dotnet/wpf · error · InvalidOperationException

SR.Media_UriNotSpecified

Error message

SR.Media_UriNotSpecified

What it means

MediaTimeline.AllocateClock throws InvalidOperationException when the timeline's Source (a URI) is null. A MediaClock cannot be allocated without a media source, so the timing engine refuses to create the clock.

Solutions

1. Set MediaTimeline.Source to a valid media URI before creating/starting clocks
2. Validate Source != null before calling CreateClock or starting the containing Storyboard
3. Ensure the Source binding resolves (check DataContext/binding errors) prior to playback

Example fix

// before
var timeline = new MediaTimeline();
var clock = timeline.CreateClock(); // InvalidOperationException
// after
var timeline = new MediaTimeline(new Uri("media\\\\video.wmv", UriKind.Relative));
var clock = timeline.CreateClock();

if (timeline == null || timeline.Source == null) throw new ArgumentException("MediaTimeline.Source must be set before creating a clock", nameof(timeline));

bool HasSource(MediaTimeline t) => t?.Source != null;

try { var clock = timeline.CreateClock(); } catch (InvalidOperationException ex) when (ex.Message.Contains("Uri")) { Log("MediaTimeline has no Source; cannot start playback"); }

When it happens

Trigger: Starting an animation/clock from a MediaTimeline whose Source property was never set - e.g. calling CreateClock(), adding the timeline to a Storyboard and playing it, or binding the timeline in XAML without a Source attribute.

Common situations: XAML MediaTimeline declared without Source, data binding failing to supply the URI before the clock is allocated, or programmatically constructing MediaTimeline and forgetting Source.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• SR.Media_NotAllowedWhileTimingEngineInControl
• Storyboard_MediaElementRequired
• Animation_AnimationTimelineTypeMismatch
• Animation_CalculatedValueIsInvalidForProperty
• Animation_ChildMustBeKeyFrame

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e4cbe84bdb7381e7. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/MediaTimeline.cs:125

        #region Timeline

        /// <summary>
        /// Called by the Clock to create a type-specific clock for this
        /// timeline.
        /// </summary>
        /// <returns>
        /// A clock for this timeline.
        /// </returns>
        /// <remarks>
        /// If a derived class overrides this method, it should only create
        /// and return an object of a class inheriting from Clock.
        /// </remarks>
        protected internal override Clock AllocateClock()
        {
            if (Source == null)
            {
                throw new InvalidOperationException(SR.Media_UriNotSpecified);
            }

            MediaClock mediaClock = new MediaClock(this);

            return mediaClock;
        }

        /// <summary>
        /// Called by the base Freezable class to make this object
        /// frozen.
        /// </summary>
        protected override bool FreezeCore(bool isChecking)
        {
            bool canFreeze = base.FreezeCore(isChecking);
            if (!canFreeze)
            {
                return false;
            }

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