Configurable command builder

src/System.CommandLine.Tests/Builder/ConfigurableBuilderTests.cs

@@ -0,0 +1,44 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System.CommandLine.Builder;
+using FluentAssertions;
+using Xunit;
+
+namespace System.CommandLine.Tests.Builder
+{
+    public class ConfigurableBuilderTests
+    {
+        [Fact]
+        public void When_a_ConfigurableCommandLineBuider_is_created_it_has_a_null_Command()
+        {
+            var builder = new ConfigurableCommandLineBuilder();
+            builder.Command.Should().BeNull();
+        }
+
+        [Fact]
+        public void ConfigurableCommandLineBuider_Command_can_be_set()
+        {
+            var builder = new ConfigurableCommandLineBuilder();
+            builder.SetCommand(new Command("Bob"));
+            builder.Command.Should().NotBeNull()
+                .And.BeEmpty("Bob");
+
+        }
+
+        [Fact]
+        public void ConfigurableCommandLineBuider_Command_should_throw_if_Command_already_set()
+        {
+            var builder = new ConfigurableCommandLineBuilder();
+            builder.SetCommand(new Command("Bob"));
+            Action create = () => builder.SetCommand(new Command("Joe"));
+
+            create.Should()
+                      .Throw<InvalidOperationException>()
+                      .Which
+                      .Message
+                      .Should()
+                      .Be("Command has already been set.");
+        }
+    }
+}

src/System.CommandLine/Builder/CommandBuilder.cs

@@ -8,15 +8,27 @@ namespace System.CommandLine.Builder
 {
     public class CommandBuilder
     {
-        public CommandBuilder(Command command) 
+        public CommandBuilder(Command command)
         {
             Command = command;
         }
 
-        public Command Command { get; }
+        private protected CommandBuilder()
+        { }
+
+        public Command? Command { get; private set; }
 
         public IEnumerable<Option> Options => Command.Children.OfType<Option>();
 
+        private protected void SetCommandInternal(Command command)
+        {
+            if (!(Command is null))
+            {
+                throw new InvalidOperationException("Command has already been set.");
+            }
+            Command = command;
+        }
+
         internal void AddCommand(Command command) => Command.AddCommand(command);
 
         internal void AddOption(Option option) => Command.AddOption(option);

src/System.CommandLine/Builder/CommandLineBuilder.cs

@@ -19,6 +19,10 @@ public CommandLineBuilder(Command? rootCommand = null)
         {
         }
 
+        private protected CommandLineBuilder()
+            : base()
+        { }
+
         public bool EnableDirectives { get; set; } = true;
 
         public bool EnablePosixBundling { get; set; } = true;

src/System.CommandLine/Builder/ConfigurableCommandLineBuilder.cs

@@ -0,0 +1,14 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace System.CommandLine.Builder
+{
+    public class ConfigurableCommandLineBuilder : CommandLineBuilder
+    {
+        public void SetCommand(Command command)
+        {
+            SetCommandInternal(command);
+        }
+
+    }
+}