dotnet/reactive · error · ArgumentNullException

Thrown when condition is null (ArgumentNullException, param…

Error message

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

What it means

DoWhile repeats the source sequence as long as the postcondition delegate holds; a null condition would make the repeat decision undefined, so the operator throws ArgumentNullException for parameter 'condition'. Fix: pass a non-null Func<bool>.

Solutions

Supply a valid Func<bool> — use () => false if 'no repeat' is intended.
Verify the delegate source (field/factory) is initialized before the call.
Add a null guard or default at the point where the condition is obtained.

Example fix

// before
Func<bool> cond = GetCondition(); // may return null
var result = source.DoWhile(cond);
// after
var result = source.DoWhile(cond ?? (() => false));

Defensive patterns

Strategy: validation

Validate before calling

if (condition is null) throw new ArgumentNullException(nameof(condition));
DoWhile(source, condition ?? (() => false));

Type guard

bool HasCondition(Func<bool> c) => c is not null;

Try / catch

try { var q = source.DoWhile(condition); }
catch (ArgumentNullException ex) when (ex.ParamName == "condition") { /* use default condition */ }

Prevention

Initialize delegate fields at declaration, not lazily.
Avoid obj?.Method passing which silently yields null.
Provide defaults (() => false) for optional conditions.
Use nullable reference types to catch uninitialized delegates.

When it happens

Trigger: Calling DoWhile(source, null) — e.g. the condition Func<bool> came from an optional configuration or a nullable delegate field.

Common situations: A delegate stored in a nullable field/property not yet initialized; a factory method returning null for the condition; passing a method group on a null object (obj?.Condition compiles to null).

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b58290c7d455b8be. Report an issue: GitHub.

Appendix: source

Thrown at Ix.NET/Source/System.Interactive/System/Linq/Operators/DoWhile.cs:23

using System.Collections.Generic;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Generates an enumerable sequence by repeating a source sequence as long as the given loop postcondition holds.
        /// </summary>
        /// <typeparam name="TResult">Result sequence element type.</typeparam>
        /// <param name="source">Source sequence to repeat while the condition evaluates true.</param>
        /// <param name="condition">Loop condition.</param>
        /// <returns>Sequence generated by repeating the given sequence until the condition evaluates to false.</returns>
        public static IEnumerable<TResult> DoWhile<TResult>(this IEnumerable<TResult> source, Func<bool> condition)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (condition == null)
                throw new ArgumentNullException(nameof(condition));

            return source.Concat(While(condition, source));
        }
    }
}

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