What causes "new ElementNotAvailableException()" in dotnet/wpf?

Catch ElementNotAvailableException during tree traversal and treat the element as removed — rescan from the root.

dotnet/wpf · error · ElementNotAvailableException

new ElementNotAvailableException()

Error message

new ElementNotAvailableException()

What it means

GetAccessibleChildren throws ElementNotAvailableException when the Win32 AccessibleChildren call returns E_INVALIDARG. The wrapper treats this as evidence the accessible object is no longer available/valid (child enumeration impossible), which in UIA terms means the element is dead.

Solutions

Catch ElementNotAvailableException during tree traversal and treat the element as removed — rescan from the root.
Re-query the element/children promptly after obtaining childCount to shrink the race window; add retries with delay.
Use UIA's raw view walker / TreeWalker instead of direct MSAA enumeration where possible, as it handles dynamic trees better.
If the target app is yours, ensure its IAccessible get_accChildCount/get_accChild agree and keep objects alive during enumeration.

Example fix

// before
var children = GetAccessibleChildren(accessible); // element died mid-call
// after
try { var children = GetAccessibleChildren(accessible); }
catch (ElementNotAvailableException)
{
    accessible = null; // element no longer available; rescan tree
}

Defensive patterns

Strategy: try-catch

Validate before calling

int count = accessible.ChildCount; // re-read and re-validate immediately before enumerating children; abort if the element reports 0 or the window is gone

Try / catch

try
{
    var children = GetAccessibleChildren(accessible);
}
catch (ElementNotAvailableException)
{
    // element vanished mid-enumeration; rescan the tree from the root
    accessible = null;
    RetryTreeWalk();
}

Prevention

Wrap every child-enumeration step in ElementNotAvailableException handling
Rescan from the root instead of reusing stale accessible references
Minimize delay between ChildCount and AccessibleChildren calls
On heavily dynamic UIs, prefer UIA TreeWalker over MSAA child enumeration
Retry the walk a bounded number of times with small delays before giving up

When it happens

Trigger: Enumerating children of an MSAA accessible whose childCount changed or whose underlying object was destroyed between querying Accessible.ChildCount and calling AccessibleChildren; passing an accessible object from a closed window/control.

Common situations: Walking UI trees of apps that rebuild controls dynamically (toolbars, virtualized lists, browser content); timing races where the container is reparented or destroyed during traversal; legacy MSAA apps with buggy child-count reporting.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/aa9d02d383402181. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/UIAutomation/UIAutomationClientSideProviders/MS/Internal/AutomationProxies/Accessible.cs:917

        // Returns an array of IAccessible interfaces and/or childID's representing the children
        internal static object[] GetAccessibleChildren(IAccessible accessibleObject, out int childrenReturned)
        {
            int childCount;
            object[] aChildren = null;

            try
            {
                childCount = accessibleObject.accChildCount;
                childrenReturned = 0;
                if (childCount > 0)
                {
                    aChildren = new object[childCount];

                    // Get the raw children because accNavigate doesn't work
                    if (UnsafeNativeMethods.AccessibleChildren(accessibleObject, 0, childCount, aChildren, out childrenReturned) == NativeMethods.E_INVALIDARG)
                    {
                        System.Diagnostics.Debug.Fail("Call to AccessibleChildren() returned E_INVALIDARG.");
                        throw new ElementNotAvailableException();
                    }
                }

                return aChildren;
            }
            catch (Exception e)
            {
                if (HandleIAccessibleException(e))
                {
                    throw;
                }
                throw new ElementNotAvailableException();
            }
        }

        #endregion

        #endregion Internal Methods

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