replace ICompletionSource with a Func

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -1,7 +1,7 @@
 System.CommandLine
-  public abstract class Argument : Symbol, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
+  public abstract class Argument : Symbol, System.CommandLine.Binding.IValueDescriptor
     public ArgumentArity Arity { get; set; }
-    public System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> Completions { get; }
+    public System.Collections.Generic.ICollection<System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem>>> Completions { get; }
     public System.Boolean HasDefaultValue { get; }
     public System.String HelpName { get; set; }
     public System.Type ValueType { get; }
@@ -12,7 +12,7 @@ System.CommandLine
     public System.Void SetDefaultValueFactory(System.Func<System.Object> defaultValueFactory)
     public System.Void SetDefaultValueFactory(System.Func<System.CommandLine.Parsing.ArgumentResult,System.Object> defaultValueFactory)
     public System.String ToString()
-  public class Argument<T> : Argument, IValueDescriptor<T>, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
+  public class Argument<T> : Argument, IValueDescriptor<T>, System.CommandLine.Binding.IValueDescriptor
     .ctor()
     .ctor(System.String name, System.String description = null)
     .ctor(System.String name, Func<T> defaultValueFactory, System.String description = null)
@@ -45,7 +45,7 @@ System.CommandLine
     public static TArgument LegalFilePathsOnly<TArgument>(this TArgument argument)
     public static ParseResult Parse(this Argument argument, System.String commandLine)
     public static ParseResult Parse(this Argument argument, System.String[] args)
-  public class Command : IdentifierSymbol, System.Collections.Generic.IEnumerable<Symbol>, System.Collections.IEnumerable, System.CommandLine.Completions.ICompletionSource
+  public class Command : IdentifierSymbol, System.Collections.Generic.IEnumerable<Symbol>, System.Collections.IEnumerable
     .ctor(System.String name, System.String description = null)
     public System.Collections.Generic.IReadOnlyList<Argument> Arguments { get; }
     public System.Collections.Generic.IEnumerable<Symbol> Children { get; }
@@ -109,9 +109,8 @@ System.CommandLine
     .ctor()
     .ctor(System.String message, System.Exception innerException)
   public static class CompletionSourceExtensions
-    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.String>> complete)
-    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem>> complete)
-    public static System.Void Add(this System.Collections.Generic.ICollection<System.CommandLine.Completions.ICompletionSource> completionSources, System.String[] completions)
+    public static System.Void Add(this System.Collections.Generic.ICollection<System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem>>> completionSources, System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.String>> complete)
+    public static System.Void Add(this System.Collections.Generic.ICollection<System.Func<System.CommandLine.Completions.CompletionContext,System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem>>> completionSources, System.String[] completions)
   public static class ConsoleExtensions
     public static System.Void Write(this IConsole console, System.String value)
     public static System.Void WriteLine(this IConsole console, System.String value)
@@ -145,7 +144,7 @@ System.CommandLine
     public System.Int32 Invoke(System.CommandLine.Invocation.InvocationContext context)
     public System.Threading.Tasks.Task<System.Int32> InvokeAsync(System.CommandLine.Invocation.InvocationContext context)
   public interface IConsole : System.CommandLine.IO.IStandardError, System.CommandLine.IO.IStandardIn, System.CommandLine.IO.IStandardOut
-  public abstract class IdentifierSymbol : Symbol, System.CommandLine.Completions.ICompletionSource
+  public abstract class IdentifierSymbol : Symbol
     public System.Collections.Generic.IReadOnlyCollection<System.String> Aliases { get; }
     public System.String Name { get; set; }
     public System.Void AddAlias(System.String alias)
@@ -188,7 +187,7 @@ System.CommandLine
     public System.String UnrecognizedCommandOrArgument(System.String arg)
     public System.String VersionOptionCannotBeCombinedWithOtherArguments(System.String optionAlias)
     public System.String VersionOptionDescription()
-  public abstract class Option : IdentifierSymbol, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
+  public abstract class Option : IdentifierSymbol, System.CommandLine.Binding.IValueDescriptor
     public System.Boolean AllowMultipleArgumentsPerToken { get; set; }
     public System.String ArgumentHelpName { get; set; }
     public ArgumentArity Arity { get; set; }
@@ -199,7 +198,7 @@ System.CommandLine
     public System.Boolean HasAliasIgnoringPrefix(System.String alias)
     public System.Void SetDefaultValue(System.Object value)
     public System.Void SetDefaultValueFactory(System.Func<System.Object> defaultValueFactory)
-  public class Option<T> : Option, IValueDescriptor<T>, System.CommandLine.Binding.IValueDescriptor, System.CommandLine.Completions.ICompletionSource
+  public class Option<T> : Option, IValueDescriptor<T>, System.CommandLine.Binding.IValueDescriptor
     .ctor(System.String name, System.String description = null)
     .ctor(System.String[] aliases, System.String description = null)
     .ctor(System.String name, Func<System.CommandLine.Parsing.ArgumentResult,T> parseArgument, System.Boolean isDefault = False, System.String description = null)
@@ -239,11 +238,11 @@ System.CommandLine
     public T GetValue<T>(Argument<T> argument)
     public T GetValue<T>(Option<T> option)
     public System.String ToString()
-  public class RootCommand : Command, System.Collections.Generic.IEnumerable<Symbol>, System.Collections.IEnumerable, System.CommandLine.Completions.ICompletionSource
+  public class RootCommand : Command, System.Collections.Generic.IEnumerable<Symbol>, System.Collections.IEnumerable
     public static System.String ExecutableName { get; }
     public static System.String ExecutablePath { get; }
     .ctor(System.String description = )
-  public abstract class Symbol, System.CommandLine.Completions.ICompletionSource
+  public abstract class Symbol
     public System.String Description { get; set; }
     public System.Boolean IsHidden { get; set; }
     public System.String Name { get; set; }
@@ -289,8 +288,6 @@ System.CommandLine.Completions
     public System.Boolean Equals(System.Object obj)
     public System.Int32 GetHashCode()
     public System.String ToString()
-  public interface ICompletionSource
-    public System.Collections.Generic.IEnumerable<CompletionItem> GetCompletions(CompletionContext context)
   public class TextCompletionContext : CompletionContext
     public System.String CommandLineText { get; }
     public System.Int32 CursorPosition { get; }

