dotnet/reactive · error · ArgumentNullException

Thrown when thenSource is null (ArgumentNullException…

Error message

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

What it means

The three-argument If operator requires thenSource as the branch returned when the condition is true. It is validated eagerly; a null thenSource throws ArgumentNullException with param name 'thenSource'.

Solutions

Pass an actual sequence for the true branch, or use the two-argument overload and supply Enumerable.Empty<TResult>() explicitly.
Coalesce: thenSource ?? Enumerable.Empty<TResult>().
Fix the source that should provide the branch sequence.

Example fix

// before
var seq = GeneratorEx.If(cond, thenSrc, elseSrc); // thenSrc is null
// after
var seq = GeneratorEx.If(cond, thenSrc ?? Enumerable.Empty<int>(), elseSrc);

Defensive patterns

Strategy: validation

Validate before calling

if (thenSource is null) thenSource = Enumerable.Empty<TResult>();

Type guard

bool HasThenSource<TResult>(IEnumerable<TResult>? s) => s is not null;

Try / catch

try { var seq = GeneratorEx.If(condition, thenSource, elseSource); }
catch (ArgumentNullException ex) when (ex.ParamName == "thenSource") { log.Error("true-branch sequence missing"); }

Prevention

Coalesce branch sequences with ?? Enumerable.Empty<TResult>().
Ensure branch-producing methods return empty rather than null.
Validate all branch sources when building conditional pipelines.

When it happens

Trigger: Calling If(condition, null, elseSource) — e.g. the 'true' branch sequence is null because a query method, cache lookup, or configuration-provided sequence was absent.

Common situations: Branch sequences composed from optional data sources; dictionary of named sequences where the key lookup missed; refactors replacing a default sequence with a nullable variable.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/72599c55e1c93db0. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/If.cs:24

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Returns an enumerable sequence based on the evaluation result of the given condition.
        /// </summary>
        /// <typeparam name="TResult">Result sequence element type.</typeparam>
        /// <param name="condition">Condition to evaluate.</param>
        /// <param name="thenSource">Sequence to return in case the condition evaluates true.</param>
        /// <param name="elseSource">Sequence to return in case the condition evaluates false.</param>
        /// <returns>Either of the two input sequences based on the result of evaluating the condition.</returns>
        public static IEnumerable<TResult> If<TResult>(Func<bool> condition, IEnumerable<TResult> thenSource, IEnumerable<TResult> elseSource)
        {
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));
            if (thenSource == null)
                throw new ArgumentNullException(nameof(thenSource));
            if (elseSource == null)
                throw new ArgumentNullException(nameof(elseSource));

            return Defer(() => condition() ? thenSource : elseSource);
        }

        /// <summary>
        /// Returns an enumerable sequence if the evaluation result of the given condition is true, otherwise returns an empty
        /// sequence.
        /// </summary>
        /// <typeparam name="TResult">Result sequence element type.</typeparam>
        /// <param name="condition">Condition to evaluate.</param>
        /// <param name="thenSource">Sequence to return in case the condition evaluates true.</param>
        /// <returns>The given input sequence if the condition evaluates true; otherwise, an empty sequence.</returns>
        public static IEnumerable<TResult> If<TResult>(Func<bool> condition, IEnumerable<TResult> thenSource)
        {
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));

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