[Feature] DefaultValue for operation parameters defined by class

Modify SwaggerGenerator so that it takes DefaultValueAttribute into account.

Given following operation:

/*
 * Matches following urls: 
 * /api/myresource/
 * /api/myresource?skip=10
 * /api/myresource?skip=10?take=20
 */
[HttpGet]
public IActionResult Index(MyQueryParams query)
{   
}

public class MyQueryParams
{
        [DefaultValue(0)]
        public int Skip { get; set; } = 0;

        [DefaultValue(10)]
        public int Top { get; set; } = 10;
}

Swashbucle should generate the parameters as Optional and with Default value:

 "get": {
    "parameters": [
        {
            "name": "skip",
            "in": "query",
            "required": false,
            "default": 0,
            "type": "integer",
            "format": "int32"
        },
        {
            "name": "top",
            "in": "query",
            "required": false,
            "default": 10,
            "type": "integer",
            "format": "int32"
        }
    ]
}

[whyleee]

@Liero you can add an operation filter as a quick fix, I wrote one that supports both property DefaultValue attributes and default values in method parameters:

// Adds Swagger support for optional parameters with default values
public class UpdateOptionalParamatersWithDefaultValues : IOperationFilter
{
    private readonly MvcJsonOptions _mvcJsonOptions;

    public UpdateOptionalParamatersWithDefaultValues(IOptions<MvcJsonOptions> mvcJsonOptions)
    {
        _mvcJsonOptions = mvcJsonOptions.Value;
    }

    public void Apply(Operation operation, OperationFilterContext context)
    {
        if (operation.Parameters == null || !operation.Parameters.Any())
        {
            return;
        }

        var parameterValuePairs = context.ApiDescription.ParameterDescriptions
            .Where(parameter => GetDefaultValueAttribute(parameter) != null || GetParameterInfo(parameter).HasDefaultValue)
            .ToDictionary(parameter => parameter.Name, GetDefaultValue);

        foreach (var parameter in operation.Parameters)
        {
            if (parameterValuePairs.TryGetValue(parameter.Name, out var defaultValue))
            {
                parameter.Extensions.Add("default", defaultValue);
                parameter.Required = false;
            }
        }
    }

    private DefaultValueAttribute GetDefaultValueAttribute(ApiParameterDescription parameter)
    {
        if (!(parameter.ModelMetadata is DefaultModelMetadata metadata) || metadata.Attributes.PropertyAttributes == null)
        {
            return null;
        }

        return metadata.Attributes.PropertyAttributes
            .OfType<DefaultValueAttribute>()
            .FirstOrDefault();
    }

    public ParameterInfo GetParameterInfo(ApiParameterDescription parameter)
    {
        return ((ControllerParameterDescriptor) parameter.ParameterDescriptor).ParameterInfo;
    }

    private object GetDefaultValue(ApiParameterDescription parameter)
    {
        var parameterInfo = GetParameterInfo(parameter);

        if (parameterInfo.HasDefaultValue)
        {
            if (parameter.Type.IsEnum)
            {
                var stringEnumConverter = _mvcJsonOptions.SerializerSettings.Converters
                    .OfType<StringEnumConverter>()
                    .FirstOrDefault();

                if (stringEnumConverter != null)
                {
                    var defaultValue = parameterInfo.DefaultValue.ToString();
                    return stringEnumConverter.CamelCaseText ? ToCamelCase(defaultValue) : defaultValue;
                }
            }

            return parameterInfo.DefaultValue;
        }

        var defaultValueAttribute = GetDefaultValueAttribute(parameter);

        return defaultValueAttribute.Value;
    }

    private string ToCamelCase(string name)
    {
        return char.ToLowerInvariant(name[0]) + name.Substring(1);
    }
}

[jondmcelroy]

Default Value Attribute is still ignored when being used as FromQuery in versions 4.0.1 and 5.0.0. @whyleee’s fix is very appreciated.