From aebb6264b17a5ec094e9034561289ce803ad6c45 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Valentin=20Breu=C3=9F?= <vbreuss@gmail.com>
Date: Tue, 22 Aug 2023 16:54:54 +0200
Subject: [PATCH] feat: add `Null` to MockFileStream to shadow the static
 `Stream.Null` field (#1025)

Fixes #1009

This is a suggestion based on the [workaround by @sheila-stewart](https://github.com/TestableIO/System.IO.Abstractions/issues/1009#issuecomment-1680946638).
---
 .../MockFileStream.cs                         | 18 ++++++++++++++++++
 .../MockFileStreamTests.cs                    | 19 +++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs b/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs
index 61b96f428..197fffd43 100644
--- a/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs
+++ b/src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs
@@ -9,6 +9,24 @@ namespace System.IO.Abstractions.TestingHelpers
     [Serializable]
     public class MockFileStream : FileSystemStream, IFileSystemAclSupport
     {
+        /// <summary>
+        ///     Wrapper around a <see cref="Stream" /> with no backing store, which
+        ///     is used as a replacement for a <see cref="FileSystemStream" />. As such
+        ///     it implements the same properties and methods as a <see cref="FileSystemStream" />.
+        /// </summary>
+        public new static FileSystemStream Null { get; } = new NullFileSystemStream();
+
+        private class NullFileSystemStream : FileSystemStream
+        {
+            /// <summary>
+            /// Initializes a new instance of <see cref="NullFileSystemStream" />.
+            /// </summary>
+            public NullFileSystemStream() : base(Null, ".", true)
+            {
+                
+            }
+        }
+
         private readonly IMockFileDataAccessor mockFileDataAccessor;
         private readonly string path;
         private readonly FileAccess access = FileAccess.ReadWrite;
diff --git a/tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs b/tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs
index 34a5e620e..8a32806d4 100644
--- a/tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs
+++ b/tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs
@@ -226,5 +226,24 @@ public void MockFileStream_FlushBool_ShouldNotChangePosition([Values] bool flush
                 Assert.AreEqual(200, stream.Position);
             }
         }
+
+        [Test]
+        public void MockFileStream_Null_ShouldReturnSingletonObject()
+        {
+            var result1 = MockFileStream.Null;
+            var result2 = MockFileStream.Null;
+
+            Assert.AreSame(result1, result2);
+        }
+
+        [Test]
+        public void MockFileStream_Null_ShouldHaveExpectedProperties()
+        {
+            var result = MockFileStream.Null;
+
+            Assert.AreEqual(result.Name, ".");
+            Assert.AreEqual(result.Length, 0);
+            Assert.AreEqual(result.IsAsync, true);
+        }
     }
 }