Annotate unsupported APIs in System.Diagnostics.Process

src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs

@@ -71,7 +71,7 @@ public Process() { }
         public long PrivateMemorySize64 { get { throw null; } }
         public System.TimeSpan PrivilegedProcessorTime { get { throw null; } }
         public string ProcessName { get { throw null; } }
-        public System.IntPtr ProcessorAffinity { get { throw null; } set { } }
+        public System.IntPtr ProcessorAffinity { [System.Runtime.Versioning.SupportedOSPlatform("windows")] [System.Runtime.Versioning.SupportedOSPlatform("linux")] get { throw null; } [System.Runtime.Versioning.SupportedOSPlatform("windows")] [System.Runtime.Versioning.SupportedOSPlatform("linux")] set { } }
         public bool Responding { get { throw null; } }
         public Microsoft.Win32.SafeHandles.SafeProcessHandle SafeHandle { get { throw null; } }
         public int SessionId { get { throw null; } }
@@ -237,7 +237,7 @@ internal ProcessThread() { }
         public int Id { get { throw null; } }
         public int IdealProcessor { set { } }
         public bool PriorityBoostEnabled { get { throw null; } set { } }
-        public System.Diagnostics.ThreadPriorityLevel PriorityLevel { get { throw null; } [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")] set { } }
+        public System.Diagnostics.ThreadPriorityLevel PriorityLevel { [System.Runtime.Versioning.SupportedOSPlatform("windows")] [System.Runtime.Versioning.SupportedOSPlatform("linux")] [System.Runtime.Versioning.SupportedOSPlatform("freebsd")] get { throw null; } [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")] set { } }
         public System.TimeSpan PrivilegedProcessorTime { get { throw null; } }
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")]
         public System.IntPtr ProcessorAffinity { set { } }

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.UnknownUnix.cs

@@ -20,12 +20,6 @@ public TimeSpan PrivilegedProcessorTime
             get { throw new PlatformNotSupportedException(); }
         }
 
-        /// <summary>Gets the time the associated process was started.</summary>
-        internal DateTime StartTimeCore
-        {
-            get { throw new PlatformNotSupportedException(); }
-        }
-
         /// <summary>
         /// Gets the amount of time the associated process has spent utilizing the CPU.
         /// It is the sum of the <see cref='System.Diagnostics.Process.UserProcessorTime'/> and

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.cs

@@ -528,6 +528,8 @@ public string ProcessName
         /// </devdoc>
         public IntPtr ProcessorAffinity
         {
+            [SupportedOSPlatform("windows")]
+            [SupportedOSPlatform("linux")]
             get
             {
                 if (!_haveProcessorAffinity)
@@ -537,6 +539,8 @@ public IntPtr ProcessorAffinity
                 }
                 return _processorAffinity;
             }
+            [SupportedOSPlatform("windows")]
+            [SupportedOSPlatform("linux")]
             set
             {
                 ProcessorAffinityCore = value;

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessManager.Unix.cs

@@ -72,9 +72,5 @@ internal static void ThrowIfRemoteMachine(string machineName)
                 throw new PlatformNotSupportedException(SR.RemoteMachinesNotSupported);
             }
         }
-        public static IntPtr GetMainWindowHandle(int processId)
-        {
-            throw new PlatformNotSupportedException(); // Window handle is a Windows concept
-        }
     }
 }

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessThread.cs

@@ -88,6 +88,9 @@ public bool PriorityBoostEnabled
         /// </devdoc>
         public ThreadPriorityLevel PriorityLevel
         {
+            [SupportedOSPlatform("windows")]
+            [SupportedOSPlatform("linux")]
+            [SupportedOSPlatform("freebsd")]
             get
             {
                 if (!_priorityLevel.HasValue)