Make non-essential types optional (#3591)

* Fixes #3568
* Added optional throwIfNotFound to reflect optional types in the TypeSystemContext interface
* Only System.Object is required - all other types are optional

4 files changed, 24 insertions, 9 deletions

diff --git a/src/Common/src/TypeSystem/Common/MetadataTypeSystemContext.cs b/src/Common/src/TypeSystem/Common/MetadataTypeSystemContext.cs
index 3c5278b08..7b82dc361 100644
--- a/src/Common/src/TypeSystem/Common/MetadataTypeSystemContext.cs
+++ b/src/Common/src/TypeSystem/Common/MetadataTypeSystemContext.cs
@@ -11,6 +11,7 @@ namespace Internal.TypeSystem
     {
         private static readonly string[] s_wellKnownTypeNames = new string[] {
             "Void",
+
             "Boolean",
             "Char",
             "SByte",
@@ -68,16 +69,27 @@ namespace Internal.TypeSystem
             // Initialize all well known types - it will save us from checking the name for each loaded type
             for (int typeIndex = 0; typeIndex < _wellKnownTypes.Length; typeIndex++)
             {
-                MetadataType type = systemModule.GetType("System", s_wellKnownTypeNames[typeIndex]);
-                type.SetWellKnownType((WellKnownType)(typeIndex + 1));
-                _wellKnownTypes[typeIndex] = type;
+                // Require System.Object to be present as a minimal sanity check. 
+                // The set of required well-known types is not strictly defined since different .NET profiles implement different subsets.
+                MetadataType type = systemModule.GetType("System", s_wellKnownTypeNames[typeIndex], typeIndex == (int)WellKnownType.Object);
+                if (type != null)
+                {
+                    type.SetWellKnownType((WellKnownType)(typeIndex + 1));
+                    _wellKnownTypes[typeIndex] = type;
+                }
             }
         }
 
-        public override DefType GetWellKnownType(WellKnownType wellKnownType)
+        public override DefType GetWellKnownType(WellKnownType wellKnownType, bool throwIfNotFound = true)
         {
             Debug.Assert(_wellKnownTypes != null, "Forgot to call SetSystemModule?");
-            return _wellKnownTypes[(int)wellKnownType - 1];
+
+            int typeIndex = (int)wellKnownType - 1;
+            DefType type = _wellKnownTypes[typeIndex];
+            if (type == null && throwIfNotFound) 
+                throw new TypeSystemException.TypeLoadException("System", s_wellKnownTypeNames[typeIndex], SystemModule);
+
+            return type;
         }
 
         protected sealed internal override bool ComputeHasStaticConstructor(TypeDesc type)
diff --git a/src/Common/src/TypeSystem/Common/TypeSystemContext.cs b/src/Common/src/TypeSystem/Common/TypeSystemContext.cs
index ea5e69591..4f9dbb141 100644
--- a/src/Common/src/TypeSystem/Common/TypeSystemContext.cs
+++ b/src/Common/src/TypeSystem/Common/TypeSystemContext.cs
@@ -57,7 +57,7 @@ namespace Internal.TypeSystem
             SystemModule = systemModule;
         }
 
-        public abstract DefType GetWellKnownType(WellKnownType wellKnownType);
+        public abstract DefType GetWellKnownType(WellKnownType wellKnownType, bool throwIfNotFound = true);
 
         public virtual ModuleDesc ResolveAssembly(AssemblyName name, bool throwIfNotFound = true)
         {
diff --git a/src/Common/src/TypeSystem/Common/TypeSystemHelpers.cs b/src/Common/src/TypeSystem/Common/TypeSystemHelpers.cs
index cf1d3db16..255eb7cfd 100644
--- a/src/Common/src/TypeSystem/Common/TypeSystemHelpers.cs
+++ b/src/Common/src/TypeSystem/Common/TypeSystemHelpers.cs
@@ -12,7 +12,7 @@ namespace Internal.TypeSystem
     {
         public static bool IsWellKnownType(this TypeDesc type, WellKnownType wellKnownType)
         {
-            return type == type.Context.GetWellKnownType(wellKnownType);
+            return type == type.Context.GetWellKnownType(wellKnownType, false);
         }
 
         public static InstantiatedType MakeInstantiatedType(this MetadataType typeDef, Instantiation instantiation)
diff --git a/src/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeLoaderTypeSystemContext.cs b/src/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeLoaderTypeSystemContext.cs
index 9a90eac5d..537e00bae 100644
--- a/src/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeLoaderTypeSystemContext.cs
+++ b/src/System.Private.TypeLoader/src/Internal/Runtime/TypeLoader/TypeLoaderTypeSystemContext.cs
@@ -107,7 +107,7 @@ namespace Internal.Runtime.TypeLoader
             return s_nativeLayoutInterfacesAlgorithm;
         }
 
-        public override DefType GetWellKnownType(WellKnownType wellKnownType)
+        public override DefType GetWellKnownType(WellKnownType wellKnownType, bool throwIfNotFound = true)
         {
             switch (wellKnownType)
             {
@@ -190,7 +190,10 @@ namespace Internal.Runtime.TypeLoader
                     return (DefType)ResolveRuntimeTypeHandle(typeof(Exception).TypeHandle);
 
                 default:
-                    throw new NotImplementedException();
+                    if (throwIfNotFound)
+                        throw new NotImplementedException();
+                    else
+                        return null;
             }
         }