From d47527d4af6066969ea9abee83ef9b172e4e1d98 Mon Sep 17 00:00:00 2001
From: Bob Arnson <bob@firegiant.com>
Date: Fri, 8 May 2020 20:25:11 -0400
Subject: Default output file if there's a single input file.

---
 src/WixToolset.Core/CommandLine/BuildCommand.cs | 75 +++++++++++++++++++++++--
 1 file changed, 71 insertions(+), 4 deletions(-)

diff --git a/src/WixToolset.Core/CommandLine/BuildCommand.cs b/src/WixToolset.Core/CommandLine/BuildCommand.cs
index a59637fe..dbdad0a9 100644
--- a/src/WixToolset.Core/CommandLine/BuildCommand.cs
+++ b/src/WixToolset.Core/CommandLine/BuildCommand.cs
@@ -74,8 +74,6 @@ namespace WixToolset.Core.CommandLine
 
             this.Platform = this.commandLine.Platform;
 
-            this.OutputFile = this.commandLine.OutputFile;
-
             this.ContentsFile = this.commandLine.ContentsFile;
 
             this.OutputsFile = this.commandLine.OutputsFile;
@@ -92,6 +90,21 @@ namespace WixToolset.Core.CommandLine
 
             this.EvaluateSourceFiles(sourceFiles, creator, out var codeFiles, out var wixipl);
 
+            this.OutputFile = this.commandLine.OutputFile;
+
+            if (String.IsNullOrEmpty(this.OutputFile))
+            {
+                if (codeFiles.Count == 1)
+                {
+                    // If output type is unknown, the extension will be replaced with the right default based on output type.
+                    this.OutputFile = Path.ChangeExtension(codeFiles[0].OutputPath, DefaultExtensionForOutputType(this.OutputType));
+                }
+                else
+                {
+                    this.Messaging.Write(ErrorMessages.MustSpecifyOutputWithMoreThanOneInput());
+                }
+            }
+
             if (this.Messaging.EncounteredError)
             {
                 return this.Messaging.LastErrorNumber;
@@ -114,7 +127,7 @@ namespace WixToolset.Core.CommandLine
 
                     if (!this.Messaging.EncounteredError)
                     {
-                        wixlib.Save(this.commandLine.OutputFile);
+                        wixlib.Save(this.OutputFile);
                     }
                 }
             }
@@ -129,9 +142,16 @@ namespace WixToolset.Core.CommandLine
 
                     if (!this.Messaging.EncounteredError)
                     {
+                        var outputExtension = Path.GetExtension(this.OutputFile);
+                        if (String.IsNullOrEmpty(outputExtension) || ".wix" == outputExtension)
+                        {
+                            var entrySectionType = wixipl.Sections.Single().Type;
+                            this.OutputFile = Path.ChangeExtension(this.OutputFile, DefaultExtensionForSectionType(entrySectionType));
+                        }
+
                         if (this.OutputType == OutputType.IntermediatePostLink)
                         {
-                            wixipl.Save(this.commandLine.OutputFile);
+                            wixipl.Save(this.OutputFile);
                         }
                         else
                         {
@@ -416,6 +436,53 @@ namespace WixToolset.Core.CommandLine
             return result?.Document;
         }
 
+        private static string DefaultExtensionForSectionType(SectionType sectionType)
+        {
+            switch (sectionType)
+            {
+                case SectionType.Bundle:
+                    return ".exe";
+                case SectionType.Module:
+                    return ".msm";
+                case SectionType.Product:
+                    return ".msi";
+                case SectionType.PatchCreation:
+                    return ".pcp";
+                case SectionType.Patch:
+                    return ".msp";
+                case SectionType.Fragment:
+                case SectionType.Unknown:
+                default:
+                    return ".wix";
+            }
+        }
+
+        private static string DefaultExtensionForOutputType(OutputType outputType)
+        {
+            switch (outputType)
+            {
+                case OutputType.Bundle:
+                    return ".exe";
+                case OutputType.Library:
+                    return ".wixlib";
+                case OutputType.Module:
+                    return ".msm";
+                case OutputType.Patch:
+                    return ".msp";
+                case OutputType.PatchCreation:
+                    return ".pcp";
+                case OutputType.Product:
+                    return ".msi";
+                case OutputType.Transform:
+                    return ".mst";
+                case OutputType.IntermediatePostLink:
+                    return ".wixipl";
+                case OutputType.Unknown:
+                default:
+                    return ".wix";
+            }
+        }
+
         private class CommandLine
         {
             private static readonly char[] BindPathSplit = { '=' };
-- 
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