define Directive symbols

src/System.CommandLine/Directive.cs

@@ -0,0 +1,46 @@
+using System.Collections.Generic;
+using System.CommandLine.Completions;
+
+namespace System.CommandLine
+{
+    /// <summary>
+    /// The purpose of directives is to provide cross-cutting functionality that can apply across command-line apps.
+    /// Because directives are syntactically distinct from the app's own syntax, they can provide functionality that applies across apps.
+    /// 
+    /// A directive must conform to the following syntax rules:
+    /// * It's a token on the command line that comes after the app's name but before any subcommands or options.
+    /// * It's enclosed in square brackets.
+    /// * It doesn't contain spaces.
+    /// </summary>
+    public class Directive : Symbol
+    {
+        /// <summary>
+        /// Initializes a new instance of the Directive class.
+        /// </summary>
+        /// <param name="name">The name of the directive. It can't contain whitespaces.</param>
+        /// <param name="description">The description of the directive, shown in help.</param>
+        public Directive(string name, string? description)
+        {
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                throw new ArgumentException("Name cannot be null, empty, or consist entirely of whitespace.");
+            }
+
+            for (var i = 0; i < name.Length; i++)
+            {
+                if (char.IsWhiteSpace(name[i]))
+                {
+                    throw new ArgumentException($"Name cannot contain whitespace: \"{name}\"", nameof(name));
+                }
+            }
+
+            Name = name;
+            Description = description;
+        }
+
+        private protected override string DefaultName => throw new NotImplementedException();
+
+        public override IEnumerable<CompletionItem> GetCompletions(CompletionContext context)
+            => Array.Empty<CompletionItem>();
+    }
+}