default value for option/argument (and show it in the help text)

src/CommandLineUtils/Abstractions/ValueParserProvider.cs

@@ -101,7 +101,7 @@ public IValueParser GetParser(Type type)
                 return parser;
             }
 
-            if (ReflectionHelper.IsNullableType(type, out var wrappedType) && wrappedType != null)
+            if (ReflectionHelper.IsNullableType(type, out var wrappedType))
             {
                 if (wrappedType.IsEnum)
                 {
@@ -114,21 +114,16 @@ public IValueParser GetParser(Type type)
                 }
             }
 
-            if (!type.IsGenericType)
+            if (ReflectionHelper.IsSpecialValueTupleType(type, out var wrappedType2))
             {
-                return null;
-            }
-
-            var typeDef = type.GetGenericTypeDefinition();
-            if (typeDef == typeof(ValueTuple<,>) && type.GenericTypeArguments[0] == typeof(bool))
-            {
-                var innerParser = GetParser(type.GenericTypeArguments[1]);
+                var innerParser = GetParser(wrappedType2);
                 if (innerParser == null)
                 {
                     return null;
                 }
                 var method = typeof(ValueTupleValueParser).GetMethod(nameof(ValueTupleValueParser.Create)).MakeGenericMethod(type.GenericTypeArguments[1]);
                 return (IValueParser)method.Invoke(null, new object[] { innerParser });
+
             }
 
             return null;

src/CommandLineUtils/CommandArgument.cs

@@ -16,6 +16,8 @@ namespace McMaster.Extensions.CommandLineUtils
     /// <seealso cref="CommandOption"/>
     public class CommandArgument
     {
+        private string? _defaultValue;
+
         /// <summary>
         /// Initializes a new instance of <see cref="CommandArgument"/>.
         /// </summary>
@@ -54,6 +56,22 @@ public CommandArgument()
         /// </summary>
         public string? Value => Values.FirstOrDefault();
 
+        /// <summary>
+        /// The default value of the argument.
+        /// </summary>
+        public string? DefaultValue
+        {
+            get => _defaultValue;
+            set
+            {
+                _defaultValue = value;
+                if (value != null)
+                {
+                    Values.Add(value);
+                }
+            }
+        }
+
         /// <summary>
         /// A collection of validators that execute before invoking <see cref="CommandLineApplication.OnExecute(Func{int})"/>.
         /// When validation fails, <see cref="CommandLineApplication.ValidationErrorHandler"/> is invoked.

src/CommandLineUtils/CommandArgument{T}.cs

@@ -20,6 +20,7 @@ public class CommandArgument<T> : CommandArgument, IInternalCommandParamOfT
     {
         private readonly List<T> _parsedValues = new List<T>();
         private readonly IValueParser<T> _valueParser;
+        private T _defaultValue;
 
         /// <summary>
         /// Initializes a new instance of <see cref="CommandArgument{T}" />
@@ -29,6 +30,7 @@ public CommandArgument(IValueParser<T> valueParser)
         {
             _valueParser = valueParser ?? throw new ArgumentNullException(nameof(valueParser));
             UnderlyingType = typeof(T);
+            SetBaseDefaultValue(default);
         }
 
         /// <summary>
@@ -41,6 +43,19 @@ public CommandArgument(IValueParser<T> valueParser)
         /// </summary>
         public IReadOnlyList<T> ParsedValues => _parsedValues;
 
+        /// <summary>
+        /// The default value of the argument.
+        /// </summary>
+        public new T DefaultValue
+        {
+            get => _defaultValue;
+            set
+            {
+                _defaultValue = value;
+                SetBaseDefaultValue(value);
+            }
+        }
+
         void IInternalCommandParamOfT.Parse(CultureInfo culture)
         {
             _parsedValues.Clear();
@@ -49,5 +64,13 @@ void IInternalCommandParamOfT.Parse(CultureInfo culture)
                 _parsedValues.Add(_valueParser.Parse(Name, t, culture));
             }
         }
+
+        void SetBaseDefaultValue(T value)
+        {
+            if (!ReflectionHelper.IsSpecialValueTupleType(typeof(T), out _))
+            {
+                base.DefaultValue = value?.ToString();
+            }
+        }
     }
 }

src/CommandLineUtils/CommandLineApplication.cs

@@ -128,6 +128,11 @@ internal CommandLineApplication(CommandLineApplication parent, string name)
 
             _conventionContext = CreateConventionContext();
 
+            this.Initialize();
+        }
+
+        internal virtual void Initialize()
+        {
             if (Parent != null)
             {
                 foreach (var convention in Parent._conventions)

src/CommandLineUtils/CommandLineApplication{T}.cs

@@ -25,7 +25,6 @@ public class CommandLineApplication<TModel> : CommandLineApplication, IModelAcce
         public CommandLineApplication()
             : base()
         {
-            Initialize();
         }
 
