RicoSuter/NSwag · error · InvalidOperationException

The parameter '" + Name + "' is required.

Error message

The parameter '" + Name + "' is required.

What it means

When a required argument's value is missing on the command line, NConsole prompts interactively via consoleHost.ReadValue. If the user (or a non-interactive host) answers '[default]', the attribute falls back to the property's current value — but when IsRequired is true there is no acceptable default, so it throws InvalidOperationException naming the required parameter.

Solutions

1. Provide the required argument explicitly on the command line (/name:value) so no prompt occurs.
2. If the argument should have a fallback, set IsRequired = false and supply a default property value in the command class.
3. In non-interactive environments, feed the value via stdin/variables instead of relying on the prompt.
4. Check the .nswag/CI config that the variable mapped to this argument isn't empty.

Example fix

// before
nswag openapi2csclient /output:Client.cs   // input prompted, [default] answered

// after
nswag openapi2csclient /input:swagger.json /output:Client.cs

var missing = requiredArgs.Where(a => !CommandLineArgs.ContainsKey(a)).ToList();
if (missing.Any())
    throw new ArgumentException($"Missing required arguments: {string.Join(", ", missing)}. Supply them on the command line; do not rely on interactive prompts.");

try { result = processor.Process(args); }
catch (InvalidOperationException ex) when (ex.Message.Contains("is required")) { Console.Error.WriteLine($"Supply the missing option: {ex.Message}"); Environment.Exit(2); }

When it happens

Trigger: Running a command without a required argument while IsRequired = true on its [Argument] attribute, and the interactive prompt returns '[default]' (e.g. piped/redirected stdin, CI with no TTY supplying the default sentinel) so no real value is captured.

Common situations: Forgetting /input or /output style required options in scripts; running NSwag in CI where interactive prompts read an empty/auto-answered stdin; marked IsRequired but never supplying the value in an .nswag variables section.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Either the argument Name or Position can be set, but not…

AI-assisted analysis of RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/09f4774da57765c5. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Commands/NConsole/ArgumentAttribute.cs:71

            {
                return ArgumentAttributeBase.ConvertToType(value, property.PropertyType);
            }
                
            if (AcceptsCommandInput && input != null)
                return input;

            if (!ArgumentAttribute.IsInteractiveMode(args) && !IsRequired)
                return property.CanRead ? property.GetValue(command) : null;

            if (ShowPrompt)
            {
                value = consoleHost.ReadValue(GetFullParameterDescription(property, command));
                if (value == "[default]")
                {
                    if (!IsRequired)
                        return property.CanRead ? property.GetValue(command) : null;

                    throw new InvalidOperationException("The parameter '" + Name + "' is required.");
                }

                return ArgumentAttributeBase.ConvertToType(value, property.PropertyType);
            }
            else
                return property.CanRead ? property.GetValue(command) : null;
        }

        private static bool IsInteractiveMode(string[] args)
        {
            return args.Length == 0;
        }

        private bool TryGetPositionalArgumentValue(string[] args, ref string used, out string value)
        {
            if (Position > 0 && Position < args.Length)
            {
                value = args[Position];

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