dotnet/wpf · error · InvalidOperationException

SR.ShareableExpressionsCannotChangeSources

Error message

SR.ShareableExpressionsCannotChangeSources

What it means

Expression.ChangeSources updates the list of DependencyObjects the expression depends on, but a shareable expression (e.g. a shared StaticResource-style expression) is immutable with respect to its sources. If the expression is marked Shareable, ChangeSources throws InvalidOperationException(SR.ShareableExpressionsCannotChangeSources) because mutating shared expressions would affect every consumer.

Solutions

Construct the expression without the Shareable ExpressionMode if its sources need to change over time
Create a new expression instance per target instead of sharing one instance
Re-evaluate/replace the whole expression (e.g. re-apply the markup extension) rather than mutating sources
Split logic: use a shareable immutable expression for static cases and a non-shareable one for dynamic cases

Example fix

// before
var expr = new ObjectRefExpression(obj, prop, ExpressionMode.Shareable);
expr.ChangeSources(target, newSources); // throws
// after
var expr = new ObjectRefExpression(obj, prop, ExpressionMode.None);
expr.ChangeSources(target, newSources); // ok

Defensive patterns

Strategy: type-guard

Validate before calling

if (expression is { Shareable: true }) throw new NotSupportedException("Cannot change sources of a shareable expression");

Type guard

static bool CanChangeSources(System.Windows.Expression e) => !e.Shareable;

Try / catch

try { expr.ChangeSources(d, newSources); }
catch (InvalidOperationException ex) when (expr.Shareable) { log.Warn("Shareable expression is immutable; recreating", ex); }

Prevention

Use Shareable mode only for expressions whose dependencies never change
Create one expression instance per target when sources are dynamic
Replace whole expressions instead of mutating shared ones

When it happens

Trigger: Calling ChangeSources (directly or via DependencyObject expression invalidation plumbing) on an expression constructed with ExpressionMode.Shareable after it has been attached and used.

Common situations: Custom markup extensions or expression subclasses that start sharing instances (shareable mode) but then attempt dynamic source changes; reusing one expression instance across multiple dependency properties while its sources evolve.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e0266cbc134f0d5f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Expression.cs:207

        /// </summary>
        /// <remarks>
        ///     Expression must be in use on provided DependencyObject/DependencyProperty.
        ///     GetSources must reflect the old sources to be replaced by the provided newSources.
        /// </remarks>
        /// <param name="d">DependencyObject whose sources are to be updated</param>
        /// <param name="dp">The property that the Expression is set to</param>
        /// <param name="newSources">New sources</param>
        internal void ChangeSources(DependencyObject d, DependencyProperty dp, DependencySource[] newSources)
        {
            if (!ForwardsInvalidations)
            {
                ArgumentNullException.ThrowIfNull(d);
                ArgumentNullException.ThrowIfNull(dp);
            }

            if (Shareable)
            {
                throw new InvalidOperationException(SR.ShareableExpressionsCannotChangeSources);
            }

            DependencyObject.ValidateSources(d, newSources, this);

            // Additional validation in callee
            if (ForwardsInvalidations)
            {
                DependencyObject.ChangeExpressionSources(this, null, null, newSources);
            }
            else
            {
                DependencyObject.ChangeExpressionSources(this, d, dp, newSources);
            }
        }


        // Determines if Expression can be attached:
        //    1) If Shareable

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