Add host option to disable AssemblyLoadContext

A host can disable the use of AssemblyLoadContext by returning
a value of "true" or "1" when GetHostOption is called with the
option name "DisableAssemblyLoadContext".

This is an escape hatch for hosts that have issues with the ALC
behavior, allowing them to opt out until the underlying issues
can be fixed.

Mono.TextTemplating/Mono.TextTemplating.CodeCompilation/CompiledAssemblyData.cs

@@ -38,7 +38,7 @@ public Assembly LoadInAssemblyLoadContext (AssemblyLoadContext loadContext)
 				return loadContext.LoadFromStream (new MemoryStream (Assembly));
 			}
 		}
-#else
+#endif
 		public Assembly LoadInCurrentAppDomain ()
 		{
 			if (DebugSymbols != null) {
@@ -47,6 +47,5 @@ public Assembly LoadInCurrentAppDomain ()
 				return System.Reflection.Assembly.Load (Assembly);
 			}
 		}
-#endif
 	}
 }

Mono.TextTemplating/Mono.TextTemplating/CompiledTemplate.TemplateAssemblyContext.cs

@@ -11,21 +11,51 @@ namespace Mono.TextTemplating;
 
 partial class CompiledTemplate
 {
-	[SuppressMessage ("Performance", "CA1822:Mark members as static", Justification = "Same API for ALC and AppDomain build variants of the class")]
-	sealed class TemplateAssemblyContext : IDisposable
+	/// <summary>
+	/// Abstracts over loading assemblies into an AssemblyLoadContext or AppDomain
+	/// and resolving assemblies from the host.
+	/// </summary>
+	abstract class TemplateAssemblyContext : IDisposable
 	{
+		public abstract Assembly LoadAssemblyFile (string assemblyPath);
+		public abstract Assembly LoadInMemoryAssembly (CompiledAssemblyData assemblyData);
+		public virtual void Dispose () { }
+
+		[SuppressMessage ("Performance", "CA1859:Use concrete types when possible for improved performance", Justification = "Conditionally compiled version of this returns multiple concrete types")]
+
+		public static TemplateAssemblyContext Create (ITextTemplatingEngineHost host, string[] referenceAssemblyFiles)
+		{
+#if FEATURE_ASSEMBLY_LOAD_CONTEXT
+			if (!host.IsAssemblyLoadContextDisabled ()) {
+				return new AssemblyLoadContextTemplateAssemblyContext (host, referenceAssemblyFiles);
+			}
+#endif
+			return new CurrentAppDomainTemplateAssemblyContext (host, referenceAssemblyFiles);
+		}
+	}
+
 #if FEATURE_ASSEMBLY_LOAD_CONTEXT
+	sealed class AssemblyLoadContextTemplateAssemblyContext : TemplateAssemblyContext
+	{
 		readonly TemplateAssemblyLoadContext templateContext;
-		public TemplateAssemblyContext (ITextTemplatingEngineHost host, string[] referenceAssemblyFiles) => templateContext = new (referenceAssemblyFiles, host);
-		public Assembly LoadAssemblyFile (string assemblyPath) => templateContext.LoadFromAssemblyPath (assemblyPath);
-		public Assembly LoadInMemoryAssembly (CompiledAssemblyData assemblyData) => assemblyData.LoadInAssemblyLoadContext (templateContext);
-		public void Dispose () { }
-#else
-		readonly CurrentDomainAssemblyResolver assemblyResolver;
-		public TemplateAssemblyContext (ITextTemplatingEngineHost host, string[] referenceAssemblyFiles) => assemblyResolver = new (referenceAssemblyFiles, host.ResolveAssemblyReference);
-		public Assembly LoadAssemblyFile (string assemblyPath) => Assembly.LoadFile (assemblyPath);
-		public Assembly LoadInMemoryAssembly (CompiledAssemblyData assemblyData) => assemblyData.LoadInCurrentAppDomain ();
-		public void Dispose () => assemblyResolver.Dispose ();
+		public AssemblyLoadContextTemplateAssemblyContext (ITextTemplatingEngineHost host, string[] referenceAssemblyFiles)
+			=> templateContext = new (referenceAssemblyFiles, host);
+		public override Assembly LoadAssemblyFile (string assemblyPath) => templateContext.LoadFromAssemblyPath (assemblyPath);
+		public override Assembly LoadInMemoryAssembly (CompiledAssemblyData assemblyData) => assemblyData.LoadInAssemblyLoadContext (templateContext);
+	}
 #endif
+
+	sealed class CurrentAppDomainTemplateAssemblyContext : TemplateAssemblyContext
+	{
+		readonly CurrentDomainAssemblyResolver assemblyResolver;
+		public CurrentAppDomainTemplateAssemblyContext (ITextTemplatingEngineHost host, string[] referenceAssemblyFiles)
+			=> assemblyResolver = new (referenceAssemblyFiles, host.ResolveAssemblyReference);
+		public override Assembly LoadAssemblyFile (string assemblyPath) => Assembly.LoadFile (assemblyPath);
+		public override Assembly LoadInMemoryAssembly (CompiledAssemblyData assemblyData) => assemblyData.LoadInCurrentAppDomain ();
+		public override void Dispose ()
+		{
+			base.Dispose ();
+			assemblyResolver.Dispose ();
+		}
 	}
 }

Mono.TextTemplating/Mono.TextTemplating/CompiledTemplate.TemplateExecutor.cs

@@ -19,7 +19,7 @@ class TemplateProcessor : MarshalByRefObject
 		[SuppressMessage ("Performance", "CA1822:Mark members as static", Justification = "Needs to be an instance for MarshalByRefObject")]
 		public string CreateAndProcess (ITextTemplatingEngineHost host, CompiledAssemblyData templateAssemblyData, string templateAssemblyFile, string fullName, CultureInfo culture, string[] referencedAssemblyFiles)
 		{
-			using var context = new TemplateAssemblyContext (host, referencedAssemblyFiles);
+			using var context = TemplateAssemblyContext.Create (host, referencedAssemblyFiles);
 
 			Assembly assembly = templateAssemblyData is not null
 				? context.LoadInMemoryAssembly (templateAssemblyData)

Mono.TextTemplating/Mono.TextTemplating/CurrentDomainAssemblyResolver.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Microsoft. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
-#if !FEATURE_ASSEMBLY_LOAD_CONTEXT
-
 using System;
 using System.IO;
 using System.Reflection;
@@ -49,5 +47,3 @@ public void Dispose ()
 		}
 	}
 }
-
-#endif

Mono.TextTemplating/Mono.TextTemplating/HostOptions.cs

@@ -0,0 +1,16 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using Microsoft.VisualStudio.TextTemplating;
+
+static class HostOptionExtensions
+{
+	const string DisableAlcOptionName = "DisableAssemblyLoadContext";
+
+	static bool IsOptionTrue (this ITextTemplatingEngineHost host, string optionName) =>
+		host.GetHostOption(optionName) is string optionVal
+			&& (optionVal == "1" || optionVal.Equals("true", StringComparison.OrdinalIgnoreCase));
+
+	public static bool IsAssemblyLoadContextDisabled (this ITextTemplatingEngineHost host) => host.IsOptionTrue (DisableAlcOptionName);
+}