dotnet/aspnetcore · error · ArgumentException

There is already a persisted object under the same key

Error message

There is already a persisted object under the same key '{key}'

What it means

Thrown by the public PersistAsJson<T> when _currentState.TryAdd(key, ...) returns false, i.e. the same key has already been persisted in this cycle. Keys are unique within a persist pass to avoid one component silently overwriting another's data. The duplicate is rejected so the conflict surfaces immediately.

Solutions

1. Namespace keys per component or feature: use $"{GetType().FullName}:state" or a constant unique to the module.
2. Ensure only one component owns each persisted key; coordinate ownership in code review.
3. If multiple values are legitimately needed, combine them into one persisted object or use distinct keys.
4. Use TryTakeFromJson on restore with the exact same namespaced key.

Example fix

// before: two components share the literal "data" key
state.PersistAsJson("data", _value);

// after: scope the key to the owning component
state.PersistAsJson($"{GetType().FullName}:data", _value);

static string ScopedKey(object owner, string key) => $"{owner.GetType().FullName}:{key}";

// then always pass ScopedKey(this, "data")

try { state.PersistAsJson(key, instance); }
catch (ArgumentException ex) when (ex.Message.Contains("already a persisted object under the same key"))
{ /* use a more specific key, or merge payloads */ }

When it happens

Trigger: Two components (or two persist callbacks) both call PersistAsJson with the same key during one persist phase; a single component registers two persist callbacks that write the same key; a key constant reused across feature modules.

Common situations: Shared key constants (e.g. "state") used by multiple components; a component rendered multiple times in the same tree each registering persistence under a fixed key; refactoring that merged two persistence modules without namespacing their keys.

Related errors

• existingState
• PersistentComponentState already initialized.
• Persisting state is only allowed during an OnPersisting…
• Registering a callback while persisting state is not…
• State already persisted.

AI-assisted analysis of dotnet/aspnetcore@3600ca084e (2026-08-11). Data as JSON: /api/errors/4723e02a9df0abb3. Report an issue: GitHub.

Appendix: source

Thrown at src/Components/Components/src/PersistentComponentState.cs:120

    /// <summary>
    /// Serializes <paramref name="instance"/> as JSON and persists it under the given <paramref name="key"/>.
    /// </summary>
    /// <typeparam name="TValue">The <paramref name="instance"/> type.</typeparam>
    /// <param name="key">The key to use to persist the state.</param>
    /// <param name="instance">The instance to persist.</param>
    [RequiresUnreferencedCode("JSON serialization and deserialization might require types that cannot be statically analyzed.")]
    public void PersistAsJson<[DynamicallyAccessedMembers(JsonSerialized)] TValue>(string key, TValue instance)
    {
        ArgumentNullException.ThrowIfNull(key);

        if (!PersistingState)
        {
            throw new InvalidOperationException("Persisting state is only allowed during an OnPersisting callback.");
        }

        if (!_currentState.TryAdd(key, JsonSerializer.SerializeToUtf8Bytes(instance, JsonSerializerOptionsProvider.Options)))
        {
            throw new ArgumentException($"There is already a persisted object under the same key '{key}'");
        }
    }

    [RequiresUnreferencedCode("JSON serialization and deserialization might require types that cannot be statically analyzed.")]
    internal void PersistAsJson(string key, object instance, [DynamicallyAccessedMembers(JsonSerialized)] Type type)
    {
        ArgumentNullException.ThrowIfNull(key);

        if (!PersistingState)
        {
            throw new InvalidOperationException("Persisting state is only allowed during an OnPersisting callback.");
        }

        if (!_currentState.TryAdd(key, JsonSerializer.SerializeToUtf8Bytes(instance, type, JsonSerializerOptionsProvider.Options)))
        {
            throw new ArgumentException($"There is already a persisted object under the same key '{key}'");
        }
    }

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