </summary>
    public class AppSettings
    {
        // Backing store — private so callers can't manipulate it directly
        private readonly Dictionary<string, string> _settings
            = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

        // String indexer — key is case-insensitive thanks to the comparer above
        public string? this[string key]
        {
            get
            {
                // Return null for missing keys instead of throwing —
                // lets callers use the null-coalescing operator cleanly
                return _settings.TryGetValue(key, out string? found) ? found : null;
            }
            set
            {
                if (string.IsNullOrWhiteSpace(key))
                    throw new ArgumentException("Setting key cannot be empty.", nameof(key));

                if (value is null)
                    // Treat assigning null as 'remove this setting'
                    _settings.Remove(key);
                else
                    _settings[key] = value;
            }
        }

        public int Count => _settings.Count;
    }

    class Program
    {
        static void Main()
        {
            var settings = new AppSettings();

            // Write settings via indexer
            settings["Theme"]           = "Dark";
            settings["MaxRetries"]      = "3";
            settings["DatabaseTimeout"] = "30";

            // Read back — case-insensitive because of our StringComparer
            Console.WriteLine($"Theme: {settings["theme"]}");          // lowercase key still works
            Console.WriteLine($"Max retries: {settings["MaxRetries"]}");

            // Null-coalescing works great with a null-returning getter
            string timeout = settings["CacheTimeout"] ?? "60";  // key doesn't exist, use default
            Console.WriteLine($"Cache timeout (default): {timeout}s");

            // Remove a setting by assigning null
            settings["Theme"] = null;
            Console.WriteLine($"Theme after removal: {settings["Theme"] ?? "(not set)"}");

            Console.WriteLine($"Total settings remaining: {settings.Count}");
        }
    }
}

using System;

namespace io.thecodeforge
{
    /// <summary>
    /// A simple 2D grid that supports both positional and Excel-style cell access.
    /// Demonstrates multi-parameter indexers AND indexer overloading.
    /// </summary>
    public class SpreadsheetGrid
    {
        private readonly string[,] _cells;
        public int Rows { get; }
        public int Columns { get; }

        public SpreadsheetGrid(int rows, int columns)
        {
            Rows    = rows;
            Columns = columns;
            _cells  = new string[rows, columns];
        }

        // OVERLOAD 1: Positional access — grid[row, col]
        // Multi-parameter indexers use a comma-separated list, just like multi-dim arrays
        public string this[int row, int col]
        {
            get
            {
                ValidateBounds(row, col);
                return _cells[row, col] ?? string.Empty;
            }
            set
            {
                ValidateBounds(row, col);
                _cells[row, col] = value;
            }
        }

        // OVERLOAD 2: Excel-style access — grid["B3"] means column B (index 1), row 3 (index 2)
        // This overload has a DIFFERENT parameter type (string vs int, int)
        // so the compiler knows which one to call
        public string this[string cellAddress]
        {
            get
            {
                (int row, int col) = ParseAddress(cellAddress);
                return this[row, col]; // delegate to the positional indexer — no code duplication
            }
            set
            {
                (int row, int col) = ParseAddress(cellAddress);
                this[row, col] = value; // same delegation pattern
            }
        }

        // Parses "B3" into (row: 2, col: 1) — zero-based
        private static (int row, int col) ParseAddress(string address)
        {
            if (string.IsNullOrEmpty(address) || address.Length < 2)
                throw new ArgumentException($"Invalid cell address: '{address}'");

            char columnLetter = char.ToUpper(address[0]);
            int col = columnLetter - 'A';                  // 'A' -> 0, 'B' -> 1, etc.
            int row = int.Parse(address[1..]) - 1;         // "3" -> row index 2 (zero-based)
            return (row, col);
        }

        private void ValidateBounds(int row, int col)
        {
            if (row < 0 || row >= Rows || col < 0 || col >= Columns)
                throw new ArgumentOutOfRangeException(
                    $"Cell ({row},{col}) is outside grid bounds ({Rows}x{Columns}).");
        }
    }

    class Program
    {
        static void Main()
        {
            var sheet = new SpreadsheetGrid(rows: 5, columns: 4);

            // Write using positional indexer (row, col) — zero-based
            sheet[0, 0] = "Name";
            sheet[0, 1] = "Score";
            sheet[1, 0] = "Alice";
            sheet[1, 1] = "95";

            // Write using Excel-style indexer — 1-based, column letter
            sheet["C1"] = "Grade";   // row 0, col 2
            sheet["C2"] = "A";       // row 1, col 2

            // Read using both styles — they access the same underlying data
            Console.WriteLine($"Positional [0,0]: {sheet[0, 0]}");
            Console.WriteLine($"Excel-style A1  : {sheet["A1"]}");
            Console.WriteLine();
            Console.WriteLine($"[0,1] = {sheet[0, 1]},  B1 = {sheet["B1"]}");  // same cell
            Console.WriteLine($"[1,0] = {sheet[1, 0]},  A2 = {sheet["A2"]}");  // same cell
            Console.WriteLine($"[1,2] = {sheet[1, 2]},  C2 = {sheet["C2"]}");  // same cell
        }
    }
}

using System;
using System.Collections.Generic;

namespace io.thecodeforge
{
    /// <summary>
    /// Interface declares a read-only string indexer.
    /// Any class implementing this can be swapped out or mocked in tests.
    /// </summary>
    public interface ICountryLookup
    {
        // Read-only indexer in an interface — only 'get' is declared
        string? this[string isoCode] { get; }
        bool Contains(string isoCode);
    }

