dotnet/reactive · error · ArgumentNullException

Thrown when source is null (ArgumentNullException, param…

Error message

Thrown when source is null (ArgumentNullException, param name: source)

What it means

IsEmpty checks whether a sequence contains no elements by delegating to Any(). The operator validates its argument eagerly, so a null source throws ArgumentNullException with paramName 'source' at the call site rather than during evaluation.

Solutions

Null-check the source first: if (data != null && data.IsEmpty()).
Coalesce to an empty sequence: (data ?? []).IsEmpty().
Fix the upstream producer to return an empty collection instead of null for the 'none' case.

Example fix

// before
if (results.IsEmpty()) ShowEmpty(); // results may be null
// after
if (results is not null && results.IsEmpty()) ShowEmpty();

Defensive patterns

Strategy: validation

Validate before calling

bool empty = source is not null && source.IsEmpty();

Type guard

static bool IsNullOrEmpty<T>(IEnumerable<T>? s) => s is null || s.IsEmpty();

Try / catch

try
{
    var empty = source.IsEmpty();
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    var empty = true; // treat null as empty
}

Prevention

Write null-safe emptiness checks as a helper (IsNullOrEmpty)
Treat null and empty distinctly at the domain boundary, then normalize to empty
Don't feed nullable API results straight into IsEmpty/Any-style guards

When it happens

Trigger: Calling source.IsEmpty() where source is null, e.g. bool empty = ParseList(xml).IsEmpty() when ParseList can return null.

Common situations: Guard checks written as if (data.IsEmpty()) where data came from an API returning null for 'no data'; migrating code from LINQ's Any() (which also throws on null) to Ix's IsEmpty without adding null handling.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b35edac6b2474f76. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/IsEmpty.cs:20

// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Determines whether an enumerable sequence is empty.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <returns>true if the sequence is empty; false otherwise.</returns>
        public static bool IsEmpty<TSource>(this IEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return !source.Any();
        }
    }
}

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