dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'observableFactory')

Error message

Value cannot be null. (Parameter 'observableFactory')

What it means

AsyncObservable.Defer requires a non-null observableFactory Func that produces the IAsyncObservable when subscribed. The library validates it eagerly and throws ArgumentNullException (Parameter 'observableFactory') because deferring a null factory would fail at subscribe time with no useful stack.

Solutions

1. Pass a valid factory lambda, e.g. () => AsyncObservable.Return(42).
2. Check that the variable holding the factory is initialized before calling Defer.
3. If the factory comes from configuration/DI, validate it resolved successfully before deferring.

Example fix

// before
Func<IAsyncObservable<int>> factory = null;
var xs = AsyncObservable.Defer(factory);
// after
var xs = AsyncObservable.Defer(() => AsyncObservable.Return(42));

if (observableFactory is null) throw new ArgumentNullException(nameof(observableFactory));
var xs = AsyncObservable.Defer(observableFactory);

static bool IsValidFactory<T>(Func<IAsyncObservable<T>> f) => f is not null;

try
{
    var xs = AsyncObservable.Defer(factory);
}
catch (ArgumentNullException ex) when (ex.ParamName == "observableFactory")
{
    // factory delegate was null; ensure it is assigned before deferring
}

When it happens

Trigger: Calling AsyncObservable.Defer<TSource>((Func<IAsyncObservable<TSource>>)null); also reachable via DeferAsync or FromAsync, which delegate to Defer, when their factory arguments are null.

Common situations: Passing a method group that resolves to null via reflection or conditional delegation; a config-selected factory string that failed to resolve; overloads where Func vs Func<ValueTask<...>> ambiguity led to casting null.

Related errors

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

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/53c79bc5c413cf00. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Defer.cs:16

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Reactive.Disposables;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> Defer<TSource>(Func<IAsyncObservable<TSource>> observableFactory)
        {
            if (observableFactory == null)
                throw new ArgumentNullException(nameof(observableFactory));

            return Defer(() => new ValueTask<IAsyncObservable<TSource>>(observableFactory()));
        }

        public static IAsyncObservable<TSource> DeferAsync<TSource>(Func<ValueTask<IAsyncObservable<TSource>>> observableFactory) => Defer(observableFactory);

        public static IAsyncObservable<TSource> Defer<TSource>(Func<ValueTask<IAsyncObservable<TSource>>> observableFactory)
        {
            if (observableFactory == null)
                throw new ArgumentNullException(nameof(observableFactory));

            return Create<TSource>(async observer =>
            {
                var source = default(IAsyncObservable<TSource>);

                try
                {
                    source = await observableFactory().ConfigureAwait(false);

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