yakovypg · yakovypg · Feb 23, 2026 · Feb 23, 2026 · Feb 23, 2026 · Feb 23, 2026
diff --git a/Core/NetArgumentParser/Attributes/EnumValueOptionAttribute.cs b/Core/NetArgumentParser/Attributes/EnumValueOptionAttribute.cs
@@ -30,7 +30,8 @@ public EnumValueOptionAttribute(
         T[]? choices = null,
         string[]? beforeParseChoices = null,
         bool addChoicesToDescription = false,
-        bool addBeforeParseChoicesToDescription = false)
+        bool addBeforeParseChoicesToDescription = false,
+        bool addDefaultValueToDescription = false)
         : base(
             defaultValue,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -45,7 +46,8 @@ public EnumValueOptionAttribute(
             choices,
             beforeParseChoices,
             addChoicesToDescription,
-            addBeforeParseChoicesToDescription)
+            addBeforeParseChoicesToDescription,
+            addDefaultValueToDescription)
     {
         UseDefaultChoices = useDefaultChoices;
     }
@@ -65,7 +67,8 @@ public EnumValueOptionAttribute(
         T[]? choices = null,
         string[]? beforeParseChoices = null,
         bool addChoicesToDescription = false,
-        bool addBeforeParseChoicesToDescription = false)
+        bool addBeforeParseChoicesToDescription = false,
+        bool addDefaultValueToDescription = false)
         : base(
             choices,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -79,7 +82,8 @@ public EnumValueOptionAttribute(
             aliases,
             beforeParseChoices,
             addChoicesToDescription,
-            addBeforeParseChoicesToDescription)
+            addBeforeParseChoicesToDescription,
+            addDefaultValueToDescription)
     {
         UseDefaultChoices = useDefaultChoices;
     }

diff --git a/Core/NetArgumentParser/Attributes/MultipleValueOptionAttribute.cs b/Core/NetArgumentParser/Attributes/MultipleValueOptionAttribute.cs
@@ -28,7 +28,8 @@ public MultipleValueOptionAttribute(
         bool ignoreOrderInChoices = false,
         string[]? aliases = null,
         ContextCaptureType contextCaptureType = ContextCaptureType.None,
-        int numberOfItemsToCapture = -1)
+        int numberOfItemsToCapture = -1,
+        bool addDefaultValueToDescription = false)
         : base(
             defaultValue,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -41,7 +42,8 @@ public MultipleValueOptionAttribute(
             ignoreCaseInChoices,
             aliases,
             choices: null,
-            beforeParseChoices: null)
+            beforeParseChoices: null,
+            addDefaultValueToDescription: addDefaultValueToDescription)
     {
         IgnoreOrderInChoices = ignoreOrderInChoices;
         ContextCapture = CreateContextCapture(contextCaptureType, numberOfItemsToCapture);
@@ -59,7 +61,8 @@ public MultipleValueOptionAttribute(
         bool ignoreOrderInChoices = false,
         string[]? aliases = null,
         ContextCaptureType contextCaptureType = ContextCaptureType.None,
-        int numberOfItemsToCapture = -1)
+        int numberOfItemsToCapture = -1,
+        bool addDefaultValueToDescription = false)
         : base(
             choices: null,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -71,7 +74,8 @@ public MultipleValueOptionAttribute(
             isFinal,
             ignoreCaseInChoices,
             aliases,
-            beforeParseChoices: null)
+            beforeParseChoices: null,
+            addDefaultValueToDescription: addDefaultValueToDescription)
     {
         IgnoreOrderInChoices = ignoreOrderInChoices;
         ContextCapture = CreateContextCapture(contextCaptureType, numberOfItemsToCapture);

