Support .NET versions different than Mutannots

Mutannot/Mutannot.fsproj

@@ -9,4 +9,8 @@
   <ItemGroup>
     <Compile Include="Program.fs" />
   </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="System.Reflection.MetadataLoadContext" Version="9.0.1" />
+  </ItemGroup>
 </Project>

Mutannot/Program.fs

@@ -1,4 +1,5 @@
 open System
+open System.Collections.Generic
 open System.Diagnostics
 open System.IO
 open System.Reflection
@@ -197,50 +198,81 @@ let ensureBuilt options project assemblyPath =
     if not (File.Exists assemblyPath) then
         fail $"Compiled test assembly not found at {assemblyPath}."
 
-let installAssemblyResolver (assemblyPath: string) =
+let requireConstructorArgumentString (args: IList<CustomAttributeTypedArgument>) index name =
+    match args[index].Value with
+    | :? string as value when not (isNull value) -> value
+    | null -> fail $"MutationCaseAttribute constructor argument '{name}' must not be null."
+    | value ->
+        fail
+            $"MutationCaseAttribute constructor argument '{name}' had unexpected type '{value.GetType().FullName}'."
+
+let requireConstructorArgumentInt32 (args: IList<CustomAttributeTypedArgument>) index name =
+    match args[index].Value with
+    | :? int as value -> value
+    | null -> fail $"MutationCaseAttribute constructor argument '{name}' must not be null."
+    | value ->
+        fail
+            $"MutationCaseAttribute constructor argument '{name}' had unexpected type '{value.GetType().FullName}'."
+
+let metadataLoadContextPaths (assemblyPath: string) =
     let assemblyDir = Path.GetDirectoryName assemblyPath
 
-    AppDomain.CurrentDomain.add_AssemblyResolve (
-        ResolveEventHandler(fun _ args ->
-            let name = AssemblyName(args.Name).Name + ".dll"
-            let candidate = Path.Combine(assemblyDir, name)
+    let runtimeAssemblies =
+        match AppContext.GetData "TRUSTED_PLATFORM_ASSEMBLIES" with
+        | :? string as value when not (String.IsNullOrWhiteSpace value) ->
+            value.Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries)
+        | _ -> fail "Unable to discover trusted platform assemblies for MetadataLoadContext."
 
-            if File.Exists candidate then
-                Assembly.LoadFrom candidate
-            else
-                null)
-    )
+    let localAssemblies =
+        seq {
+            yield assemblyPath
+            yield! Directory.EnumerateFiles(assemblyDir, "*.dll")
+            yield! Directory.EnumerateFiles(assemblyDir, "*.exe")
+        }
+
+    Seq.append runtimeAssemblies localAssemblies
+    |> Seq.distinct
+    |> Seq.toArray
+
+let createMetadataLoadContext (assemblyPath: string) =
+    let resolver = PathAssemblyResolver(metadataLoadContextPaths assemblyPath)
+    let coreAssemblyName = typeof<obj>.Assembly.GetName().Name
+
+    new MetadataLoadContext(resolver, coreAssemblyName)
 
 let mutationCases options project assemblyPath =
     ensureBuilt options project assemblyPath
-    installAssemblyResolver assemblyPath
 
-    let asm = Assembly.LoadFrom assemblyPath
+    use mlc = createMetadataLoadContext assemblyPath
+    let asm = mlc.LoadFromAssemblyPath assemblyPath
 
     asm.GetTypes()
     |> Array.collect (fun t ->
+        let declaringType =
+            match t.FullName with
+            | null -> t.Name
+            | name -> name
+
         t.GetMethods(BindingFlags.Public ||| BindingFlags.NonPublic ||| BindingFlags.Static ||| BindingFlags.Instance)
         |> Array.collect (fun m ->
             m.GetCustomAttributesData()
-            |> Seq.filter (fun attr -> attr.AttributeType.FullName = "Mutannot.MutationCaseAttribute")
-            |> Seq.map (fun attr ->
-                let args = attr.ConstructorArguments
+            |> Seq.choose (fun attr ->
+                if attr.AttributeType.FullName <> "Mutannot.MutationCaseAttribute" then
+                    None
+                else
+                    let args = attr.ConstructorArguments
 
-                if args.Count <> 5 then
-                    failwithf "Unexpected MutationCaseAttribute shape on %s.%s" t.FullName m.Name
+                    if args.Count <> 5 then
+                        fail $"Unexpected MutationCaseAttribute shape on {declaringType}.{m.Name}"
 
-                let declaringType =
-                    match t.FullName with
-                    | null -> t.Name
-                    | name -> name
-
-                { Id = unbox<string> args[0].Value
-                  File = unbox<string> args[1].Value
-                  Line = unbox<int> args[2].Value
-                  Find = unbox<string> args[3].Value
-                  Replace = unbox<string> args[4].Value
-                  TestName = m.Name
-                  DeclaringType = declaringType })
+                    Some
+                        { Id = requireConstructorArgumentString args 0 "id"
+                          File = requireConstructorArgumentString args 1 "file"
+                          Line = requireConstructorArgumentInt32 args 2 "line"
+                          Find = requireConstructorArgumentString args 3 "find"
+                          Replace = requireConstructorArgumentString args 4 "replace"
+                          TestName = m.Name
+                          DeclaringType = declaringType })
             |> Seq.toArray))
     |> Array.sortBy (fun mutation -> mutation.Id)