39 items listed (if you ignore #12)
13 primitives
52 in total
List compiled from the code in SystemDomain::LoadBaseSystemClasses()
System.ObjectSystem.__Canon(special class, used as a generic 'placeholder')System.ValueTypeSystem.EnumSystem.RuntimeTypeSystem.ArraySystem.SZArrayHelper(internal array helper class)System.ByReference<T>(new class, to makeSpan<T>possible)System.Nullable<T>System.Object[](i.e. theObjectarray class)System.TypedReference- All the built-in 'primitive' or 'value' types (13 in total):
System.Void(void)System.Boolean(bool)System.Char(char)System.SByte(sbyte)System.Byte(byte)System.Int16(short)System.UInt16(ushort)System.Int32(int)System.UInt32(uint)System.Int64(long)System.UInt64(ulong)System.Single(float)System.Double(double)
System.IntPtrSystem.UIntPtrSystem.DelegateSystem.MulticastDelegateSystem.Collections.Generic.IEnumerable<T>System.Collections.Generic.ICollection<T>System.Collections.Generic.IList<T>System.Collections.Generic.IReadonlyCollection<T>System.Collections.Generic.IReadonlyList<T>System.StringSystem.Byte[](i.e. theBytearray class)System.ExceptionSystem.OutOfMemoryExceptionSystem.StackOverflowExceptionSystem.ExecutionEngineExceptionSystem.ThreadAbortExceptionSystem.Threading.ThreadSystem.__ComObject(#ifdef FEATURE_COMINTEROP)System.Runtime.InteropServices.WindowsRuntime.RuntimeClassSystem.Collections.Generic.IDictionary<T,U>System.Collections.Generic.IReadOnlyDictionary<T,U>System.Collections.Generic.AttributeSystem.EventHandler<T>System.Collections.IEnumerableSystem.Collections.ICollectionSystem.Collections.IListSystem.IDisposable(#endif // FEATURE_COMINTEROP)System.Runtime.CompilerServices.ICastable(#ifdef FEATURE_ICASTABLE)