Dictionary contracts and equality rules

The idea

A Dictionary<TKey,TValue> associates unique keys with values under an equality comparer. For textual identifiers, choose case sensitivity intentionally. StringComparer.OrdinalIgnoreCase is useful when a technical identifier should ignore case without applying a culture-specific sorting policy. The comparer is part of the collection’s contract, not a display preference.

TryGetValue reports presence separately from the returned value. A missing integer value and a stored zero can both produce zero in the out variable, so the boolean result matters. The indexer getter throws for absence; its setter adds or replaces. Add rejects an existing key. Use these distinct behaviors deliberately when counting, updating, or detecting duplicates.

using System;
using System.Collections.Generic;
var counts = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);
foreach (var word in new[] {"Red", "red", "Blue"}) {
  counts.TryGetValue(word, out int old);
  counts[word] = old + 1;
}
Console.WriteLine($"{counts.Count} {counts["RED"]}");

2 2

Red, red, and RED identify the same key under the supplied comparer.