Fix unreported bug: FileStream created with FileMode.Open or Truncate should throw an exception if the file is missing

Author: pianomanjh

System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamFactoryTests.cs

@@ -84,6 +84,22 @@ public void MockFileStreamFactory_CreateInNonExistingDirectory_ShouldThrowDirect
             Assert.Throws<DirectoryNotFoundException>(() => fileStreamFactory.Create(XFS.Path(@"C:\Test\NonExistingDirectory\some_random_file.txt"), fileMode));
         }
 
+        [Test]
+        [TestCase(FileMode.Open)]
+        [TestCase(FileMode.Truncate)]
+        public void MockFileStreamFactory_OpenNonExistingFile_ShouldThrowFileNotFoundException(FileMode fileMode)
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+            fileSystem.AddDirectory(XFS.Path(@"C:\Test"));
+
+            // Act
+            var fileStreamFactory = new MockFileStreamFactory(fileSystem);
+
+            // Assert
+            Assert.Throws<FileNotFoundException>(() => fileStreamFactory.Create(XFS.Path(@"C:\Test\some_random_file.txt"), fileMode));
+        }
+
         [Test]
         [TestCase(FileMode.CreateNew)]
         public void MockFileStreamFactory_CreateExistingFile_Should_Throw_IOException(FileMode fileMode)

System.IO.Abstractions.TestingHelpers/MockFile.cs

@@ -417,7 +417,7 @@ private Stream OpenInternal(
             else if (mode == FileMode.Append)
                 streamType = MockFileStream.StreamType.APPEND;
 
-            return new MockFileStream(mockFileDataAccessor, path, streamType, options);
+            return new MockFileStream(mockFileDataAccessor, path, streamType, options, mode);
         }
 
         public override Stream OpenRead(string path)

System.IO.Abstractions.TestingHelpers/MockFileStream.cs

@@ -73,7 +73,9 @@ public MockFileStream(
                     throw CommonExceptions.CouldNotFindPartOfPath(path);
                 }
 
-                if (StreamType.READ.Equals(streamType))
+                if (StreamType.READ.Equals(streamType)
+                    || fileMode.Equals(FileMode.Open)
+                    || fileMode.Equals(FileMode.Truncate))
                 {
                     throw CommonExceptions.FileNotFound(path);
                 }