Use Muxer from .NET SDK for dotnet discovery

release_notes.md

@@ -13,3 +13,4 @@
 - Refactor to use msbuild for determining .NET target framework & add support multiple TFMs (#4715)
   - When using `func init --docker-only` on a .NET project with multiple target frameworks, the CLI will now
    prompt the user to select which target framework to use for the Dockerfile.
+- Enhanced dotnet installation discovery by adopting the same `Muxer` logic used by the .NET SDK itself (#4732)

src/Cli/func/Helpers/DotnetHelpers.cs

@@ -28,9 +28,15 @@ private static Task<int> RunDotnetNewAsync(string args)
 
         public static void EnsureDotnet()
         {
-            if (!CommandChecker.CommandExists("dotnet"))
+            try
+            {
+                _ = DotnetMuxer.GetMuxerPath();
+            }
+            catch (InvalidOperationException ex)
             {
-                throw new CliException("dotnet sdk is required for dotnet based functions. Please install https://microsoft.com/net");
+                throw new CliException(
+                    "Unable to locate the .NET SDK. Install the .NET SDK from https://aka.ms/dotnet-download or configure PATH | DOTNET_ROOT | DOTNET_HOST_PATH so that 'dotnet' is discoverable.",
+                    ex);
             }
         }
 

src/Cli/func/Helpers/DotnetMuxer.cs

@@ -0,0 +1,61 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+// Adapted from: https://github.com/dotnet/sdk/blob/main/src/Cli/Microsoft.DotNet.Cli.Utils/Muxer.cs
+using System.Runtime.InteropServices;
+
+namespace Azure.Functions.Cli.Helpers;
+
+internal static class DotnetMuxer
+{
+    private static readonly string _muxerName = "dotnet";
+    private static readonly string _exeSuffix = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ".exe" : string.Empty;
+
+    /// <summary>
+    /// Locates the dotnet muxer (dotnet executable).
+    /// </summary>
+    /// <returns>The full path to the dotnet executable.</returns>
+    /// <exception cref="InvalidOperationException">Thrown when the dotnet executable cannot be located.</exception>
+    public static string GetMuxerPath()
+    {
+        string muxerPath;
+
+        // Most scenarios are running dotnet.dll as the app
+        // Root directory with muxer should be two above app base: <root>/sdk/<version>
+        string rootPath = Path.GetDirectoryName(Path.GetDirectoryName(AppContext.BaseDirectory.TrimEnd(Path.DirectorySeparatorChar)));
+        if (rootPath is not null)
+        {
+            muxerPath = Path.Combine(rootPath, $"{_muxerName}{_exeSuffix}");
+            if (File.Exists(muxerPath))
+            {
+                return muxerPath;
+            }
+        }
+
+        // Best-effort search for muxer.
+        muxerPath = Environment.ProcessPath;
+
+        // The current process should be dotnet in most normal scenarios except when dotnet.dll is loaded in a custom host like the testhost
+        if (muxerPath is not null && !Path.GetFileNameWithoutExtension(muxerPath).Equals("dotnet", StringComparison.OrdinalIgnoreCase))
+        {
+            // SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
+            muxerPath = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH");
+            if (muxerPath is null)
+            {
+                // fallback to DOTNET_ROOT which typically holds some dotnet executable
+                string root = Environment.GetEnvironmentVariable("DOTNET_ROOT");
+                if (root is not null)
+                {
+                    muxerPath = Path.Combine(root, $"dotnet{_exeSuffix}");
+                }
+            }
+        }
+
+        if (muxerPath is null)
+        {
+            throw new InvalidOperationException("Unable to locate dotnet multiplexer");
+        }
+
+        return muxerPath;
+    }
+}

test/Cli/Func.UnitTests/HelperTests/DotnetHelpersTests.cs

@@ -8,6 +8,14 @@ namespace Azure.Functions.Cli.UnitTests.HelperTests
 {
     public class DotnetHelpersTests
     {
+        [Fact]
+        public void EnsureDotnet_DoesNotThrow_WhenDotnetExists()
+        {
+            // dotnet is always installed in the test environment
+            var exception = Record.Exception(() => DotnetHelpers.EnsureDotnet());
+            Assert.Null(exception);
+        }
+
         [Theory]
         [InlineData("BlobTrigger", "blob")]
         [InlineData("HttpTrigger", "http")]

test/Cli/Func.UnitTests/HelperTests/DotnetMuxerTests.cs

@@ -0,0 +1,76 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+using Azure.Functions.Cli.Helpers;
+using Xunit;
+
+namespace Azure.Functions.Cli.UnitTests.HelperTests
+{
+    public class DotnetMuxerTests
+    {
+        [Fact]
+        public void GetMuxerPath_ReturnsMuxerPath_WhenDotnetExists()
+        {
+            // Act
+            var path = DotnetMuxer.GetMuxerPath();
+
+            // Assert
+            Assert.NotNull(path);
+            Assert.False(string.IsNullOrWhiteSpace(path));
+        }
+
+        [Fact]
+        public void GetMuxerPath_ContainsDotnetExecutable()
+        {
+            // Act
+            var path = DotnetMuxer.GetMuxerPath();
+
+            // Assert
+            Assert.Contains("dotnet", path, StringComparison.OrdinalIgnoreCase);
+        }
+
+        [Fact]
+        public void GetMuxerPath_PointsToExecutableFile()
+        {
+            // Act
+            var path = DotnetMuxer.GetMuxerPath();
+
+            // Assert
+            // The path should exist as a file
+            Assert.True(
+                File.Exists(path) || File.Exists(path + ".exe"),
+                $"Expected muxer path '{path}' to exist");
+        }
+
+        [Fact]
+        public void GetMuxerPath_PointsToFunctionalDotnetExecutable()
+        {
+            // Act
+            var path = DotnetMuxer.GetMuxerPath();
+
+            // Assert - invoke dotnet --version to verify it's functional
+            var startInfo = new System.Diagnostics.ProcessStartInfo
+            {
+                FileName = path,
+                Arguments = "--version",
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                UseShellExecute = false,
+                CreateNoWindow = true
+            };
+
+            using var process = System.Diagnostics.Process.Start(startInfo);
+            Assert.NotNull(process);
+
+            var output = process.StandardOutput.ReadToEnd();
+            process.WaitForExit();
+
+            // Assert
+            Assert.Equal(0, process.ExitCode);
+            Assert.False(string.IsNullOrWhiteSpace(output), "Expected dotnet --version to produce output");
+
+            // Verify output looks like a version number (e.g., "8.0.100")
+            Assert.Matches(@"^\d+\.\d+\.\d+", output.Trim());
+        }
+    }
+}