dotnet/wpf · error · InvalidOperationException

CannotChangeAfterSealed

CannotChangeAfterSealed

Error message

SR.Format(SR.CannotChangeAfterSealed, "SeekStoryboard")

What it means

SeekStoryboard.Offset's setter throws InvalidOperationException once the SeekStoryboard object is sealed (read-only). ControllableStoryboardAction objects become sealed when they are in use inside a Storyboard/trigger tree, freezing their configuration.

Solutions

Create a new SeekStoryboard instance for each seek and set Offset before use.
Set Offset immediately after construction, before adding the action to any trigger or storyboard.
If sharing is needed, keep a prototype and clone it per use.

Example fix

// before
sharedSeek.Offset = TimeSpan.FromSeconds(3); // throws after first use

// after
var seek = new SeekStoryboard { Storyboard = targetStoryboard, Offset = TimeSpan.FromSeconds(3), Origin = TimeSeekOrigin.BeginTime };

Defensive patterns

Strategy: type-guard

Validate before calling

if (seek.IsSealed) seek = new SeekStoryboard { Storyboard = seek.Storyboard, Offset = newOffset, Origin = seek.Origin };

Type guard

static SeekStoryboard EnsureMutable(SeekStoryboard s, TimeSpan newOffset) => s.IsSealed ? new SeekStoryboard { Storyboard = s.Storyboard, Offset = newOffset, Origin = s.Origin } : s;

Try / catch

try { seek.Offset = newOffset; } catch (InvalidOperationException) { seek = new SeekStoryboard { /* reconfigure */ }; }

Prevention

Treat storyboard action objects as immutable once used
Create a fresh SeekStoryboard per seek operation
Set all properties immediately after construction

When it happens

Trigger: Setting the Offset property on a SeekStoryboard instance that has already been sealed — typically after it has been applied to a running storyboard or shared/attached in a template.

Common situations: Reusing a single SeekStoryboard instance across multiple seek operations from code; mutating a template-defined SeekStoryboard at runtime.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b533577ae9ed4b86. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Media/Animation/SeekStoryboard.cs:37

    /// </summary>
    public sealed class SeekStoryboard : ControllableStoryboardAction
{
    /// <summary>
    ///     A time offset to use for this action.  If it is never explicitly
    /// specified, it will be zero.
    /// </summary>
    // [DefaultValue(TimeSpan.Zero)] - not usable because TimeSpan.Zero is not a constant expression.
    public TimeSpan Offset
    {
        get
        {
            return _offset;
        }
        set
        {
            if (IsSealed)
            {
                throw new InvalidOperationException(SR.Format(SR.CannotChangeAfterSealed, "SeekStoryboard"));
            }
            // TimeSpan is a struct and can't be null - hence no ArgumentNullException check.
            _offset = value;
        }
    }

    /// <summary>
    /// This method is used by TypeDescriptor to determine if this property should
    /// be serialized.
    /// </summary>
    // Because we can't use [DefaultValue(TimeSpan.Zero)] - TimeSpan.Zero is not a constant expression.
    [EditorBrowsable(EditorBrowsableState.Never)]
    public bool ShouldSerializeOffset()
    {
        return !(TimeSpan.Zero.Equals(_offset));
    }

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