         /// <summary>
@@ -35,7 +34,6 @@ public CommandLineApplication()
         public CommandLineApplication(IConsole console)
             : base(console)
         {
-            Initialize();
         }
 
         /// <summary>
@@ -46,7 +44,6 @@ public CommandLineApplication(IConsole console)
         public CommandLineApplication(IConsole console, string workingDirectory)
             : base(console, workingDirectory)
         {
-            Initialize();
         }
 
         /// <summary>
@@ -64,18 +61,17 @@ public CommandLineApplication(IConsole console, string workingDirectory)
         public CommandLineApplication(IHelpTextGenerator helpTextGenerator, IConsole console, string workingDirectory)
             : base(helpTextGenerator, console, workingDirectory)
         {
-            Initialize();
         }
 
         internal CommandLineApplication(CommandLineApplication parent, string name)
             : base(parent, name)
         {
-            Initialize();
         }
 
-        private void Initialize()
+        internal override void Initialize()
         {
             _lazy = new Lazy<TModel>(CreateModel);
+            base.Initialize();
         }
 
         private static TModel DefaultModelFactory()

src/CommandLineUtils/CommandOption.cs

@@ -17,6 +17,8 @@ namespace McMaster.Extensions.CommandLineUtils
     /// </summary>
     public class CommandOption
     {
+        private string? _defaultValue;
+
         /// <summary>
         /// Initializes a new <see cref="CommandOption"/>.
         /// </summary>
@@ -101,6 +103,22 @@ internal CommandOption(CommandOptionType type)
         /// </summary>
         public List<string?> Values { get; } = new List<string?>();
 
+        /// <summary>
+        /// The default value of the option.
+        /// </summary>
+        public string? DefaultValue
+        {
+            get => _defaultValue;
+            set
+            {
+                _defaultValue = value;
+                if (value != null)
+                {
+                    Values.Add(value);
+                }
+            }
+        }
+
         /// <summary>
         /// Defines the type of the option.
         /// </summary>

src/CommandLineUtils/CommandOption{T}.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Reflection;
 using McMaster.Extensions.CommandLineUtils.Abstractions;
 
 namespace McMaster.Extensions.CommandLineUtils
@@ -19,6 +20,7 @@ public class CommandOption<T> : CommandOption, IInternalCommandParamOfT
     {
         private readonly List<T> _parsedValues = new List<T>();
         private readonly IValueParser<T> _valueParser;
+        private T _defaultValue;
 
         /// <summary>
         /// Initializes a new instance of <see cref="CommandOption{T}" />
@@ -31,6 +33,7 @@ public CommandOption(IValueParser<T> valueParser, string template, CommandOption
         {
             _valueParser = valueParser ?? throw new ArgumentNullException(nameof(valueParser));
             UnderlyingType = typeof(T);
+            SetBaseDefaultValue(default);
         }
 
         /// <summary>
@@ -43,6 +46,19 @@ public CommandOption(IValueParser<T> valueParser, string template, CommandOption
         /// </summary>
         public IReadOnlyList<T> ParsedValues => _parsedValues;
 
+        /// <summary>
+        /// The default value of the option.
+        /// </summary>
+        public new T DefaultValue
+        {
+            get => _defaultValue;
+            set
+            {
+                _defaultValue = value;
+                SetBaseDefaultValue(value);
+            }
+        }
+
         void IInternalCommandParamOfT.Parse(CultureInfo culture)
         {
             _parsedValues.Clear();
@@ -51,5 +67,13 @@ void IInternalCommandParamOfT.Parse(CultureInfo culture)
                 _parsedValues.Add(_valueParser.Parse(LongName ?? ShortName ?? SymbolName, t, culture));
             }
         }
+
+        void SetBaseDefaultValue(T value)
+        {
+            if (!ReflectionHelper.IsSpecialValueTupleType(typeof(T), out _))
+            {
+                base.DefaultValue = value?.ToString();
+            }
+        }
     }
 }

src/CommandLineUtils/Conventions/ArgumentAttributeConvention.cs

@@ -95,6 +95,13 @@ public virtual void Apply(ConventionContext context)
             argPropOrder.Add(argumentAttr.Order, prop);
             argOrder.Add(argumentAttr.Order, argument);
 
