What causes "Value cannot be null. (Parameter 'timeSelector')" in dotnet/reactive?

Supply a non-null timeSelector, e.g. state => TimeSpan.FromMilliseconds(50).

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'timeSelector')

Error message

Value cannot be null. (Parameter 'timeSelector')

What it means

Generate requires a timeSelector delegate that maps each state to the delay before the next element. When it is null, this overload throws ArgumentNullException immediately. This instance comes from the synchronous 5-parameter Generate overload (no scheduler).

Solutions

Supply a non-null timeSelector, e.g. state => TimeSpan.FromMilliseconds(50).
For constant delays, pass a lambda returning a fixed TimeSpan instead of null.
Verify you are invoking the overload matching your delegate types (sync vs ValueTask-returning).

Example fix

// before
var src = AsyncObservable.Generate(0, i => i < 5, i => i + 1, i => i, (Func<int, TimeSpan>)null);
// after
var src = AsyncObservable.Generate(0, i => i < 5, i => i + 1, i => i, _ => TimeSpan.FromSeconds(1));

Defensive patterns

Strategy: validation

Validate before calling

if (timeSelector is null) timeSelector = _ => TimeSpan.Zero;
var src = AsyncObservable.Generate(initialState, condition, iterate, resultSelector, timeSelector);

Type guard

static bool HasTimeSelector<TState>(Func<TState,TimeSpan> t) => t != null;

Try / catch

try
{
    var src = AsyncObservable.Generate(initialState, condition, iterate, resultSelector, timeSelector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "timeSelector")
{
    // substitute a default delay and retry construction
}

Prevention

Default the delay to TimeSpan.Zero at the call site instead of relying on the library.
Keep timing lambdas near the Generate call, not in nullable remote config.
Use named arguments to make every delegate slot explicit.

When it happens

Trigger: Calling AsyncObservable.Generate(initialState, condition, iterate, resultSelector, timeSelector) with timeSelector == null, typically because a TimeSpan-returning lambda was omitted or a null variable was passed.

Common situations: Dynamically composed pipelines where timing lambdas come from settings that failed to load; wrong overload picked (using the sync overload where the async ValueTask overload was intended) so the argument slot is null.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/3616effa588533fc. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Generate.cs:73

                throw new ArgumentNullException(nameof(iterate));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector, scheduler));
        }

        public static IAsyncObservable<TResult> Generate<TState, TResult>(TState initialState, Func<TState, bool> condition, Func<TState, TState> iterate, Func<TState, TResult> resultSelector, Func<TState, TimeSpan> timeSelector)
        {
            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));

            return Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector, timeSelector));
        }

        public static IAsyncObservable<TResult> Generate<TState, TResult>(TState initialState, Func<TState, bool> condition, Func<TState, TState> iterate, Func<TState, TResult> resultSelector, Func<TState, TimeSpan> timeSelector, IAsyncScheduler scheduler)
        {
            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 Create<TResult>(observer => AsyncObserver.Generate(observer, initialState, condition, iterate, resultSelector, timeSelector, scheduler));

View on GitHub (pinned to 94b5d5ab91)