🐛 WriteTo truncates output file

docs/articles/core/binary/datastream.md

@@ -94,6 +94,8 @@ internally maps to the .NET enumerations `FileMode` and `FileAccess` as follow:
 > hundreds of `DataStream` on different files ready without running out of
 > resources in the operative system.
 
+<!-- ignore warning -->
+
 > [!TIP]  
 > `FileOpenMode` is handy enumeration that covers most use cases. If you require
 > any other combination of `FileMode` and `FileAccess` you can create the
@@ -119,7 +121,9 @@ applies to the content that targets this `DataStream`, not the entire parent
 
 > [!NOTE]  
 > The method ignores the current position, it will always start writing from the
-> start to the end. It will **restore** the current position after comparing.
+> start to the end. It will **restore** the current position after writing. It
+> will also start **truncate** the output file. It will not append or preserve
+> any existing content.
 
 The path should point to the output file. If there is any directory that doesn't
 exist, it will create them first.

src/Yarhl.UnitTests/IO/DataStreamTests.cs

@@ -1368,17 +1368,44 @@ public void WriteToWhenLengthZeroCreatesEmptyFile()
         {
             string tempFile = Path.Combine(
                 Path.GetTempPath(),
-                Path.GetRandomFileName(),
                 Path.GetRandomFileName());
 
-            DataStream stream = new DataStream();
-            stream.WriteTo(tempFile);
+            try {
+                var stream = new DataStream();
+                stream.WriteTo(tempFile);
 
-            Assert.That(File.Exists(tempFile), Is.True);
-            FileStream fs = new FileStream(tempFile, FileMode.Open, FileAccess.Read);
-            Assert.AreEqual(0, fs.Length);
-            fs.Dispose();
-            File.Delete(tempFile);
+                Assert.That(File.Exists(tempFile), Is.True);
+
+                using var fs = new FileStream(tempFile, FileMode.Open, FileAccess.Read);
+                Assert.That(fs.Length, Is.EqualTo(0));
+            } finally {
+                File.Delete(tempFile);
+            }
+        }
+
+        [Test]
+        public void WriteToWhenTruncatesExistingFile()
+        {
+            string tempFile = Path.Combine(
+                Path.GetTempPath(),
+                Path.GetRandomFileName());
+
+            byte[] expected = new byte[] { 0xCA, 0xFE };
+            using var stream = new DataStream();
+            stream.Write(expected);
+
+            try {
+                // Pre-fill the output file with some long content
+                File.WriteAllText(tempFile, "hello world!");
+
+                // Overwrite with 2 bytes stream content
+                stream.WriteTo(tempFile);
+
+                byte[] actual = File.ReadAllBytes(tempFile);
+                Assert.That(actual, Is.EquivalentTo(expected));
+            } finally {
+                File.Delete(tempFile);
+            }
         }
 
         [Test]

src/Yarhl/IO/DataStream.cs

@@ -616,7 +616,7 @@ public void WriteSegmentTo(long start, long length, string fileOut)
             }
 
             // We use FileStream so it creates a file even when the length is zero
-            using var segment = new FileStream(fileOut, FileMode.OpenOrCreate, FileAccess.Write);
+            using var segment = new FileStream(fileOut, FileMode.Create, FileAccess.Write);
             WriteSegmentTo(start, length, segment);
         }