DictionaryExtensions.GetOrAdd.cs

[PublicAPI]
public static partial class DictionaryExtensions
{
    /// <summary>
    ///     Adds a key/value pair to the <see cref="T:System.Collections.Generic.IDictionary`2" /> by using the specified
    ///     function, if the key does not already exist.
    /// </summary>
    /// <param name="dictionary">the dictionary to get or add from</param>
    /// <param name="key">The key of the element to add.</param>
    /// <param name="valueFactory">The function used to generate a value for the key</param>
    /// <returns>
    ///     The value for the key. This will be either the existing value for the key if the key is already in the
    ///     dictionary, or the new value for the key as returned by valueFactory if the key was not in the dictionary.
    /// </returns>
    /// <exception cref="T:System.ArgumentNullException">
    ///     <paramref name="key" /> or <paramref name="valueFactory" /> is <see langword="null" />.
    /// </exception>
    /// <exception cref="T:System.OverflowException">
    ///     The dictionary already contains the maximum number of elements (
    ///     <see cref="F:System.Int32.MaxValue" />).
    /// </exception>
    [ContractAnnotation("dictionary:null => halt; key:null => halt; valueFactory:null => halt")]
    public static TValue GetOrAdd<TKey, TValue>([NotNull] this IDictionary<TKey, TValue> dictionary, [NotNull] TKey key, [NotNull] Func<TKey, TValue> valueFactory)
    {
        if (dictionary == null)
            throw new ArgumentNullException(nameof(dictionary));
        if (key == null)
            throw new ArgumentNullException(nameof(key));
        if (valueFactory == null)
            throw new ArgumentNullException(nameof(valueFactory));

        if (dictionary.TryGetValue(key, out var result))
            return result;

        result = valueFactory(key);
        dictionary.Add(key, result);
        return result;
    }
}