Add support for opening a file in Truncate mode

Author: robkeim

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

@@ -12,7 +12,6 @@ namespace System.IO.Abstractions.TestingHelpers.Tests
 
     public class MockFileCreateTests
     {
-
         [Test]
         public void Mockfile_Create_ShouldCreateNewStream()
         {
@@ -165,5 +164,58 @@ public void MockFile_Create_ShouldThrowDirectoryNotFoundExceptionIfCreatingAndPa
             var exception = Assert.Throws<DirectoryNotFoundException>(action);
             Assert.That(exception.Message, Does.StartWith("Could not find a part of the path"));
         }
+
+        [Test]
+        public void MockFile_Create_TruncateShouldWriteNewContents()
+        {
+            // Arrange
+            const string testFileName = @"c:\someFile.txt";
+            var fileSystem = new MockFileSystem();
+            
+            using (var stream = fileSystem.FileStream.Create(testFileName, FileMode.Create, FileAccess.Write))
+            {
+                using (var writer = new StreamWriter(stream))
+                {
+                    writer.Write("original_text");
+                }
+            }
+
+            // Act
+            using (var stream = fileSystem.FileStream.Create(testFileName, FileMode.Truncate, FileAccess.Write))
+            {
+                using (var writer = new StreamWriter(stream))
+                {
+                    writer.Write("new_text");
+                }
+            }
+
+            // Assert
+            Assert.That(fileSystem.File.ReadAllText(testFileName), Is.EqualTo("new_text"));
+        }
+
+        [Test]
+        public void MockFile_Create_TruncateShouldClearFileContentsOnOpen()
+        {
+            // Arrange
+            const string testFileName = @"c:\someFile.txt";
+            var fileSystem = new MockFileSystem();
+
+            using (var stream = fileSystem.FileStream.Create(testFileName, FileMode.Create, FileAccess.Write))
+            {
+                using (var writer = new StreamWriter(stream))
+                {
+                    writer.Write("original_text");
+                }
+            }
+
+            // Act
+            using (var stream = fileSystem.FileStream.Create(testFileName, FileMode.Truncate, FileAccess.Write))
+            {
+                // Opening the stream is enough to reset the contents
+            }
+
+            // Assert
+            Assert.That(fileSystem.File.ReadAllText(testFileName), Is.EqualTo(string.Empty));
+        }
     }
 }

System.IO.Abstractions.TestingHelpers/MockFileStream.cs

@@ -12,7 +12,8 @@ public enum StreamType
         {
             READ,
             WRITE,
-            APPEND
+            APPEND,
+            TRUNCATE
         }
 
         public MockFileStream(IMockFileDataAccessor mockFileDataAccessor, string path, StreamType streamType)
@@ -24,7 +25,7 @@ public MockFileStream(IMockFileDataAccessor mockFileDataAccessor, string path, S
             {
                 /* only way to make an expandable MemoryStream that starts with a particular content */
                 var data = mockFileDataAccessor.GetFile(path).Contents;
-                if (data != null && data.Length > 0)
+                if (data != null && data.Length > 0 && streamType != StreamType.TRUNCATE)
                 {
                     Write(data, 0, data.Length);
                     Seek(0, StreamType.APPEND.Equals(streamType)

System.IO.Abstractions.TestingHelpers/MockFileStreamFactory.cs

@@ -74,6 +74,10 @@ private static MockFileStream.StreamType GetStreamType(FileMode mode, FileAccess
             {
                 return MockFileStream.StreamType.APPEND;
             }
+            else if (mode == FileMode.Truncate)
+            {
+                return MockFileStream.StreamType.TRUNCATE;
+            }
             else
             {
                 return MockFileStream.StreamType.WRITE;