diff --git a/Core/NetArgumentParser/Attributes/ValueOptionAttribute.cs b/Core/NetArgumentParser/Attributes/ValueOptionAttribute.cs
@@ -30,7 +30,8 @@ public ValueOptionAttribute(
         T[]? choices = null,
         string[]? beforeParseChoices = null,
         bool addChoicesToDescription = false,
-        bool addBeforeParseChoicesToDescription = false)
+        bool addBeforeParseChoicesToDescription = false,
+        bool addDefaultValueToDescription = false)
         : this(
             choices,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -44,7 +45,8 @@ public ValueOptionAttribute(
             aliases,
             beforeParseChoices,
             addChoicesToDescription,
-            addBeforeParseChoicesToDescription)
+            addBeforeParseChoicesToDescription,
+            addDefaultValueToDescription)
     {
         DefaultValue = new DefaultOptionValue<T>(defaultValue);
     }
@@ -62,7 +64,8 @@ public ValueOptionAttribute(
         string[]? aliases = null,
         string[]? beforeParseChoices = null,
         bool addChoicesToDescription = false,
-        bool addBeforeParseChoicesToDescription = false)
+        bool addBeforeParseChoicesToDescription = false,
+        bool addDefaultValueToDescription = false)
         : this(
             choices: null,
             longName ?? throw new ArgumentNullException(nameof(longName)),
@@ -76,7 +79,8 @@ public ValueOptionAttribute(
             aliases,
             beforeParseChoices,
             addChoicesToDescription,
-            addBeforeParseChoicesToDescription)
+            addBeforeParseChoicesToDescription,
+            addDefaultValueToDescription)
     {
     }
 
@@ -93,7 +97,8 @@ public ValueOptionAttribute(
         string[]? aliases = null,
         string[]? beforeParseChoices = null,
         bool addChoicesToDescription = false,
-        bool addBeforeParseChoicesToDescription = false)
+        bool addBeforeParseChoicesToDescription = false,
+        bool addDefaultValueToDescription = false)
         : base(
             longName ?? throw new ArgumentNullException(nameof(longName)),
             shortName ?? throw new ArgumentNullException(nameof(shortName)),
@@ -109,6 +114,7 @@ public ValueOptionAttribute(
         IgnoreCaseInChoices = ignoreCaseInChoices;
         AddChoicesToDescription = addChoicesToDescription;
         AddBeforeParseChoicesToDescription = addBeforeParseChoicesToDescription;
+        AddDefaultValueToDescription = addDefaultValueToDescription;
 
         Choices = choices;
         BeforeParseChoices = beforeParseChoices;
@@ -118,6 +124,7 @@ public ValueOptionAttribute(
     public bool IgnoreCaseInChoices { get; }
     public bool AddChoicesToDescription { get; }
     public bool AddBeforeParseChoicesToDescription { get; }
+    public bool AddDefaultValueToDescription { get; }
 
     public IEnumerable<T>? Choices { get; }
     public IEnumerable<string>? BeforeParseChoices { get; }
@@ -149,6 +156,9 @@ public override ICommonOption CreateOption(object source, PropertyInfo propertyI
             null,
             t => propertyInfo.SetValue(source, t));
 
+        if (AddDefaultValueToDescription)
+            valueOption.AddDefaultValueToDescription();
+
         if (AddBeforeParseChoicesToDescription && BeforeParseChoices is not null && BeforeParseChoices.Any())
             valueOption.AddBeforeParseChoicesToDescription();
         else if (AddChoicesToDescription && Choices is not null && Choices.Any())

diff --git a/Core/NetArgumentParser/Options/DefaultValueAlreadyAddedToDescriptionException.cs b/Core/NetArgumentParser/Options/DefaultValueAlreadyAddedToDescriptionException.cs
@@ -0,0 +1,29 @@
+using System;
+using System.ComponentModel;
+using System.Runtime.Serialization;
+
+namespace NetArgumentParser.Options;
+
+[Serializable]
+public class DefaultValueAlreadyAddedToDescriptionException : Exception
+{
+    public DefaultValueAlreadyAddedToDescriptionException() { }
+
+    public DefaultValueAlreadyAddedToDescriptionException(string? message)
+        : base(message) { }
+
+    public DefaultValueAlreadyAddedToDescriptionException(string? message, Exception? innerException)
+        : base(message ?? GetDefaultMessage(), innerException) { }
+
+#if NET8_0_OR_GREATER
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.", DiagnosticId = "SYSLIB0051", UrlFormat = "https://aka.ms/dotnet-warnings/{0}")]
+#endif
+    protected DefaultValueAlreadyAddedToDescriptionException(SerializationInfo info, StreamingContext context)
+        : base(info, context) { }
+
+    private static string GetDefaultMessage()
+    {
+        return "Default value have already been added to description.";
+    }
+}
diff --git a/Core/NetArgumentParser/Options/ValueOption.cs b/Core/NetArgumentParser/Options/ValueOption.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
@@ -15,6 +16,7 @@ public class ValueOption<T> : CommonOption, IValueOption<T>
 {
     private List<T> _choices;
     private List<string> _beforeParseChoices;
+    private bool _areDefaultValueAddedToDescription;
     private bool _areChoicesAddedToDescription;
 
