DotVariantSchemaTransformer.cs

/// <summary>
/// Automatically detects dotVariant union types and properly re-schemas
/// them to instead be registered with oneOf OpenApi spec
/// </summary>
public class DotVariantSchemaTransformer : IOpenApiSchemaTransformer
{
    public async Task TransformAsync(
        OpenApiSchema schema, 
        OpenApiSchemaTransformerContext context, 
        CancellationToken cancellationToken
    )
    {
        // Step one: confirm it is a public class
        var type = context.JsonTypeInfo.Type;
       
        if (!type.IsClass || !type.IsPublic)
        {
            return;
        }

        // Step 2: It should have a void Visit(...) function
        var visit = type.GetMethods(BindingFlags.Public | BindingFlags.Instance)
            .FirstOrDefault(m => 
                m.Name == "Visit" &&
                !m.IsGenericMethod &&
                m.ReturnType == typeof(void)
            );

        if (visit == null)
        {
            return;
        }

        var unionTypes = new HashSet<Type>();

        // Step 3: all params of the visit function should be Action<SomeType>
        // Extract out all those SomeTypes
        foreach (var p in visit.GetParameters())
        {
            if (!p.ParameterType.IsGenericType) 
            {
                return;
            }

            var def = p.ParameterType.GetGenericTypeDefinition();
            if (def != typeof(Action<>)) 
            {
                return;
            }

            var unionType = p.ParameterType.GetGenericArguments()[0];

            unionTypes.Add(unionType);
        }

        // Step 4: clear out the non-exposed properties that show up defaul
        // on a dotVariant type (IsEmpty, Index)
        schema.Properties = null;

        // Step 5: Convert all the extracted types and register them as OneOfs
        schema.OneOf = [];
        foreach(var unionType in unionTypes)
        {
            schema.OneOf.Add(
                await context.GetOrCreateSchemaAsync(
                    unionType, cancellationToken: cancellationToken
                )
            );
        }
    }

}