C#

Quote:

if I had to implement "lazy" loading like feature here, I would simply just set it to null

// C# 7: using expression-body member as 'setter or 'getter now allowed in properties that are not read-only
public List<PersonNode> AllNodes
{
    get => _allNodes ?? (_allNodes = new List<PersonNode>());
    private set => _allNodes = value;
}

SELECT TOP 10 FROM table_name ... 

BillWoodruff wrote:

instantiating one with 'new does not allocate memory for what it wraps

public class LazyDictionary<TKey, TValue> : IReadOnlyDictionary<TKey, TValue>
{
    private readonly ConcurrentDictionary<TKey, TValue> _values;
    private readonly Func<TKey, TValue> _valueFactory;
    private readonly Func<TKey, bool> _keyValidator;
    
    public LazyDictionary(Func<TKey, TValue> valueFactory, Func<TKey, bool> keyValidator = null, IEqualityComparer<TKey> keyComparer = null)
    {
        if (valueFactory == null) throw new ArgumentNullException(nameof(valueFactory));
        if (keyComparer == null) keyComparer = EqualityComparer<TKey>.Default;
        _values = new ConcurrentDictionary<TKey, TValue>(keyComparer);
        _valueFactory = valueFactory;
        _keyValidator = keyValidator;
    }
    
    private bool KeyIsValid(TKey key) => _keyValidator == null || _keyValidator(key);
    
    private TValue GetOrAdd(TKey key) => _values.GetOrAdd(key, _valueFactory);
    
    public int Count => _values.Count;
    
    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() => _values.GetEnumerator();
    
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    
    public IEnumerable<TKey> Keys => _values.Keys;
    
    public IEnumerable<TValue> Values => _values.Values;
    
    public bool ContainsKey(TKey key) => KeyIsValid(key);
    
    public bool TryGetValue(TKey key, out TValue value)
    {
        if (!ContainsKey(key))
        {
            value = default(TValue);
            return false;
        }
        
        value = GetOrAdd(key);
        return true;
    }
    
    public TValue this[TKey key]
    {
        get
        {
            if (!ContainsKey(key)) throw new KeyNotFoundException();
            return GetOrAdd(key);
        }
    }
}

public IReadOnlyDictionary<Division, IList<Person>> DivisionToStaff { get; }
   = new LazyDictionary<Division, IList<Person>>(d => new List<Person>(), d => Enum.IsDefined(typeof(Division), d));

...

DivisionToStaff[personInstance.Division].Add(personInstance);

Member 13196574 wrote:

it would be wonderful to find any example or tutorial

Pete O'Hanlon wrote:

What do you need from us?

Quote:

Shazam uses a smartphone or computer's built-in microphone to gather a brief sample of audio being played. It creates an acoustic fingerprint based on the sample and compares it against a central database for a match.