RicoSuter/NSwag · error · InvalidOperationException

No dependency resolver available to create a command…

Error message

No dependency resolver available to create a command without default constructor.

What it means

CreateCommand instantiates the resolved command type by reflection, preferring a parameterless constructor. If the only accessible constructor has parameters and no IDependencyResolver was configured on the processor, it cannot supply dependencies and throws this InvalidOperationException.

Solutions

1. Add a parameterless constructor to the command class, or
2. Configure a dependency resolver on the command host before processing (e.g. a class implementing IDependencyResolver returning GetService instances).
3. Register the command through the overload that accepts a factory/instance instead of relying on reflection.

Example fix

// before
var host = new NSwagCommandLineHost(); // no dependency resolver
// after
host.DependencyResolver = new SimpleInjectorDependencyResolver(container);

var ctor = commandType.GetConstructors();
if (!ctor.Any(c => c.GetParameters().Length == 0) && hostDependencyResolver == null)
    throw new InvalidOperationException($"{commandType} needs a dependency resolver or a parameterless constructor.");

try { return host.GetCommand(args); }
catch (InvalidOperationException ex) when (ex.Message.Contains("No dependency resolver"))
{ throw new StartupException($"Register a dependency resolver for {commandType.Name}.", ex); }

When it happens

Trigger: A registered command type exposes only constructors with parameters while CommandLineProcessorHost/processor was built without a dependency resolver; CreateCommand then fails at the _dependencyResolver == null check.

Common situations: Custom NConsole commands with injected dependencies registered in a host created via the default constructor; library upgrades changing a built-in command's constructor signature.

Understand the failure class

Background: "not installed", "pip install", "required for": how missing-dependency errors surface across open-source libraries — this error's family across 34 libraries.

Related errors

• Cannot create an instance of
• API Explorer not registered in DI.
• The NSwag DI services are not registered: Call…
• The OpenAPI/Swagger document
• The specified runtime in the document

AI-assisted analysis of RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/191eb9aa75af93fa. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Commands/NConsole/CommandLineProcessor.cs:271

            }
            else
            {
                throw new InvalidOperationException($"Could not retrieve command from arguments {string.Join(", ", args)}");
            }
        }

        /// <exception cref="InvalidOperationException">No dependency resolver available to create a command without default constructor.</exception>
        private IConsoleCommand CreateCommand(Type commandType)
        {
            var constructors = commandType.GetTypeInfo().DeclaredConstructors;
            IConsoleCommand command;

            if (constructors.Any())
            {
                var constructor = constructors.First(c => !c.IsStatic);

                if (constructor.GetParameters().Length > 0 && _dependencyResolver == null)
                    throw new InvalidOperationException("No dependency resolver available to create a command without default constructor.");

                var parameters = constructor.GetParameters()
                    .Select(param => _dependencyResolver.GetService(param.ParameterType))
                    .ToArray();

                command = (IConsoleCommand)constructor.Invoke(parameters);
            }
            else
            {
                if (_dependencyResolver == null)
                {
                    throw new InvalidOperationException($"Cannot create an instance of {commandType} because it does not " +
                                                        $"have any accessible constructors and no dependency resolver is available.");
                }

                command = (IConsoleCommand)_dependencyResolver.GetService(commandType);
            }

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