@@ -162,6 +162,10 @@ IEnumerable<string> IBuildActions.EnumerateDirectories(string dir)
 
         bool IBuildActions.IsRunningOnAppleSilicon() => IsRunningOnAppleSilicon;
 
+        public bool IsMonoInstalled { get; set; }
+
+        bool IBuildActions.IsMonoInstalled() => IsMonoInstalled;
+
         public string PathCombine(params string[] parts)
         {
             return string.Join(IsWindows ? '\\' : '/', parts.Where(p => !string.IsNullOrWhiteSpace(p)));
@@ -856,11 +860,32 @@ public void TestDirsProjWindows()
         }
 
         [Fact]
-        public void TestDirsProjLinux()
+        public void TestDirsProjLinux_WithMono()
         {
+            actions.IsMonoInstalled = true;
+
             actions.RunProcess[@"nuget restore C:\Project/dirs.proj -DisableParallelProcessing"] = 1;
             actions.RunProcess[@"mono scratch/.nuget/nuget.exe restore C:\Project/dirs.proj -DisableParallelProcessing"] = 0;
             actions.RunProcess[@"msbuild C:\Project/dirs.proj /t:rebuild"] = 0;
+
+            var autobuilder = TestDirsProjLinux();
+            TestAutobuilderScript(autobuilder, 0, 3);
+        }
+
+        [Fact]
+        public void TestDirsProjLinux_WithoutMono()
+        {
+            actions.IsMonoInstalled = false;
+
+            actions.RunProcess[@"dotnet msbuild /t:restore C:\Project/dirs.proj"] = 0;
+            actions.RunProcess[@"dotnet msbuild C:\Project/dirs.proj /t:rebuild"] = 0;
+
+            var autobuilder = TestDirsProjLinux();
+            TestAutobuilderScript(autobuilder, 0, 2);
+        }
+
+        private CSharpAutobuilder TestDirsProjLinux()
+        {
             actions.FileExists["csharp.log"] = true;
             actions.FileExists[@"C:\Project/a/test.csproj"] = true;
             actions.FileExists[@"C:\Project/dirs.proj"] = true;
@@ -889,8 +914,7 @@ public void TestDirsProjLinux()
 </Project>");
             actions.LoadXml[@"C:\Project/dirs.proj"] = dirsproj;
 
-            var autobuilder = CreateAutoBuilder(false);
-            TestAutobuilderScript(autobuilder, 0, 3);
+            return CreateAutoBuilder(false);
         }
 
         [Fact]
 
@@ -150,6 +150,10 @@ IEnumerable<string> IBuildActions.EnumerateDirectories(string dir)
 
         bool IBuildActions.IsRunningOnAppleSilicon() => IsRunningOnAppleSilicon;
 
+        public bool IsMonoInstalled { get; set; }
+
+        bool IBuildActions.IsMonoInstalled() => IsMonoInstalled;
+
         string IBuildActions.PathCombine(params string[] parts)
         {
             return string.Join(IsWindows ? '\\' : '/', parts.Where(p => !string.IsNullOrWhiteSpace(p)));
 
