What causes "Output converter IDs cannot be empty." in elsa-workflows/elsa-core?

Pass a non-empty, meaningful converter ID string

elsa-workflows/elsa-core · error · ArgumentException

Output converter IDs cannot be empty.

Error message

Output converter IDs cannot be empty.

What it means

AddOutputConverter registers an output converter with an ID; ValidateUniqueId rejects empty/whitespace IDs with ArgumentException before any duplicate check. This keeps converter IDs stable and valid for lookup. It is a guard against misconfigured registrations.

Solutions

Pass a non-empty, meaningful converter ID string
If the ID comes from configuration, fail fast with a clear config error instead of registering
Guard with string.IsNullOrWhiteSpace before calling AddOutputConverter

Example fix

// before
services.AddOutputConverter(new OutputConverterDescriptor { Id = config["Converter:Id"]! });
// after
var id = config["Converter:Id"];
if (string.IsNullOrWhiteSpace(id)) throw new InvalidOperationException("Output converter id must be configured.");
services.AddOutputConverter(new OutputConverterDescriptor { Id = id });

Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrWhiteSpace(id)) throw new ArgumentException("Converter id must be non-empty.", nameof(id));

Try / catch

try { services.AddOutputConverter(descriptor); } catch (ArgumentException ex) when (ex.ParamName == "id") { logger.LogError(ex, "Invalid output converter id"); }

Prevention

Never build IDs from possibly-empty config values without checking
Centralize converter registration so IDs are reviewed in one place

When it happens

Trigger: Calling services.AddOutputConverter(new OutputConverterDescriptor { Id = "" }) or AddOutputConverter(id: " ", ...) with a null, empty, or whitespace-only ID string.

Common situations: Building the descriptor from config where the ID key is missing; string interpolation producing an empty value; copy-pasting a registration template without filling in the ID.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of elsa-workflows/elsa-core@fe9217bdfa (2026-09-13). Data as JSON: /api/errors/2b2021f69bf22fcb. Report an issue: GitHub.

Appendix: source

Thrown at src/modules/Elsa.Workflows.Core/Extensions/OutputConverterServiceCollectionExtensions.cs:42

        OutputConverterRegistry.ValidateDescriptor(descriptor);
        ValidateUniqueId(services, descriptor.Id);

        var registration = new OutputConverterRegistration(descriptor, descriptor.Id, serviceLifetime);
        services.AddSingleton(registration);
        services.Add(ServiceDescriptor.DescribeKeyed(
            typeof(IOutputConverter),
            descriptor.Id,
            typeof(TConverter),
            serviceLifetime));
        services.TryAddSingleton<IOutputConverterRegistry, OutputConverterRegistry>();
        return services;
    }

    private static void ValidateUniqueId(IServiceCollection services, string id)
    {
        if (string.IsNullOrWhiteSpace(id))
            throw new ArgumentException("Output converter IDs cannot be empty.", nameof(id));

        var existingRegistration = services
            .Where(x => x.ServiceType == typeof(OutputConverterRegistration))
            .Select(x => x.ImplementationInstance)
            .OfType<OutputConverterRegistration>()
            .FirstOrDefault(x => string.Equals(x.Descriptor.Id, id, StringComparison.OrdinalIgnoreCase));

        if (existingRegistration != null)
            throw new InvalidOperationException($"Output converter ID '{id}' is already registered or differs from '{existingRegistration.Descriptor.Id}' only by case.");
    }
}

View on GitHub (pinned to fe9217bdfa)