    /// <summary>
    /// Concrete implementation — backed by a static dictionary.
    /// Could be swapped for a database-backed version without changing callers.
    /// </summary>
    public class CountryCodeLookup : ICountryLookup
    {
        private readonly Dictionary<string, string> _countries;

        public CountryCodeLookup()
        {
            // Pre-populated with a handful of ISO 3166-1 alpha-2 codes
            _countries = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
            {
                ["US"] = "United States",
                ["GB"] = "United Kingdom",
                ["DE"] = "Germany",
                ["JP"] = "Japan",
                ["BR"] = "Brazil"
            };
        }

        // Implements the interface's read-only indexer
        // The class itself only exposes get — there's no way for callers to pollute the data
        public string? this[string isoCode]
            => _countries.TryGetValue(isoCode, out string? name) ? name : null;

        public bool Contains(string isoCode) => _countries.ContainsKey(isoCode);
    }

    /// <summary>
    /// Consumes ICountryLookup — it only knows about the interface,
    /// not the concrete class. Fully testable.
    /// </summary>
    public class CountryReportGenerator
    {
        private readonly ICountryLookup _lookup;

        public CountryReportGenerator(ICountryLookup lookup)
        {
            _lookup = lookup;
        }

        public void PrintCountryName(string isoCode)
        {
            // Uses the indexer through the interface reference
            string display = _lookup[isoCode] ?? $"Unknown ({isoCode})";
            Console.WriteLine($"{isoCode.ToUpper()} => {display}");
        }
    }

    class Program
    {
        static void Main()
        {
            ICountryLookup lookup = new CountryCodeLookup();
            var generator = new CountryReportGenerator(lookup);

            generator.PrintCountryName("US");
            generator.PrintCountryName("gb");   // lowercase — case-insensitive
            generator.PrintCountryName("JP");
            generator.PrintCountryName("ZZ");   // not in the lookup

            // Direct interface usage — read-only, clean API
            Console.WriteLine();
            Console.WriteLine($"Contains 'DE': {lookup.Contains("DE")}");
            Console.WriteLine($"DE full name : {lookup["DE"]}");
        }
    }
}

using System;
using System.Collections.Generic;
using System.Threading;

namespace io.thecodeforge
{
    /// <summary>
    /// A thread-safe settings store using ReaderWriterLockSlim.
    /// Optimized for many reads, few writes.
    /// </summary>
    public class ThreadSafeConfig
    {
        private readonly Dictionary<string, string> _store
            = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
        private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();

        public string? this[string key]
        {
            get
            {
                _lock.EnterReadLock();
                try
                {
                    return _store.TryGetValue(key, out string? value) ? value : null;
                }
                finally
                {
                    _lock.ExitReadLock();
                }
            }
            set
            {
                _lock.EnterWriteLock();
                try
                {
                    if (value is null)
                        _store.Remove(key);
                    else
                        _store[key] = value;
                }
                finally
                {
                    _lock.ExitWriteLock();
                }
            }
        }

        public int Count
        {
            get
            {
                _lock.EnterReadLock();
                try { return _store.Count; }
                finally { _lock.ExitReadLock(); }
            }
        }
    }
}

// io.thecodeforge — csharp tutorial

public class ConfigStore
{
    private string[] _values = new string[10];

    // Public get, private set — external reads, internal writes only
    public string this[int index]
    {
        get => _values[index];
        private set => _values[index] = value;
    }

    public void Initialize()
    {
        // Only this method can write via the indexer
        this[0] = "connection_string";
        this[1] = "readonly_user";
    }
}

// Usage
var config = new ConfigStore();
config.Initialize();
Console.WriteLine(config[0]);  // connection_string
// config[0] = "new_value"; // Compile error: setter is private

// io.thecodeforge — csharp tutorial

public class SparseMatrix
{
    private Dictionary<(int, int), double> _store = new();

    // Row/column access
    public double this[int row, int col]
    {
        get => _store.TryGetValue((row, col), out var val) ? val : 0.0;
        set => _store[(row, col)] = value;
    }

    // Flat index access (row-major)
    public double this[int flatIndex]
    {
        get
        {
            int row = flatIndex / 100;
            int col = flatIndex % 100;
            return this[row, col];
        }
        set
        {
            int row = flatIndex / 100;
            int col = flatIndex % 100;
            this[row, col] = value;
        }
    }
}

// Usage
var matrix = new SparseMatrix();
matrix[0, 0] = 1.5;
Console.WriteLine(matrix[0]);  // Outputs 1.5

// io.thecodeforge — csharp tutorial

using System.Collections.Generic;

public class RegionalSalesMap
{
    private readonly Dictionary<(string Region, string Quarter), decimal> _data = new();

    public decimal this[string region, string quarter]
    {
        get => _data.TryGetValue((region, quarter), out var val) ? val : 0m;
        set => _data[(region, quarter)] = value;
    }
}

// Usage:
var sales = new RegionalSalesMap();
sales["EU", "Q3"] = 142000.50m;
System.Console.WriteLine(sales["EU", "Q3"]);  // 142000.50
System.Console.WriteLine(sales["APAC", "Q1"]); // 0

// io.thecodeforge — csharp tutorial

// DO use indexers for:
// - In-memory caches:   cache["userId"]
// - Config maps:        config["ConnectionString"]
// - Lookup tables:      matrix[row, col]

// DON'T use indexers for:
// - Database queries:   use FetchUser(id) not user[id]
// - Heavy computation:  use ComputeRiskScore() not risk[policyId]
// - Async operations:   GetAsync() never indexers

// Senior rule: if the getter can't return in < 1ms,
// don't call it with bracket syntax.