@@ -10,15 +10,15 @@ internal static class MsBuildCommandExtensions
         /// <summary>
         /// Appends a call to msbuild.
         /// </summary>
-        /// <param name="cmdBuilder"></param>
-        /// <param name="builder"></param>
         /// <returns></returns>
-        public static CommandBuilder MsBuildCommand(this CommandBuilder cmdBuilder, IAutobuilder<AutobuildOptionsShared> builder)
+        public static CommandBuilder MsBuildCommand(this CommandBuilder cmdBuilder, IAutobuilder<AutobuildOptionsShared> builder, bool preferDotnet)
         {
             // mono doesn't ship with `msbuild` on Arm-based Macs, but we can fall back to
             // msbuild that ships with `dotnet` which can be invoked with `dotnet msbuild`
             // perhaps we should do this on all platforms?
-            return builder.Actions.IsRunningOnAppleSilicon()
+            // Similarly, there's no point in trying to rely on mono if it's not installed.
+            // In which case we can still fall back to `dotnet msbuild`.
+            return preferDotnet
                 ? cmdBuilder.RunCommand("dotnet").Argument("msbuild")
                 : cmdBuilder.RunCommand("msbuild");
         }
@@ -75,13 +75,16 @@ BuildScript GetNugetRestoreScript() =>
                         QuoteArgument(projectOrSolution.FullPath).
                         Argument("-DisableParallelProcessing").
                         Script;
+
+                var preferDotnet = builder.Actions.IsRunningOnAppleSilicon() || !builder.Actions.IsWindows() && !builder.Actions.IsMonoInstalled();
+
                 var nugetRestore = GetNugetRestoreScript();
                 var msbuildRestoreCommand = new CommandBuilder(builder.Actions).
-                    MsBuildCommand(builder).
+                    MsBuildCommand(builder, preferDotnet).
                     Argument("/t:restore").
                     QuoteArgument(projectOrSolution.FullPath);
 
-                if (builder.Actions.IsRunningOnAppleSilicon())
+                if (preferDotnet)
                 {
                     // On Apple Silicon, only try package restore with `dotnet msbuild /t:restore`
                     ret &= BuildScript.Try(msbuildRestoreCommand.Script);
@@ -119,7 +122,7 @@ BuildScript GetNugetRestoreScript() =>
                     command.RunCommand("set Platform=&& type NUL", quoteExe: false);
                 }
 
-                command.MsBuildCommand(builder);
+                command.MsBuildCommand(builder, preferDotnet);
                 command.QuoteArgument(projectOrSolution.FullPath);
 
                 var target = "rebuild";
 
@@ -125,6 +125,11 @@ public interface IBuildActions
         /// <returns>True if we are running on Apple Silicon.</returns>
         bool IsRunningOnAppleSilicon();
 
+        /// <summary>
+        /// Checks if Mono is installed.
+        /// </summary>
+        bool IsMonoInstalled();
+
         /// <summary>
         /// Combine path segments, Path.Combine().
         /// </summary>
@@ -261,6 +266,25 @@ bool IBuildActions.IsRunningOnAppleSilicon()
             }
         }
 
+        bool IBuildActions.IsMonoInstalled()
+        {
+            var thisBuildActions = (IBuildActions)this;
+
+            if (thisBuildActions.IsWindows())
+            {
+                return false;
+            }
+
+            try
+            {
+                return 0 == thisBuildActions.RunProcess("mono", "--version", workingDirectory: null, env: null);
+            }
+            catch (Exception)
+            {
+                return false;
+            }
+        }
+
         string IBuildActions.PathCombine(params string[] parts) => Path.Combine(parts);
 
         void IBuildActions.WriteAllText(string filename, string contents) => File.WriteAllText(filename, contents);
 
@@ -1,8 +1,2 @@
-import pytest
-import runs_on
-
-
-# Skipping the test on macos-15, as we're running into trouble.
-@pytest.mark.only_if(not runs_on.macos_15)
 def test(codeql, csharp):
     codeql.database.create(_assert_failure=True)
Original file line number	Diff line number	Diff line change
`@@ -150,6 +150,10 @@ IEnumerable<string> IBuildActions.EnumerateDirectories(string dir)`
`150`	`150`
`151`	`151`	`bool IBuildActions.IsRunningOnAppleSilicon() => IsRunningOnAppleSilicon;`
`152`	`152`
	`153`	`+ public bool IsMonoInstalled { get; set; }`
	`154`	`+`
	`155`	`+ bool IBuildActions.IsMonoInstalled() => IsMonoInstalled;`
	`156`	`+`
`153`	`157`	`string IBuildActions.PathCombine(params string[] parts)`
`154`	`158`	`{`
`155`	`159`	`return string.Join(IsWindows ? '\\' : '/', parts.Where(p => !string.IsNullOrWhiteSpace(p)));`