src/System.CommandLine/Argument.cs

@@ -17,7 +17,7 @@ public abstract class Argument : Symbol, IValueDescriptor
         private Func<ArgumentResult, object?>? _defaultValueFactory;
         private ArgumentArity _arity;
         private TryConvertArgument? _convertArguments;
-        private List<ICompletionSource>? _completions = null;
+        private List<Func<CompletionContext, IEnumerable<CompletionItem>>>? _completions = null;
         private List<Action<ArgumentResult>>? _validators = null;
 
         /// <summary>
@@ -74,7 +74,7 @@ internal TryConvertArgument? ConvertArguments
         /// <summary>
         /// Gets the collection of completion sources for the argument.
         /// </summary>
-        public ICollection<ICompletionSource> Completions =>
+        public ICollection<Func<CompletionContext, IEnumerable<CompletionItem>>> Completions =>
             _completions ??= new ()
             {
                 CompletionSource.ForType(ValueType)
@@ -192,7 +192,7 @@ internal void AddAllowedValues(IReadOnlyList<string> values)
         public override IEnumerable<CompletionItem> GetCompletions(CompletionContext context)
         {
             return Completions
-                   .SelectMany(source => source.GetCompletions(context))
+                   .SelectMany(source => source.Invoke(context))
                    .Distinct()
                    .OrderBy(c => c.SortText, StringComparer.OrdinalIgnoreCase);
         }

src/System.CommandLine/CompletionSourceExtensions.cs

@@ -18,7 +18,7 @@ public static class CompletionSourceExtensions
         /// <param name="completionSources">The list of completion sources to add to.</param>
         /// <param name="complete">The delegate to be called when calculating completions.</param>
         public static void Add(
-            this ICollection<ICompletionSource> completionSources,
+            this ICollection<Func<CompletionContext, IEnumerable<CompletionItem>>> completionSources,
             Func<CompletionContext, IEnumerable<string>> complete)
         {
             if (completionSources is null)
@@ -31,29 +31,7 @@ public static void Add(
                 throw new ArgumentNullException(nameof(complete));
             }
 
-            completionSources.Add(new AnonymousCompletionSource(complete));
-        }
-
-        /// <summary>
-        /// Adds a completion source using a delegate.
-        /// </summary>
-        /// <param name="completionSources">The list of completion sources to add to.</param>
-        /// <param name="complete">The function to be called when calculating completions.</param>
-        public static void Add(
-            this ICollection<ICompletionSource> completionSources,
-            Func<CompletionContext, IEnumerable<CompletionItem>> complete)
-        {
-            if (completionSources is null)
-            {
-                throw new ArgumentNullException(nameof(completionSources));
-            }
-
-            if (complete is null)
-            {
-                throw new ArgumentNullException(nameof(complete));
-            }
-
-            completionSources.Add(new AnonymousCompletionSource(complete));
+            completionSources.Add(context => complete(context).Select(value => new CompletionItem(value)));
         }
 
