dotnet/wpf · error · InvalidOperationException

throw new…

Error message

throw new System.InvalidOperationException(SR.Enumerator_VerifyContext);

What it means

WeakReferenceListEnumerator.Current throws InvalidOperationException(SR.Enumerator_VerifyContext) when the strong reference has not been established, meaning MoveNext was not called before reading Current (or the enumerator was used out of order). It enforces the standard IEnumerator protocol.

Solutions

Call MoveNext() and check it returns true before reading Current
Restructure as foreach or a while(en.MoveNext()) loop
Do not read Current again after the loop completes
Create a fresh enumerator instead of resetting/reusing a finished one

Example fix

// before
var e = list.GetEnumerator();
object v = e.Current;
// after
var e = list.GetEnumerator();
if (e.MoveNext()) { object v = e.Current; }

Defensive patterns

Strategy: try-catch

Validate before calling

// protocol check: only read Current after MoveNext()==true
// bool canRead = enumerator.MoveNext();

Type guard

static bool TryCurrent(System.Collections.IEnumerator e, out object v) {
    v = null;
    if (!e.MoveNext()) return false;
    v = e.Current; return true;
}

Try / catch

try { v = e.Current; }
catch (InvalidOperationException) { /* MoveNext not called/failed: reset loop */ }

Prevention

Always gate Current on MoveNext()'s return value
Prefer foreach over manual enumerator loops
Never reuse exhausted enumerators

When it happens

Trigger: Accessing the Current property of WeakReferenceListEnumerator immediately after creation or after MoveNext returned false, without a successful MoveNext establishing _StrongReference.

Common situations: Manual foreach-equivalent loops over weak-reference lists where MoveNext's return value is ignored; resuming an exhausted enumerator; sharing one enumerator across threads.

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/69dfb790a2a42a54. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/MS/Internal/WeakReferenceEnumerator.cs:37

    internal struct WeakReferenceListEnumerator : IEnumerator
    {
        public WeakReferenceListEnumerator( ArrayList List)
        {
            _i = 0;
            _List = List;
            _StrongReference = null;
        }

        object IEnumerator.Current
        {  get{ return Current; } }
        
        public object Current
        {
            get
            {
                if( null == _StrongReference )
                {
                    throw new System.InvalidOperationException(SR.Enumerator_VerifyContext);
                }
                return _StrongReference;
            }
        }

        public bool MoveNext()
        {
            object obj=null;
            while( _i < _List.Count )
            {
                WeakReference weakRef = (WeakReference) _List[ _i++ ];
                obj = weakRef.Target;
                if(null != obj)
                    break;
            }
            _StrongReference = obj;
            return (null != obj);
        }

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