An an override of Flush() to GZipOutputStream to ensure the headers are writen before flushing

src/ICSharpCode.SharpZipLib/GZip/GzipOutputStream.cs

@@ -160,6 +160,20 @@ protected override void Dispose(bool disposing)
 			}
 		}
 
+		/// <summary>
+		/// Flushes the stream by ensuring the header is written, and then calling <see cref="DeflaterOutputStream.Flush">Flush</see>
+		/// on the deflater.
+		/// </summary>
+		public override void Flush()
+		{
+			if (state_ == OutputState.Header)
+			{
+				WriteHeader();
+			}
+
+			base.Flush();
+		}
+
 		#endregion Stream overrides
 
 		#region DeflaterOutputStream overrides

test/ICSharpCode.SharpZipLib.Tests/GZip/GZipTests.cs

@@ -76,6 +76,37 @@ public void DelayedHeaderWriteNoData()
 			Assert.IsTrue(data.Length > 0);
 		}
 
+
+		/// <summary>
+		/// Variant of DelayedHeaderWriteNoData testing flushing for https://github.com/icsharpcode/SharpZipLib/issues/382
+		/// </summary>
+		[Test]
+		[Category("GZip")]
+		public void DelayedHeaderWriteFlushNoData()
+		{
+			var ms = new MemoryStream();
+			Assert.AreEqual(0, ms.Length);
+
+			using (GZipOutputStream outStream = new GZipOutputStream(ms) { IsStreamOwner = false })
+			{
+				// #382 - test flushing the stream before writing to it.
+				outStream.Flush();
+			}
+
+			ms.Seek(0, SeekOrigin.Begin);
+
+			// Test that the gzip stream can be read
+			var readStream = new MemoryStream();
+			using (GZipInputStream inStream = new GZipInputStream(ms))
+			{
+				inStream.CopyTo(readStream);
+			}
+
+			byte[] data = readStream.ToArray();
+
+			Assert.That(data, Is.Empty, "Should not have any decompressed data");
+		}
+
 		/// <summary>
 		/// Writing GZip headers is delayed so that this stream can be used with HTTP/IIS.
 		/// </summary>
@@ -99,6 +130,38 @@ public void DelayedHeaderWriteWithData()
 			Assert.IsTrue(data.Length > 0);
 		}
 
+		/// <summary>
+		/// variant of DelayedHeaderWriteWithData to test https://github.com/icsharpcode/SharpZipLib/issues/382
+		/// </summary>
+		[Test]
+		[Category("GZip")]
+		public void DelayedHeaderWriteFlushWithData()
+		{
+			var ms = new MemoryStream();
+			Assert.AreEqual(0, ms.Length);
+			using (GZipOutputStream outStream = new GZipOutputStream(ms) { IsStreamOwner = false })
+			{
+				Assert.AreEqual(0, ms.Length);
+
+				// #382 - test flushing the stream before writing to it.
+				outStream.Flush();
+				outStream.WriteByte(45);
+			}
+
+			ms.Seek(0, SeekOrigin.Begin);
+
+			// Test that the gzip stream can be read
+			var readStream = new MemoryStream();
+			using (GZipInputStream inStream = new GZipInputStream(ms))
+			{
+				inStream.CopyTo(readStream);
+			}
+
+			// Check that the data was read
+			byte[] data = readStream.ToArray();
+			CollectionAssert.AreEqual(new byte[] { 45 }, data, "Decompressed data should match initial data");
+		}
+
 		[Test]
 		[Category("GZip")]
 		public void ZeroLengthInputStream()