Handle empty argv in RootCommand.ExecutablePath

src/System.CommandLine.Tests/RootCommandTests.cs

@@ -1,6 +1,8 @@
 // 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;
+using System.Reflection;
 using FluentAssertions;
 using Xunit;
 
@@ -48,5 +50,22 @@ public void Setting_HelpName_does_not_change_Name()
 
             rootCommand.Name.Should().Be(RootCommand.ExecutableName);
         }
+
+        [Fact]
+        public void ExecutablePath_falls_back_to_empty_string_when_command_line_args_are_empty()
+        {
+            GetExecutablePath(Array.Empty<string>()).Should().Be("");
+        }
+
+        [Fact]
+        public void ExecutablePath_uses_first_command_line_arg()
+        {
+            GetExecutablePath(new[] { "my-tool", "--help" }).Should().Be("my-tool");
+        }
+
+        private static string GetExecutablePath(string[] commandLineArgs)
+            => (string)typeof(RootCommand)
+                .GetMethod("GetExecutablePath", BindingFlags.NonPublic | BindingFlags.Static)!
+                .Invoke(null, new object[] { commandLineArgs })!;
     }
 }

src/System.CommandLine/RootCommand.cs

@@ -74,7 +74,10 @@ public static string ExecutableName
         /// <summary>
         /// The path to the currently running executable.
         /// </summary>
-        public static string ExecutablePath => _executablePath ??= Environment.GetCommandLineArgs()[0];
+        public static string ExecutablePath => _executablePath ??= GetExecutablePath(Environment.GetCommandLineArgs());
+
+        private static string GetExecutablePath(IReadOnlyList<string> commandLineArgs)
+            => commandLineArgs.Count > 0 ? commandLineArgs[0] : string.Empty;
 
         private static string? ToolCommandName
         {