dotnet/wpf · error · InvalidOperationException

SR.NestedBeginInitNotSupported

Error message

SR.NestedBeginInitNotSupported

What it means

ResourceDictionary.BeginInit throws InvalidOperationException (SR.NestedBeginInitNotSupported) when called while another initialization is already pending (IsInitializePending). Nested Begin/EndInit pairs on the same instance are not permitted.

Solutions

  1. Ensure every BeginInit is paired with exactly one EndInit, including on exception paths (try/finally).
  2. Check IsInitializePending before calling BeginInit, or guard with a bool flag in your own code.
  3. If an earlier EndInit was skipped due to an exception, restructure to use try/finally so the state resets.

Example fix

// before
dictionary.BeginInit();
LoadThemes(dictionary);
dictionary.BeginInit(); // nested -> throws
// after
dictionary.BeginInit();
try { LoadThemes(dictionary); }
finally { dictionary.EndInit(); }
Defensive patterns

Strategy: try-catch

Validate before calling

if (dictionary.IsInitializePending) throw new InvalidOperationException("BeginInit already called; call EndInit before initializing again");

Type guard

bool CanBeginInit(ResourceDictionary rd) => !rd.IsInitializePending;

Try / catch

try { dictionary.BeginInit(); /* configure */ } catch (InvalidOperationException ex) when (ex.Message.Contains("Nested BeginInit")) { /* an init is already in flight; join it or wait */ }

Prevention

When it happens

Trigger: Calling BeginInit twice on the same ResourceDictionary without a matching EndInit in between — e.g. nested load helpers or re-entrant initialization code.

Common situations: Two theme-loading routines both calling BeginInit on the same shared dictionary; exception in the middle of a Begin/EndInit block leaving IsInitializePending stuck, then retrying initialization.

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/987cfd58807ae1f6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ResourceDictionary.cs:992

        }

        #endregion IEnumerable

        #region ISupportInitialize

        /// <summary>
        ///     Mark the begining of the Init phase
        /// </summary>
        /// <remarks>
        ///     BeginInit and EndInit follow a transaction model. BeginInit marks the
        ///     dictionary uninitialized and EndInit marks it initialized.
        /// </remarks>
        public void BeginInit()
        {
            // Nested BeginInits on the same instance aren't permitted
            if (IsInitializePending)
            {
                throw new InvalidOperationException(SR.NestedBeginInitNotSupported);
            }

            IsInitializePending = true;
            IsInitialized = false;
        }

        /// <summary>
        ///     Fire Invalidation at the end of Init phase
        /// </summary>
        /// <remarks>
        ///     BeginInit and EndInit follow a transaction model. BeginInit marks the
        ///     dictionary uninitialized and EndInit marks it initialized.
        /// </remarks>
        public void EndInit()
        {
            // EndInit without a BeginInit isn't permitted
            if (!IsInitializePending)
            {

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