dotnet/reactive · error · ArgumentNullException

timeSelector

Error message

timeSelector

What it means

The scheduler-based timed Observable.Generate overload throws ArgumentNullException when timeSelector (Func<TState, DateTimeOffset>) is null. timeSelector yields the absolute due time for each element; without it scheduling is impossible. Validation is eager, thrown synchronously at the call site.

Solutions

1. Supply an absolute-time selector, e.g. state => startTime.AddSeconds(state).
2. Use the TimeSpan-based overload (relative times) if that matches your data instead.
3. Default to a constant increment: state => scheduler.Now.Add(TimeSpan.FromSeconds(1)) when per-state timing is unnecessary.

Example fix

// before
var xs = Observable.Generate(0, i => i < 5, i => i + 1, i => i, null, Scheduler.Default);
// after
var start = DateTimeOffset.Now;
var xs = Observable.Generate(0, i => i < 5, i => i + 1, i => i, i => start.AddSeconds(i), Scheduler.Default);

if (timeSelector == null)
    throw new ArgumentNullException(nameof(timeSelector));
var xs = Observable.Generate(initialState, condition, iterate, resultSelector, timeSelector, scheduler);

bool HasTimeSelector<TState>(Func<TState, DateTimeOffset> timeSelector) => timeSelector != null;

try
{
    var xs = Observable.Generate(state, cond, iter, sel, timeSel, scheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "timeSelector")
{
    logger.LogError(ex, "Generate timeSelector (DateTimeOffset) was null");
}

When it happens

Trigger: Calling Observable.Generate<TState, TResult>(initialState, condition, iterate, resultSelector, timeSelector, scheduler) with timeSelector == null, e.g. when the absolute-time function is derived from user settings that are absent, or an overload mix-up passed a TimeSpan-based selector where DateTimeOffset was expected and the variable stayed null.

Common situations: Switching between the TimeSpan-based and DateTimeOffset-based Generate overloads during migration; calendar/simulation schedulers where the time function comes from injected services that returned null.

Related errors

• action
• action (Parameter 'action')
• action (Parameter 'action')
• action (Value cannot be null)
• ArgumentNullException(nameof(first))

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/16011a8b96a98f81. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Time.cs:760

        {
            if (condition == null)
            {
                throw new ArgumentNullException(nameof(condition));
            }

            if (iterate == null)
            {
                throw new ArgumentNullException(nameof(iterate));
            }

            if (resultSelector == null)
            {
                throw new ArgumentNullException(nameof(resultSelector));
            }

            if (timeSelector == null)
            {
                throw new ArgumentNullException(nameof(timeSelector));
            }

            if (scheduler == null)
            {
                throw new ArgumentNullException(nameof(scheduler));
            }

            return s_impl.Generate(initialState, condition, iterate, resultSelector, timeSelector, scheduler);
        }

        #endregion

        #region + Interval +

        /// <summary>
        /// Returns an observable sequence that produces a value after each period.
        /// </summary>
        /// <param name="period">Period for producing the values in the resulting sequence. If this value is equal to TimeSpan.Zero, the timer will recur as fast as possible.</param>

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