     public ValueOption(
@@ -152,6 +154,50 @@ public bool TrySetConverter(IValueConverter converter)
         return false;
     }
 
+    public void AddDefaultValueToDescription(
+        string separator = ", ",
+        string prefix = " [default=",
+        string postfix = "]",
+        string arraySeparator = "; ",
+        string arrayPrefix = "[",
+        string arrayPostfix = "]",
+        string nullPresenter = "null")
+    {
+        ExtendedArgumentNullException.ThrowIfNull(separator, nameof(separator));
+        ExtendedArgumentNullException.ThrowIfNull(prefix, nameof(prefix));
+        ExtendedArgumentNullException.ThrowIfNull(postfix, nameof(postfix));
+        ExtendedArgumentNullException.ThrowIfNull(arraySeparator, nameof(arraySeparator));
+        ExtendedArgumentNullException.ThrowIfNull(arrayPrefix, nameof(arrayPrefix));
+        ExtendedArgumentNullException.ThrowIfNull(arrayPostfix, nameof(arrayPostfix));
+        ExtendedArgumentNullException.ThrowIfNull(nullPresenter, nameof(nullPresenter));
+
+        if (_areDefaultValueAddedToDescription)
+            throw new DefaultValueAlreadyAddedToDescriptionException();
+
+        string defaultValueString;
+
+        if (DefaultValue is null)
+        {
+            defaultValueString = nullPresenter;
+        }
+        else if (DefaultValue.Value is IEnumerable defaultValueEnumerable)
+        {
+            defaultValueString = ExtendedString.JoinWithExpand(
+                separator,
+                arraySeparator,
+                arrayPrefix,
+                arrayPostfix,
+                defaultValueEnumerable.Cast<object>());
+        }
+        else
+        {
+            defaultValueString = DefaultValue.Value?.ToString() ?? nullPresenter;
+        }
+
+        Description += $"{prefix}{defaultValueString}{postfix}";
+        _areDefaultValueAddedToDescription = true;
+    }
+
     public void AddChoicesToDescription(
         string separator = ", ",
         string prefix = " (",

diff --git a/Documentation/AdditionalFeatures.md b/Documentation/AdditionalFeatures.md
@@ -7,6 +7,7 @@
 *    [Parse Known Arguments](#parse-known-arguments)
 *    [Skip Arguments](#skip-arguments)
 *    [Getting Info About Handled Options And Subcommands](#getting-info-about-handled-options-and-subcommands)
+*    [Add Default Value To Description](#add-default-value-to-description)
 *    [Add Choices To Description](#add-choices-to-description)
 
