// O(1) time complexity for existence check
var allowedIds = new HashSet<int> { 1, 2, 3, 4, 5 };

if (allowedIds.Contains(userId))
{
    // Allowed user
}

// Set operations
setA.IntersectWith(setB); // Intersection
setA.UnionWith(setB);     // Union
setA.ExceptWith(setB);    // Difference

using System.Collections.Frozen;

// Immutable fast lookup (read-only scenarios)
var allowedExtensions = new[] { ".jpg", ".png", ".gif" }
    .ToFrozenSet(StringComparer.OrdinalIgnoreCase);

if (allowedExtensions.Contains(fileExtension))
{
    // Allowed extension
}

// ❌ Two lookups
if (dict.ContainsKey(key))
{
    var value = dict[key];
}

// ✅ Single lookup
if (dict.TryGetValue(key, out var value))
{
    // Use value
}

// Lookup with default value
var value = dict.GetValueOrDefault(key, defaultValue);

// Case-insensitive string comparison
var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
set.Add("Hello");
set.Contains("HELLO"); // true