Add new Marshal.InitHandle API.

Fix SetHandle accessibility

Fix test.

Use Marshal.InitHandle.

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.cs

@@ -34,11 +34,5 @@ public SafeProcessHandle(IntPtr existingHandle, bool ownsHandle)
         {
             SetHandle(existingHandle);
         }
-
-        internal void InitialSetHandle(IntPtr h)
-        {
-            Debug.Assert(IsInvalid, "Safe handle should only be set once");
-            base.handle = h;
-        }
     }
 }

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeThreadHandle.cs

@@ -25,12 +25,6 @@ public SafeThreadHandle()
         {
         }
 
-        internal void InitialSetHandle(IntPtr h)
-        {
-            Debug.Assert(IsInvalid, "Safe handle should only be set once");
-            base.SetHandle(h);
-        }
-
         public override bool IsInvalid
         {
             get { return handle == IntPtr.Zero || handle == new IntPtr(-1); }

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

@@ -610,7 +610,7 @@ ref processInfo      // pointer to PROCESS_INFORMATION
                     }
 
                     if (processInfo.hProcess != IntPtr.Zero && processInfo.hProcess != new IntPtr(-1))
-                        procSH.InitialSetHandle(processInfo.hProcess);
+                        Marshal.InitHandle(procSH, processInfo.hProcess);
                     if (processInfo.hThread != IntPtr.Zero && processInfo.hThread != new IntPtr(-1))
                         Interop.Kernel32.CloseHandle(processInfo.hThread);
 

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Marshal.cs

@@ -996,5 +996,12 @@ internal static unsafe uint SysStringByteLen(IntPtr s)
 
         [SupportedOSPlatform("windows")]
         public static Type? GetTypeFromCLSID(Guid clsid) => GetTypeFromCLSID(clsid, null, throwOnError: false);
+
+        /// <summary>
+        /// Initializes the underlying handle of a newly created <see cref="SafeHandle" /> to the provided value.
+        /// </summary>
+        /// <param name="safeHandle"><see cref="SafeHandle"/> instance to update</param>
+        /// <param name="handle">Pre-existing handle</param>
+        public static void InitHandle(SafeHandle safeHandle, IntPtr handle) => safeHandle.SetHandle(handle);
     }
 }

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/SafeHandle.cs

@@ -74,7 +74,7 @@ protected SafeHandle(IntPtr invalidHandleValue, bool ownsHandle)
         }
 #endif
 
-        protected void SetHandle(IntPtr handle) => this.handle = handle;
+        protected internal void SetHandle(IntPtr handle) => this.handle = handle;
 
         public IntPtr DangerousGetHandle() => handle;
 

src/libraries/System.Runtime.InteropServices/ref/System.Runtime.InteropServices.cs

@@ -601,6 +601,7 @@ public static void GetNativeVariantForObject<T>(T? obj, System.IntPtr pDstNative
         public static string GetTypeInfoName(System.Runtime.InteropServices.ComTypes.ITypeInfo typeInfo) { throw null; }
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")]
         public static object GetUniqueObjectForIUnknown(System.IntPtr unknown) { throw null; }
+        public static void InitHandle(SafeHandle safeHandle, IntPtr handle) { }
         public static bool IsComObject(object o) { throw null; }
         public static bool IsTypeVisibleFromCom(System.Type t) { throw null; }
         [System.ComponentModel.EditorBrowsableAttribute(System.ComponentModel.EditorBrowsableState.Never)]

src/libraries/System.Runtime.InteropServices/tests/System.Runtime.InteropServices.Tests.csproj

@@ -93,6 +93,7 @@
     <Compile Include="System\Runtime\InteropServices\Marshal\GetTypeInfoNameTests.cs" />
     <Compile Include="System\Runtime\InteropServices\Marshal\GetUniqueObjectForIUnknownTests.cs" />
     <Compile Include="System\Runtime\InteropServices\Marshal\GetUniqueObjectForIUnknownTests.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
+    <Compile Include="System\Runtime\InteropServices\Marshal\InitHandleTests.cs" />
     <Compile Include="System\Runtime\InteropServices\Marshal\IsComObjectTests.cs" />
     <Compile Include="System\Runtime\InteropServices\Marshal\IsComObjectTests.Windows.cs" Condition="'$(TargetsWindows)' == 'true'" />
     <Compile Include="System\Runtime\InteropServices\Marshal\ReleaseTests.cs" />
@@ -176,4 +177,4 @@
     <Compile Include="System\Runtime\InteropServices\TypeLibVarAttributeTests.cs" />
     <Compile Include="System\Runtime\InteropServices\TypeLibVersionAttributeTests.cs" />
   </ItemGroup>
-</Project>
+</Project>

src/libraries/System.Runtime.InteropServices/tests/System/Runtime/InteropServices/Marshal/InitHandleTests.cs

@@ -0,0 +1,30 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace System.Runtime.InteropServices.Tests
+{
+    public class InitHandleTests
+    {
+        [Fact]
+        public void InitHandle_SetsHandle()
+        {
+            var safeHandle = new TestSafeHandle();
+            nint underlyingHandle = 42;
+
+            Marshal.InitHandle(safeHandle, underlyingHandle);
+
+            Assert.Equal((IntPtr)underlyingHandle, safeHandle.DangerousGetHandle());
+        }
+
+        class TestSafeHandle : SafeHandle
+        {
+            public TestSafeHandle() : base(IntPtr.Zero, true) { }
+
+            public override bool IsInvalid => handle == IntPtr.Zero;
+
+            protected override bool ReleaseHandle() => true;
+        }
+    }
+}