dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'sources')

Error message

Value cannot be null. (Parameter 'sources')

What it means

The params overload OnErrorResumeNext(params IEnumerable<TSource>[] sources) throws ArgumentNullException when the sources array itself is null. This is eager argument validation so failure occurs at call time, not during enumeration.

Solutions

1. Pass an initialized array; use an empty array if there are no sequences.
2. Coalesce with a fallback: sources ?? new IEnumerable<int>[0].
3. Fix the upstream builder that produced a null array.

Example fix

// before
var combined = xs.OnErrorResumeNext(seqArray); // seqArray may be null
// after
var combined = xs.OnErrorResumeNext(seqArray ?? new IEnumerable<int>[0]);

if (sources == null) sources = Array.Empty<IEnumerable<int>>();
var combined = xs.OnErrorResumeNext(sources);

static bool HasSources(IEnumerable<int>[]? sources) => sources is { Length: > 0 };

try
{
    var combined = xs.OnErrorResumeNext(sources);
}
catch (ArgumentNullException ex) when (ex.ParamName == "sources")
{
    // handle missing array
}

When it happens

Trigger: Calling OnErrorResumeNext((IEnumerable<int>[])null) or passing a null array variable into the params overload.

Common situations: Building an array of sequences dynamically (e.g. from a collection of tasks or files) where the array was never initialized.

Related errors

• ArgumentNullException(nameof(comparer))
• Value cannot be null. (Parameter 'second')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/3148498f47fba528. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/OnErrorResumeNext.cs:38

            if (first == null)
                throw new ArgumentNullException(nameof(first));
            if (second == null)
                throw new ArgumentNullException(nameof(second));

            return OnErrorResumeNextCore(new[] { first, second });
        }

        /// <summary>
        /// Creates a sequence that concatenates the given sequences, regardless of whether an error occurs in any of the
        /// sequences.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="sources">Source sequences.</param>
        /// <returns>Sequence concatenating the elements of the given sequences, ignoring errors.</returns>
        public static IEnumerable<TSource> OnErrorResumeNext<TSource>(params IEnumerable<TSource>[] sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return OnErrorResumeNextCore(sources);
        }

        /// <summary>
        /// Creates a sequence that concatenates the given sequences, regardless of whether an error occurs in any of the
        /// sequences.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="sources">Source sequences.</param>
        /// <returns>Sequence concatenating the elements of the given sequences, ignoring errors.</returns>
        public static IEnumerable<TSource> OnErrorResumeNext<TSource>(this IEnumerable<IEnumerable<TSource>> sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return OnErrorResumeNextCore(sources);
        }

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