 ## Negative Numbers & Scientific Notation
@@ -116,6 +117,50 @@ ParseArgumentsResult result = parser.ParseKnownArguments(args, out _);
 // result.HandledSubcommands: compressSubcommand
 ```
 
+## Add Default Value To Description
+You can add default value to the value option description automatically. To do this, you can use `AddDefaultValueToDescription()` method as follows:
+
+```cs
+var angleOption = new ValueOption<int>(
+    defaultValue: new DefaultOptionValue<int>(45),
+    longName: "angle",
+    description: "Angle");
+
+angleOption.AddDefaultValueToDescription();
+
+var namesOption = new MultipleValueOption<List<string>>(
+    defaultValue: new DefaultOptionValue<IList<List<string>>>([["Leo", "Max"], ["Eva", "Zoe"]]),
+    longName: "input",
+    description: "Names");
+
+namesOption.AddDefaultValueToDescription(
+    separator: ", ",
+    prefix: " (default: ",
+    postfix: ")",
+    arraySeparator: "; ",
+    arrayPrefix: "[",
+    arrayPostfix: "]",
+    nullPresenter: "null");
+
+var fileModeOption = new ValueOption<FileMode>(
+    defaultValue: new DefaultOptionValue<FileMode>(FileMode.Open),
+    longName: "mode",
+    description: "File mode");
+
+fileModeOption.AddDefaultValueToDescription(
+    prefix: ". Default ",
+    postfix: string.Empty);
+
+// Angle [default=45]
+Console.WriteLine(angleOption.Description);
+
+// Names (default: [John, Max]; [John1, Max1])
+Console.WriteLine(namesOption.Description);
+
+// File mode. Default Open
+Console.WriteLine(fileModeOption.Description);
+```
+
 ## Add Choices To Description
 You can add choices to the value option description automatically. To do this, you can use `AddChoicesToDescription()` method as follows:
 

diff --git a/Documentation/ParserGenerationUsingAttributes.md b/Documentation/ParserGenerationUsingAttributes.md
@@ -74,7 +74,8 @@ internal class CustomParserConfig
         choices: [FileMode.Create, FileMode.Open],
         beforeParseChoices: ["Create", "Open"],
         addChoicesToDescription: false,
-        addBeforeParseChoicesToDescription: true)
+        addBeforeParseChoicesToDescription: true,
+        addDefaultValueToDescription: false)
     ]
     public FileMode Mode { get; set; }
 
@@ -143,15 +144,16 @@ internal class CustomParserConfig
         choices: [0, 45, 90],
         beforeParseChoices: ["0", "45", "90"],
         addChoicesToDescription: true,
-        addBeforeParseChoicesToDescription: false)
+        addBeforeParseChoicesToDescription: false,
+        addDefaultValueToDescription: true)
     ]
     public double? Angle { get; set; }
 }
 
 internal record Point(double X, double Y, double Z);
 ```
 
-Note that attributes allow you to add choices and before parse choices to the option description (if the corresponding option supports it) using `addChoicesToDescription` and `addBeforeParseChoicesToDescription` parameters, so you don't need to find the generated options to do this.
+Note that attributes allow you to add default value, choices and before parse choices to the option description (if the corresponding option supports it) using `addDefaultValueToDescription`, `addChoicesToDescription`, and `addBeforeParseChoicesToDescription` parameters, so you don't need to find the generated options to do this.
 
 ### Group Attributes
 Goups can be configured using `OptionGroupAttribute` attribute. In addition to specifying the group header and description, you should specify the group ID. It is necessary for the correct placement of options, since groups can have the same header. Options that you want to put in the same group must be marked with an attribute with the same ID. You should't specify header and description for all group attributes with same id. It is enough to do this for only one attribute.

diff --git a/Examples/NetArgumentParser.Examples.ParserGenerationUsingAttributes/Program.cs b/Examples/NetArgumentParser.Examples.ParserGenerationUsingAttributes/Program.cs
@@ -105,7 +105,8 @@ public CustomParserConfig()
         choices: [FileMode.Create, FileMode.Open],
         beforeParseChoices: ["Create", "Open"],
         addChoicesToDescription: false,
-        addBeforeParseChoicesToDescription: true)
+        addBeforeParseChoicesToDescription: true,
+        addDefaultValueToDescription: false)
     ]
     [OptionGroup("complex-values", "", "")]
     public FileMode Mode { get; set; }
@@ -178,7 +179,8 @@ public CustomParserConfig()
         choices: [0, 45, 90],
         beforeParseChoices: ["0", "45", "90"],
         addChoicesToDescription: true,
-        addBeforeParseChoicesToDescription: false)
+        addBeforeParseChoicesToDescription: false,
+        addDefaultValueToDescription: true)
     ]
     [OptionGroup("values", "", "")]
     public double? Angle { get; set; }