handle UNC and device paths

src/libraries/Common/tests/Common.Tests.csproj

@@ -109,6 +109,8 @@
     <Compile Include="$(CommonPath)System\Net\StreamBuffer.cs" Link="Common\System\Net\StreamBuffer.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsWindows)'=='true'">
+    <Compile Include="$(CommonPath)Interop\Windows\Interop.UNICODE_STRING.cs"
+         Link="Common\Interop\Windows\Interop.UNICODE_STRING.cs" />
     <Compile Include="$(CoreLibSharedDir)System\IO\PathInternal.Windows.cs"
              Link="System\IO\PathInternal.Windows.cs" />
     <Compile Include="$(CommonPath)Interop\Windows\Interop.Libraries.cs"

src/libraries/Common/tests/Tests/System/IO/PathInternal.Windows.Tests.cs

@@ -3,6 +3,9 @@
 
 using System;
 using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
 using Xunit;
 
 namespace Tests.System.IO
@@ -257,5 +260,45 @@ public void GetRootLengthDevice(string path, int length)
             Assert.Equal(length + PathInternal.ExtendedPathPrefix.Length, PathInternal.GetRootLength(@"\\?\" + path));
             Assert.Equal(length + PathInternal.ExtendedPathPrefix.Length, PathInternal.GetRootLength(@"\\.\" + path));
         }
+
+        public static TheoryData<string, string> DosToNtPathTest_Data => new TheoryData<string, string>
+        {
+            { @"C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // typical path
+            { @"\\?\C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // NtPath with \\?\ prefix
+            { @"\\.\device\file.cs", @"\??\device\file.cs" }, // device path with \\.\ prefix
+            { @"\\server\file.cs", @"\??\UNC\server\file.cs" }, // UNC path with \\ prefix
+            { @"\\?\UNC\server\file", @"\??\UNC\server\file" }, // extended UNC prefix
+            { @"\??\C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // NtPath with \??\ prefix (no changes required)
+            { @"C:\", @"\??\C:\" }, // a short path
+            { @"\\s", @"\??\UNC\s" }, // short UNC path
+            { @"\\s\", @"\??\UNC\s\" }, // short UNC path with trailing
+            { $@"C:\{string.Join("\\", Enumerable.Repeat("a", PathInternal.MaxShortPath + 1))}",  $@"\??\C:\{string.Join("\\", Enumerable.Repeat("a", PathInternal.MaxShortPath + 1))}"}, // long path
+        };
+
+        [Theory, MemberData(nameof(DosToNtPathTest_Data))]
+        public void DosToNtPathTest(string path, string expected)
+        {
+            // first of all, we use an internal Windows API to ensure that expected value is valid
+            if (path.Length < PathInternal.MaxShortPath) // RtlDosPathNameToRelativeNtPathName_U_WithStatus does not support long paths
+            {
+                RtlDosPathNameToRelativeNtPathName_U_WithStatus(path, out Interop.UNICODE_STRING ntFileName, out IntPtr _, IntPtr.Zero);
+                try
+                {
+                    Assert.Equal(expected, Marshal.PtrToStringUni(ntFileName.Buffer));
+                }
+                finally
+                {
+                    Marshal.ZeroFreeGlobalAllocUnicode(ntFileName.Buffer);
+                }
+            }
+
+            // after that, we test our implementation
+            var vsb = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);
+            PathInternal.DosToNtPath(path, ref vsb);
+            Assert.Equal(expected, vsb.ToString());
+
+            [DllImport(Interop.Libraries.NtDll, CharSet = CharSet.Unicode)]
+            static extern int RtlDosPathNameToRelativeNtPathName_U_WithStatus(string DosFileName, out Interop.UNICODE_STRING NtFileName, out IntPtr FilePart, IntPtr RelativeName);
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Windows.cs

@@ -56,31 +56,12 @@ internal static unsafe SafeFileHandle Open(string fullPath, FileMode mode, FileA
 
         private static IntPtr NtCreateFile(string fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, long preallocationSize)
         {
-            uint ntStatus;
-            IntPtr fileHandle;
+            var vsb = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);
 
-            const string MandatoryNtPrefix = @"\??\";
-            if (fullPath.StartsWith(MandatoryNtPrefix, StringComparison.Ordinal))
-            {
-                (ntStatus, fileHandle) = Interop.NtDll.NtCreateFile(fullPath, mode, access, share, options, preallocationSize);
-            }
-            else
-            {
-                var vsb = new ValueStringBuilder(stackalloc char[256]);
-                vsb.Append(MandatoryNtPrefix);
+            PathInternal.DosToNtPath(fullPath, ref vsb);
 
-                if (fullPath.StartsWith(@"\\?\", StringComparison.Ordinal)) // NtCreateFile does not support "\\?\" prefix, only "\??\"
-                {
-                    vsb.Append(fullPath.AsSpan(4));
-                }
-                else
-                {
-                    vsb.Append(fullPath);
-                }
-
-                (ntStatus, fileHandle) = Interop.NtDll.NtCreateFile(vsb.AsSpan(), mode, access, share, options, preallocationSize);
-                vsb.Dispose();
-            }
+            (uint ntStatus, IntPtr fileHandle) = Interop.NtDll.NtCreateFile(vsb.AsSpan(), mode, access, share, options, preallocationSize);
+            vsb.Dispose();
 
             switch (ntStatus)
             {

src/libraries/System.Private.CoreLib/src/System/IO/PathInternal.Windows.cs

@@ -47,6 +47,7 @@ internal static partial class PathInternal
         internal const string DirectorySeparatorCharAsString = "\\";
 
         internal const string ExtendedPathPrefix = @"\\?\";
+        internal const string NtPrefix = @"\??\";
         internal const string UncPathPrefix = @"\\";
         internal const string UncExtendedPrefixToInsert = @"?\UNC\";
         internal const string UncExtendedPathPrefix = @"\\?\UNC\";
@@ -409,5 +410,34 @@ internal static bool IsEffectivelyEmpty(ReadOnlySpan<char> path)
             }
             return true;
         }
+
+        // this method works only for `fullPath` returned by Path.GetFullPath
+        // currently we don't have interest in supporting relative paths
+        internal static void DosToNtPath(ReadOnlySpan<char> fullPath, ref ValueStringBuilder vsb)
+        {
+            vsb.Append(NtPrefix);
+
+            if (fullPath.Length >= 3 && fullPath[0] == '\\' && fullPath[1] == '\\')
+            {
+                // \\.\ (Device) or \\?\ (NtPath)
+                if (fullPath.Length >= 4 && fullPath[3] == '\\' && (fullPath[2] == '.' || fullPath[2] == '?'))
+                {
+                    vsb.Append(fullPath.Slice(NtPrefix.Length));
+                }
+                else // \\ (UNC)
+                {
+                    vsb.Append(@"UNC\");
+                    vsb.Append(fullPath.Slice(2));
+                }
+            }
+            else if (fullPath.Length >= 4 && fullPath[0] == '\\' && fullPath[1] == '?' && fullPath[2] == '?' && fullPath[3] == '\\') // \??\
+            {
+                vsb.Append(fullPath.Slice(NtPrefix.Length));
+            }
+            else
+            {
+                vsb.Append(fullPath);
+            }
+        }
     }
 }