OrchardCMS/OrchardCore · error · ArgumentException

The type must inherit from ContentPart

Error message

The type must inherit from ContentPart

What it means

ContentOptions.GetOrAddContentPart throws ArgumentException when the given Type does not derive from ContentPart. Only ContentPart subclasses can have content part options registered in the content metadata options.

Solutions

1. Make sure the type passed to AddContentPart<T> inherits from ContentPart.
2. If you meant to register a field, call AddContentField<T> instead.
3. Check for accidental use of a ContentPartOption/other wrapper type as the generic argument.

Example fix

// before
services.AddContentPart<MyField>(); // MyField : ContentField

// after
services.AddContentField<MyField>();

if (!typeof(ContentPart).IsAssignableFrom(typeof(T)))
    throw new ArgumentException("T must inherit from ContentPart");

bool IsContentPart(Type t) => typeof(ContentPart).IsAssignableFrom(t) && !t.IsAbstract; // adjust as needed

try
{
    services.AddContentPart<T>();
}
catch (ArgumentException e)
{
    throw new InvalidOperationException("AddContentPart requires a ContentPart subclass", e);
}

When it happens

Trigger: Calling AddContentPart<T> (or ContentOptions.GetOrAddContentPart directly) with a type that is not a ContentPart subclass, e.g. a ContentField, ContentPartOption wrapper, interface, or plain POCO.

Common situations: Copy-paste registering a field type with AddContentPart, wrong generic parameter in Startup.cs, registering a view model or record instead of the part class.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• The type must inherit from ContentField
• Failed casting content to
• Content part name must start with a letter
• Content part name contains invalid characters
• Content part name is reserved for internal use

AI-assisted analysis of OrchardCMS/OrchardCore@4306c0717f (2026-09-13). Data as JSON: /api/errors/9ae3982a19ceed79. Report an issue: GitHub.

Appendix: source

Thrown at src/OrchardCore/OrchardCore.ContentManagement.Abstractions/ContentOptions.cs:40

    public IReadOnlyList<ContentFieldOption> ContentFieldOptions => _contentFields;

    internal void AddPartHandler(Type contentPartType, Type handlerType)
    {
        var option = GetOrAddContentPart(contentPartType);
        option.AddHandler(handlerType);
    }

    internal void RemovePartHandler(Type contentPartType, Type handlerType)
    {
        var option = GetOrAddContentPart(contentPartType);
        option.RemoveHandler(handlerType);
    }

    internal ContentPartOption GetOrAddContentPart(Type contentPartType)
    {
        if (!contentPartType.IsSubclassOf(typeof(ContentPart)))
        {
            throw new ArgumentException("The type must inherit from " + nameof(ContentPart));
        }

        var option = _contentParts.FirstOrDefault(x => x.Type == contentPartType);
        if (option == null)
        {
            option = new ContentPartOption(contentPartType);
            _contentParts.Add(option);
        }

        return option;
    }

    internal void AddFieldHandler(Type contentFieldType, Type handlerType)
    {
        var option = GetOrAddContentField(contentFieldType);
        option.AddHandler(handlerType);
    }

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