RicoSuter/NSwag · error · ArgumentNullException

items

Error message

items

What it means

ObservableDictionary.AddRange throws ArgumentNullException when the items dictionary passed in is null. The method requires a non-null IDictionary<TKey,TValue> because it iterates and inspects the items count and keys immediately. This is a fail-fast guard so callers get a clear error at the call site instead of a NullReferenceException deeper in the add logic.

Solutions

  1. Ensure the source dictionary is initialized before calling AddRange
  2. Coalesce null to an empty dictionary with ?? new Dictionary<TKey,TValue>()
  3. Guard with an if (items != null) check before calling AddRange

Example fix

// before
dict.AddRange(externalItems);
// after
if (externalItems != null)
{
    dict.AddRange(externalItems);
}
Defensive patterns

Strategy: validation

Validate before calling

if (items == null) throw new ArgumentNullException(nameof(items)); // or skip the call
dict.AddRange(items);

Type guard

bool canAddRange = items != null;

Try / catch

try { dict.AddRange(items); } catch (ArgumentNullException) { /* initialize items */ }

Prevention

When it happens

Trigger: Calling AddRange(null) on an ObservableDictionary, e.g. when building a dictionary from a nullable collection or a method that returned null.

Common situations: Populating OpenApiDocument extensions or parameters from a variable that was never initialized, or deserializing optional config that came back null and passing it straight to AddRange.

Related errors

AI-assisted analysis of RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/6840874fdc09db87. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Core/Collections/ObservableDictionary.cs:76

        /// <summary>Initializes a new instance of the <see cref="ObservableDictionary{TKey, TValue}"/> class. </summary>
        /// <param name="capacity">The capacity. </param>
        /// <param name="comparer">The comparer. </param>
        public ObservableDictionary(int capacity, IEqualityComparer<TKey> comparer)
        {
            _dictionary = new Dictionary<TKey, TValue>(capacity, comparer);
        }

        /// <summary>Gets the underlying dictonary. </summary>
        protected Dictionary<TKey, TValue> Dictionary => _dictionary;

        /// <summary>Adds multiple key-value pairs the the dictionary. </summary>
        /// <param name="items">The key-value pairs. </param>
        public void AddRange(IDictionary<TKey, TValue> items)
        {
            if (items == null)
            {
                throw new ArgumentNullException(nameof(items));
            }

            if (items.Count > 0)
            {
                if (_dictionary.Count > 0)
                {
                    if (items.Keys.Any(k => _dictionary.ContainsKey(k)))
                    {
                        throw new ArgumentException("An item with the same key has already been added.");
                    }

                    foreach (var pair in items)
                    {
                        _dictionary.Add(pair.Key, pair.Value);
                    }
                }
                else
                {

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