+            var getter = ReflectionHelper.GetPropertyGetter(prop);
+
+            if (!ReflectionHelper.IsSpecialValueTupleType(prop.PropertyType, out _))
+            {
+                argument.DefaultValue = getter.Invoke(convention.ModelAccessor.GetModel())?.ToString();
+            }
+
             var setter = ReflectionHelper.GetPropertySetter(prop);
 
             if (argument.MultipleValues)

src/CommandLineUtils/Conventions/OptionAttributeConventionBase.cs

@@ -57,6 +57,18 @@ private protected void AddOption(ConventionContext context, CommandOption option
                 context.Application._longOptions.Add(option.LongName, prop);
             }
 
+            if (option.OptionType != CommandOptionType.NoValue)
+            {
+                var getter = ReflectionHelper.GetPropertyGetter(prop);
+
+                var value = getter.Invoke(modelAccessor.GetModel());
+
+                if (!ReflectionHelper.IsSpecialValueTupleType(prop.PropertyType, out _))
+                {
+                    option.DefaultValue = value?.ToString();
+                }
+            }
+
             var setter = ReflectionHelper.GetPropertySetter(prop);
 
             switch (option.OptionType)

src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs

@@ -214,6 +214,11 @@ public virtual void Generate(CommandLineApplication application, TextWriter outp
                         }
                     }
 
+                    if (!string.IsNullOrEmpty(arg.DefaultValue))
+                    {
+                        description += $"\nDefault value is: {arg.DefaultValue}.";
+                    }
+
                     var wrappedDescription = IndentWriter?.Write(description);
                     var message = string.Format(outputFormat, arg.Name, wrappedDescription);
 
@@ -266,6 +271,11 @@ public virtual void Generate(CommandLineApplication application, TextWriter outp
                         }
                     }
 
+                    if (!string.IsNullOrEmpty(opt.DefaultValue))
+                    {
+                        description += $"\nDefault value is: {opt.DefaultValue}.";
+                    }
+
                     var wrappedDescription = IndentWriter?.Write(description);
                     var message = string.Format(outputFormat, Format(opt), wrappedDescription);
 
@@ -412,9 +422,9 @@ private string[] ExtractNamesFromEnum(Type type)
                 return ExtractNamesFromEnum(wrappedType);
             }
 
-            if (ReflectionHelper.IsSpecialValueTupleType(type, out var fieldInfo))
+            if (ReflectionHelper.IsSpecialValueTupleType(type, out var wrappedType2))
             {
-                return ExtractNamesFromEnum(fieldInfo.FieldType);
+                return ExtractNamesFromEnum(wrappedType2);
             }
 
             if (type.IsEnum)

src/CommandLineUtils/Internal/CommandOptionTypeMapper.cs

@@ -55,28 +55,24 @@ public CommandOptionType GetOptionType(Type clrType, ValueParserProvider? valueP
                 return CommandOptionType.SingleValue;
             }
 
-            if (clrType.IsGenericType)
+            if (ReflectionHelper.IsNullableType(clrType, out var wrappedType))
             {
-                var typeDef = clrType.GetGenericTypeDefinition();
-                if (typeDef == typeof(Nullable<>))
-                {
-                    return GetOptionType(clrType.GetGenericArguments().First(), valueParsers);
-                }
+                return GetOptionType(wrappedType, valueParsers);
+            }
 
-                if (typeDef == typeof(Tuple<,>) && clrType.GenericTypeArguments[0] == typeof(bool))
+            if (ReflectionHelper.IsSpecialValueTupleType(clrType, out var wrappedType2))
+            {
+                if (GetOptionType(wrappedType2, valueParsers) == CommandOptionType.SingleValue)
                 {
-                    if (GetOptionType(clrType.GenericTypeArguments[1], valueParsers) == CommandOptionType.SingleValue)
-                    {
-                        return CommandOptionType.SingleOrNoValue;
-                    }
+                    return CommandOptionType.SingleOrNoValue;
                 }
+            }
 
-                if (typeDef == typeof(ValueTuple<,>) && clrType.GenericTypeArguments[0] == typeof(bool))
+            if (ReflectionHelper.IsSpecialTupleType(clrType, out var wrappedType3))
+            {
+                if (GetOptionType(wrappedType3, valueParsers) == CommandOptionType.SingleValue)
                 {
-                    if (GetOptionType(clrType.GenericTypeArguments[1], valueParsers) == CommandOptionType.SingleValue)
-                    {
-                        return CommandOptionType.SingleOrNoValue;
-                    }
+                    return CommandOptionType.SingleOrNoValue;
                 }
             }
 

src/CommandLineUtils/Internal/Delegates.cs

@@ -4,4 +4,6 @@
 namespace McMaster.Extensions.CommandLineUtils
 {
     internal delegate void SetPropertyDelegate(object obj, object? value);
+
+    internal delegate object GetPropertyDelegate(object obj);
 }