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

Pass a valid non-null Func to Observable.Start.

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'function')

Error message

Value cannot be null. (Parameter 'function')

What it means

Observable.Start<TResult>(Func<TResult>) invokes the given function synchronously-or-on-schedule and publishes its result as an observable. Rx validates the function eagerly and throws ArgumentNullException with Parameter 'function' when null, since there is no computation to run.

Solutions

Pass a valid non-null Func<TResult> to Observable.Start.
Ensure any delegate field/dependency is initialized before calling Start.
If the computation may be absent, fall back to Observable.Empty<TResult>() or a default result instead of Start(null).

Example fix

// before
Func<int> compute = config.Computations["fast"] ?? null;
Observable.Start(compute);
// after
Func<int> compute = config.Computations["fast"];
Observable.Start(() => compute != null ? compute() : 0);

Defensive patterns

Strategy: validation

Validate before calling

if (function is null) throw new ArgumentNullException(nameof(function));
var observable = Observable.Start(function);

Type guard

static bool IsValidFunction<TResult>(Func<TResult> function) => function is not null;

Try / catch

try { var obs = Observable.Start(function); }
catch (ArgumentNullException ex) when (ex.ParamName == "function") { obs = Observable.Empty<TResult>(); }

Prevention

Check delegate-producing lookups (dictionaries, DI) for misses before calling Start.
Use Observable.Empty<TResult>() or a default when no computation is available.
Enable nullable reference types to catch unassigned Func fields at compile time.

When it happens

Trigger: Calling Observable.Start(function) with a null Func<TResult> (line 991 in Observable.Async.cs), e.g. a null delegate field, an unassigned lambda variable, or a factory that returned null.

Common situations: Dependency-injected computation delegate not registered; conditional logic skipped assignment; passing the result of a lookup (e.g. dictionary of delegates) that missed.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/09af9b901e7c8eb5. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Async.cs:991

        /// <summary>
        /// Invokes the specified function asynchronously, surfacing the result through an observable sequence.
        /// </summary>
        /// <typeparam name="TResult">The type of the result returned by the function.</typeparam>
        /// <param name="function">Function to run asynchronously.</param>
        /// <returns>An observable sequence exposing the function's result value, or an exception.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="function"/> is null.</exception>
        /// <remarks>
        /// <list type="bullet">
        /// <item><description>The function is called immediately, not during the subscription of the resulting sequence.</description></item>
        /// <item><description>Multiple subscriptions to the resulting sequence can observe the function's result.</description></item>
        /// </list>
        /// </remarks>
        public static IObservable<TResult> Start<TResult>(Func<TResult> function)
        {
            if (function == null)
            {
                throw new ArgumentNullException(nameof(function));
            }

            return s_impl.Start(function);
        }

        /// <summary>
        /// Invokes the specified function asynchronously on the specified scheduler, surfacing the result through an observable sequence
        /// </summary>
        /// <typeparam name="TResult">The type of the result returned by the function.</typeparam>
        /// <param name="function">Function to run asynchronously.</param>
        /// <param name="scheduler">Scheduler to run the function on.</param>
        /// <returns>An observable sequence exposing the function's result value, or an exception.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="function"/> or <paramref name="scheduler"/> is null.</exception>
        /// <remarks>
        /// <list type="bullet">
        /// <item><description>The function is called immediately, not during the subscription of the resulting sequence.</description></item>
        /// <item><description>Multiple subscriptions to the resulting sequence can observe the function's result.</description></item>
        /// </list>

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