dotnet-core/release-notes/3.0/preview/api-diff/preview6/3.0-preview6_System.md

System

 namespace System {
     public static class Environment {
+        public static long TickCount64 { get; }
     }
     public static class GC {
+        public static GCMemoryInfo GetGCMemoryInfo();
+        public static long GetTotalAllocatedBytes(bool precise = false);
     }
+    public readonly struct GCMemoryInfo {
+        public long FragmentedBytes { get; }
+        public long HeapSizeBytes { get; }
+        public long HighMemoryLoadThresholdBytes { get; }
+        public long MemoryLoadBytes { get; }
+        public long TotalAvailableMemoryBytes { get; }
+    }
     public abstract class StringComparer : IComparer, IComparer<string>, IEqualityComparer, IEqualityComparer<string> {
-        bool System.Collections.IEqualityComparer.Equals(object x, object y);

-        int System.Collections.IEqualityComparer.GetHashCode(object obj);

     }
 }