✅ Is it possible to use union type in C#?

public class UISchemaGenerator
{
    private readonly ConcurrentDictionary<(CustomFieldSet, CultureInfo), UISchemaSpecification> _cache =
        new();

    public UISchemaSpecification Generate(
        CustomFieldSet fieldSet,
        MandatorySecurityScope scope,
        CultureInfo culture = default)
    {
        culture ??= CultureInfo.CurrentCulture;

        return _cache.GetOrAdd(
            key: (fieldSet, culture),
            valueFactory: _ => InternalGenerator(fieldSet, scope, culture));
    }

    private UISchemaSpecification InternalGenerator(
        CustomFieldSet fieldSet,
        MandatorySecurityScope scope,
        CultureInfo culture)
    {
        var schemaBuilder = new UISchemaBuilder();

        foreach (var group in fieldSet.Groups)
        {
            schemaBuilder
                .StartGroup(group.Title)
                    .AddFields(scope, group.Fields)
                .EndGroup();
        }

        return schemaBuilder.Build();
    }

    private UISchemaSpecification InternalGenerator(
        CustomField.CustomField fieldSet,
        MandatorySecurityScope scope,
        CultureInfo culture)
    {
        var schemaBuilder = new UISchemaBuilder()
            .AddField(fieldSet)
            .Build();

        return schemaBuilder;
    }
}

public UISchemaSpecification Generate(
   Union<CustomField, CustomFieldSet> fieldSet,
   MandatorySecurityScope scope,
   CultureInfo culture = default)
{
    // if someone passed CustomField then call InternalGenerator for CustomField
    // if someone passed CustomFieldSetthen call InternalGenerator for CustomFieldSet
}