RicoSuter/NSwag · error · InvalidOperationException

Cannot create an instance of

Error message

Cannot create an instance of {commandType} because it does not have any accessible constructors and no dependency resolver is available.

What it means

CreateCommand throws this InvalidOperationException when the command type has no accessible constructors at all (e.g. private, abstract, or static-only) AND no dependency resolver is available to have the DI container construct it. It is the terminal fallback of command instantiation.

Solutions

Make the command class concrete with at least one public constructor (ideally parameterless).
Provide a dependency resolver so the container can create the instance.
Verify the registered Type is the concrete command class, not an abstract base or interface.

Example fix

// before
public abstract class MyCommand : IConsoleCommand { private MyCommand() {} }
// after
public class MyCommand : IConsoleCommand { public MyCommand() {} }

Defensive patterns

Strategy: validation

Validate before calling

if (commandType.IsAbstract || commandType.GetConstructors(BindingFlags.Public | BindingFlags.Instance).Length == 0)
    throw new ArgumentException($"{commandType} must be concrete with a public constructor.");

Type guard

bool IsInstantiable(Type t) => !t.IsAbstract && !t.IsInterface && t.GetConstructors(BindingFlags.Public | BindingFlags.Instance).Any(c => !c.IsStatic);

Try / catch

try { return host.GetCommand(args); }
catch (InvalidOperationException ex) when (ex.Message.Contains("does not have any accessible constructors"))
{ throw new StartupException($"{commandType} is not constructible; add a public constructor or a resolver.", ex); }

Prevention

Register concrete classes only, never abstract bases or interfaces.
Include a public (parameterless) constructor on every custom command.
Exercise command creation in CI smoke tests.

When it happens

Trigger: The command type resolved from the registry exposes no public constructors and _dependencyResolver is null; reflection cannot Invoke any constructor and DI cannot be used.

Common situations: Registering an abstract base command class or a type with only private constructors; accidentally registering the wrong Type in a custom host.

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

AI-assisted analysis of RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/5d0c8f6750ac4502. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Commands/NConsole/CommandLineProcessor.cs:283

            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);
            }

            return command;
        }
    }
}

View on GitHub (pinned to 63daf8fcc3)