dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'progress')

Error message

Value cannot be null. (Parameter 'progress')

What it means

This ToObservable(IAsyncActionWithProgress, IProgress) overload requires a non-null progress reporter so progress notifications can be forwarded to observers. After validating 'source', it validates 'progress' and throws ArgumentNullException naming 'progress' when null. The library deliberately rejects null instead of silently dropping progress reports.

Solutions

1. Pass a real IProgress<TProgress> implementation, e.g. new Progress<TProgress>(p => ...).
2. Use the parameterless ToObservable() overload if you do not need forwarded progress; ToObservableProgress() reports progress to observers directly.
3. Ensure any progress field/property is initialized before the conversion call.

Example fix

// before
action.ToObservable(null).Subscribe(...);
// after
action.ToObservable(new Progress<MyProgress>(p => Console.WriteLine(p))).Subscribe(...);

if (progress is null) throw new ArgumentNullException(nameof(progress));

bool HasProgress<TProgress>(IProgress<TProgress>? p) => p is not null;

try { action.ToObservable(progress).Subscribe(...); }
catch (ArgumentNullException ex) when (ex.ParamName == "progress") { action.ToObservable().Subscribe(...); }

When it happens

Trigger: Passing null (or an uninitialized IProgress<TProgress>) as the second argument to ToObservable on an IAsyncActionWithProgress<TProgress>.

Common situations: Developers copying the parameterless overload's call shape and passing null for progress, or a Progress<T> field not yet initialized when the extension is invoked.

Related errors

• source
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'scheduler')
• Value cannot be null. (Parameter 'observer')
• comparer

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c45e1cbd6de93719. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive.WindowsRuntime/AsyncInfoObservableExtensions.cs:84

        /// <summary>
        /// Converts a Windows Runtime asynchronous action to an observable sequence, reporting its progress through the supplied progress object.
        /// Each observer subscribed to the resulting observable sequence will be notified about the action's successful or exceptional completion.
        /// </summary>
        /// <typeparam name="TProgress">The type of the reported progress objects.</typeparam>
        /// <param name="source">Asynchronous action to convert.</param>
        /// <param name="progress">Progress object to receive progress notifications on.</param>
        /// <returns>An observable sequence that produces a unit value when the asynchronous action completes, or propagates the exception produced by the asynchronous action.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="progress"/> is null.</exception>
        public static IObservable<Unit> ToObservable<TProgress>(this IAsyncActionWithProgress<TProgress> source, IProgress<TProgress> progress)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

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

            return source.ToObservable_(progress);
        }

        /// <summary>
        /// Converts a Windows Runtime asynchronous action to an observable sequence reporting its progress.
        /// Each observer subscribed to the resulting observable sequence will be notified about the action's successful or exceptional completion.
        /// </summary>
        /// <typeparam name="TProgress">The type of the reported progress objects.</typeparam>
        /// <param name="source">Asynchronous action to convert.</param>
        /// <returns>An observable sequence that produces progress values from the asynchronous action and notifies observers about the action's completion.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static IObservable<TProgress> ToObservableProgress<TProgress>(this IAsyncActionWithProgress<TProgress> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));

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