         /// <summary>
@@ -62,7 +40,7 @@ public static void Add(
         /// <param name="completionSources">The list of completion sources to add to.</param>
         /// <param name="completions">A list of strings to be suggested for command line completions.</param>
         public static void Add(
-            this ICollection<ICompletionSource> completionSources,
+            this ICollection<Func<CompletionContext, IEnumerable<CompletionItem>>> completionSources,
             params string[] completions)
         {
             if (completionSources is null)
@@ -75,7 +53,7 @@ public static void Add(
                 throw new ArgumentNullException(nameof(completions));
             }
 
-            completionSources.Add(new AnonymousCompletionSource(_ => completions.Select(s => new CompletionItem(s))));
+            completionSources.Add(context => completions.Select(value => new CompletionItem(value)));
         }
     }
 }

src/System.CommandLine/Completions/CompletionSource.cs

@@ -9,67 +9,38 @@
 namespace System.CommandLine.Completions
 {
     /// <summary>
-    /// Provides extension methods supporting <see cref="ICompletionSource"/> and command line tab completion.
+    /// Provides extension methods supporting command line tab completion.
     /// </summary>
     internal static class CompletionSource
     {
-        private static readonly ConcurrentDictionary<Type, ICompletionSource> _completionSourcesByType = new();
+        private static readonly ConcurrentDictionary<Type, Func<CompletionContext, IEnumerable<CompletionItem>>> _completionSourcesByType = new();
 
         /// <summary>
         /// Gets a completion source that provides completions for a type (e.g. enum) with well-known values.
         /// </summary>
-        internal static ICompletionSource ForType(Type type)
+        internal static Func<CompletionContext, IEnumerable<CompletionItem>> ForType(Type type)
         {
-            return _completionSourcesByType.GetOrAdd(type, t => new CompletionSourceForType(t));
+            return _completionSourcesByType.GetOrAdd(type, t => GetCompletionSourceForType(t));
         }
 
-        internal static ICompletionSource Empty { get; } = new AnonymousCompletionSource(static _ => Array.Empty<CompletionItem>());
-
-        private class CompletionSourceForType : ICompletionSource
+        private static Func<CompletionContext, IEnumerable<CompletionItem>> GetCompletionSourceForType(Type type)
         {
-            private readonly Type _type;
-            private ICompletionSource? _innerCompletionSource;
+            Type actualType = type.TryGetNullableType(out var nullableType) ? nullableType : type;
 
-            public CompletionSourceForType(Type type)
+            if (actualType.IsEnum)
             {
-                _type = type;
+                return _ => Enum.GetNames(actualType).Select(n => new CompletionItem(n));
             }
-
-            public IEnumerable<CompletionItem> GetCompletions(CompletionContext context)
+            else if (actualType == typeof(bool))
             {
-                if (_innerCompletionSource is null)
+                return static _ => new CompletionItem[]
                 {
-                    _innerCompletionSource = CreateForType(_type);
-                }
-
-                return _innerCompletionSource.GetCompletions(context);
+                    new(bool.TrueString),
+                    new(bool.FalseString)
+                };
             }
 
-            private static ICompletionSource CreateForType(Type type)
-            {
-                if (type.TryGetNullableType(out var nullableType))
-                {
-                    return CreateForType(nullableType);
-                }
-
-                if (type.IsEnum)
-                {
-                    return new AnonymousCompletionSource(_ => GetEnumNames());
-
-                    IEnumerable<CompletionItem> GetEnumNames() => Enum.GetNames(type).Select(n => new CompletionItem(n));
-                }
-
-                if (type == typeof(bool))
-                {
-                    return new AnonymousCompletionSource(static  _ => new CompletionItem[]
-                    {
-                        new(bool.TrueString),
-                        new(bool.FalseString)
-                    });
-                }
-
-                return Empty;
-            }
+            return static _ => Array.Empty<CompletionItem>();
         }
     }
 }

src/System.CommandLine/ParseResult.cs

@@ -220,8 +220,8 @@ public IEnumerable<CompletionItem> GetCompletions(
             }
 
             var completions =
-                currentSymbol is ICompletionSource currentCompletionSource
-                    ? currentCompletionSource.GetCompletions(context)
+                currentSymbol is not null
+                    ? currentSymbol.GetCompletions(context)
                     : Array.Empty<CompletionItem>();
 
             completions =

src/System.CommandLine/Symbol.cs

@@ -9,7 +9,7 @@ namespace System.CommandLine
     /// <summary>
     /// Defines a named symbol that resides in a hierarchy with parent and child symbols.
     /// </summary>
-    public abstract class Symbol : ICompletionSource
+    public abstract class Symbol
     {
         private string? _name;
         private ParentNode? _firstParent;