RicoSuter/NSwag · error · ArgumentException

The argument 'Input' was empty.

Error message

The argument 'Input' was empty.

What it means

GetInputSwaggerDocument resolves the Swagger/OpenAPI document to process. When no document was loaded in memory, it falls back to the Input property; if that string is null or empty, it throws ArgumentException. It simply means no input document (URL, file path, or inline JSON) was supplied.

Solutions

  1. Pass an explicit input: a swagger file path, an http(s) URL, or inline JSON via the Input property (/input:...).
  2. If input comes from a variable/config, validate it is non-empty before invoking NSwag.
  3. Check shell quoting — an argument swallowed by quotes/whitespace can collapse to an empty value.
  4. If the document should come from runtime generation (aspnetcore/webapi), use the matching command rather than a document-input command.

Example fix

// before
nswag openapi2csclient /output:ApiClient.cs

// after
nswag openapi2csclient /input:swagger.json /output:ApiClient.cs
Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrWhiteSpace(input) && document == null)
    throw new ArgumentException("Provide /input as a swagger file path, URL, or inline JSON before running the command.");

Try / catch

try { await command.GetInputSwaggerDocumentAsync(); }
catch (ArgumentException ex) when (ex.Message.Contains("argument 'Input' was empty")) { Console.Error.WriteLine("Missing --input: pass a file path, URL, or inline JSON."); }

Prevention

When it happens

Trigger: Calling a command (e.g. openapi2csclient, webapi2openapi with /output) without setting the Input argument — e.g. `nswag openapi2csclient /output:Client.cs` with no /input, or programmatically creating a command and never assigning Input.

Common situations: Omitting /input on the command line; referencing a settings file that doesn't set the input variable; empty environment-variable expansion like /input:%SWAGGER_URL% resolving to nothing; calling the NSwag.Commands API in code with defaults.

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 RicoSuter/NSwag@63daf8fcc3 (2026-09-14). Data as JSON: /api/errors/e871c7d04c1c4cf3. Report an issue: GitHub.

Appendix: source

Thrown at src/NSwag.Commands/Commands/InputOutputCommandBase.cs:39

        public object Input { get; set; }

        [Argument(Name = "ServiceHost", IsRequired = false, Description = "Overrides the service host of the web document (optional, use '.' to remove the hostname).")]
        public string ServiceHost { get; set; }

        [Argument(Name = "ServiceSchemes", IsRequired = false, Description = "Overrides the allowed schemes of the web service (optional, comma separated, 'http', 'https', 'ws', 'wss').")]
        public string[] ServiceSchemes { get; set; }

        /// <exception cref="ArgumentException">The argument 'Input' was empty.</exception>
        protected async Task<OpenApiDocument> GetInputSwaggerDocument()
        {
            var document = Input as OpenApiDocument;
            if (document == null)
            {
                var input = Input.ToString();

                if (string.IsNullOrEmpty(input))
                {
                    throw new ArgumentException("The argument 'Input' was empty.");
                }

                document = await ReadSwaggerDocumentAsync(input);
            }

            if (ServiceHost == ".")
            {
                document.Host = string.Empty;
                document.Schemes.Clear();
            }
            else
            {
                if (!string.IsNullOrEmpty(ServiceHost))
                {
                    document.Host = ServiceHost;
                }

                if (ServiceSchemes != null && ServiceSchemes.Length > 0)

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