Allow empty CscToolPath

src/Compilers/Core/MSBuildTask/DotnetHost.cs

@@ -12,16 +12,23 @@ internal sealed class DotnetHost
 
         public string CommandLineArgs { get; }
 
-        private DotnetHost(string toolNameOpt, string pathToToolOpt, string commandLineArgs)
+        /// <summary>
+        /// True if this tool is a managed executable, and will be invoked with `dotnet`.
+        /// False if this executable is being invoked directly.
+        /// </summary>
+        public bool IsManagedTool { get; }
+
+        private DotnetHost(string toolNameOpt, string pathToToolOpt, string commandLineArgs, bool isManagedTool)
         {
             ToolNameOpt = toolNameOpt;
             PathToToolOpt = pathToToolOpt;
             CommandLineArgs = commandLineArgs;
+            IsManagedTool = isManagedTool;
         }
 
         public static DotnetHost CreateUnmanagedToolInvocation(string pathToTool, string commandLineArgs)
         {
-            return new DotnetHost(null, pathToTool, commandLineArgs);
+            return new DotnetHost(null, pathToTool, commandLineArgs, isManagedTool: false);
         }
 
         public static DotnetHost CreateManagedToolInvocation(string toolNameWithoutExtension, string commandLineArgs)
@@ -45,7 +52,7 @@ public static DotnetHost CreateManagedToolInvocation(string toolNameWithoutExten
                 pathToToolOpt = Utilities.GenerateFullPathToTool(toolName);
             }
 
-            return new DotnetHost(toolName, pathToToolOpt, commandLineArgs);
+            return new DotnetHost(toolName, pathToToolOpt, commandLineArgs, isManagedTool: true);
         }
 
         private static bool IsCliHost(out string pathToDotnet)

src/Compilers/Core/MSBuildTask/ManagedCompiler.cs

@@ -440,13 +440,13 @@ private DotnetHost DotnetHostInfo
 
                         if (ToolExe != _dotnetHostInfo.ToolNameOpt)
                         {
-                            _dotnetHostInfo = DotnetHost.CreateUnmanagedToolInvocation(ToolPath, commandLine);
+                            _dotnetHostInfo = DotnetHost.CreateUnmanagedToolInvocation(ToolExe, commandLine);
                         }
                     }
                     else
                     {
                         // Explicitly provided ToolPath, don't try to figure anything out
-                        _dotnetHostInfo = DotnetHost.CreateUnmanagedToolInvocation(ToolPath, commandLine);
+                        _dotnetHostInfo = DotnetHost.CreateUnmanagedToolInvocation(Path.Combine(ToolPath, ToolExe), commandLine);
                     }
                 }
                 return _dotnetHostInfo;
@@ -488,7 +488,7 @@ protected override int ExecuteTool(string pathToTool, string responseFileCommand
             try
             {
                 if (!UseSharedCompilation ||
-                !string.IsNullOrEmpty(ToolPath) ||
+                !DotnetHostInfo.IsManagedTool ||
                 !BuildServerConnection.IsCompilerServerSupported)
                 {
                     return base.ExecuteTool(pathToTool, responseFileCommands, commandLineCommands);