Fixed #32 by improving the UUIDConverter.

src/main/java/com/github/msemys/esjc/util/UUIDConverter.java

@@ -9,14 +9,32 @@ public static byte[] toBytes(UUID uuid) {
         ByteBuffer bb = ByteBuffer.allocate(16);
         bb.putLong(uuid.getMostSignificantBits());
         bb.putLong(uuid.getLeastSignificantBits());
-        return bb.array();
+        return adaptEndianness(bb.array());
     }
 
     public static UUID toUUID(byte[] bytes) {
-        ByteBuffer bb = ByteBuffer.wrap(bytes);
+        ByteBuffer bb = ByteBuffer.wrap(adaptEndianness(bytes));
         long mostSignificantBits = bb.getLong();
         long leastSignificantBits = bb.getLong();
         return new UUID(mostSignificantBits, leastSignificantBits);
     }
 
+    /*
+     * Adapt between mixed-endian UUID encoding (C#/EventStore) and big-endian (Java).
+     * https://stackoverflow.com/questions/45671415/c-sharp-why-does-system-guid-flip-the-bytes-in-a-byte-array
+     */
+    private static byte[] adaptEndianness(byte[] bytes) {
+        switchBytes(bytes, 0, 3);
+        switchBytes(bytes, 1, 2);
+        switchBytes(bytes, 4, 5);
+        switchBytes(bytes, 6, 7);
+        return bytes;
+    }
+
+    private static void switchBytes(byte[] bytes, int i, int j) {
+        byte b = bytes[i];
+        bytes[i] = bytes[j];
+        bytes[j] = b;
+    }
+
 }

src/test/java/com/github/msemys/esjc/AbstractEventStoreTest.java

@@ -1,6 +1,11 @@
 package com.github.msemys.esjc;
 
+
 import com.github.msemys.esjc.runner.EventStoreRunnerWithParametersFactory;
+import com.github.msemys.esjc.http.HttpClient;
+import com.github.msemys.esjc.util.Throwables;
+import org.junit.After;
+import org.junit.Before;
 import org.junit.Rule;
 import org.junit.rules.TestName;
 import org.junit.runner.RunWith;
@@ -38,6 +43,12 @@ public static Object[] parameters() {
         };
     }
 
+    protected static final HttpClient httpClient = HttpClient.newBuilder()
+        .address("127.0.0.1", 2113)
+        .build();
+
+    protected static final UserCredentials eventstoreCredentials = new UserCredentials("admin", "changeit");
+
     protected final EventStore eventstore;
 
     @Rule

src/test/java/com/github/msemys/esjc/ITReadEvent.java

@@ -1,9 +1,14 @@
 package com.github.msemys.esjc;
 
+import io.netty.handler.codec.http.FullHttpRequest;
 import org.junit.Test;
 
 import java.util.List;
+import java.util.UUID;
+import java.util.concurrent.ExecutionException;
 
+import static com.github.msemys.esjc.http.HttpClient.newRequest;
+import static io.netty.handler.codec.http.HttpMethod.POST;
 import static java.util.Arrays.asList;
 import static org.junit.Assert.*;
 
@@ -93,6 +98,26 @@ public void returnsExistingEvent() {
         assertNotNull(result.event.originalEvent().created);
     }
 
+    @Test
+    public void returnsCorrectEventId() throws ExecutionException, InterruptedException {
+        final String stream = generateStreamName();
+        final UUID eventId = UUID.randomUUID();
+
+        // write event via http API
+        String uri = "/streams/" + stream;
+        FullHttpRequest request = newRequest(POST, uri, "{}", "application/json", eventstoreCredentials);
+        request.headers().set("ES-EventId", eventId.toString());
+        request.headers().set("ES-EventType", "event0");
+        httpClient.send(request).get();
+
+        // read event via tcp
+        EventReadResult result = eventstore.readEvent(stream, 0, true).join();
+
+        // compare event IDs
+        assertEquals(EventReadStatus.Success, result.status);
+        assertEquals(eventId, result.event.originalEvent().eventId);
+    }
+
     @Test
     public void retrievesJsonFlagProperly() {
         final String stream = generateStreamName();

src/test/java/com/github/msemys/esjc/tcp/TcpPackageTest.java

@@ -84,7 +84,7 @@ public void createsNonAuthorizedTcpPackageFromByteArray() {
 
         assertEquals(TcpCommand.HeartbeatRequestCommand, tcpPackage.command);
         assertEquals(TcpFlag.None, tcpPackage.flag);
-        assertEquals("0dc092bd-aeb3-4ad4-a6cf-4448e4ca50cb", tcpPackage.correlationId.toString());
+        assertEquals("bd92c00d-b3ae-d44a-a6cf-4448e4ca50cb", tcpPackage.correlationId.toString());
         assertNull(tcpPackage.login);
         assertNull(tcpPackage.password);
         assertEquals(DATA, new String(tcpPackage.data, UTF_8));
@@ -100,7 +100,7 @@ public void createsAuthorizedTcpPackageFromByteArray() {
 
         assertEquals(TcpCommand.HeartbeatRequestCommand, tcpPackage.command);
         assertEquals(TcpFlag.Authenticated, tcpPackage.flag);
-        assertEquals("beca0c9b-4d2d-4c10-9bd8-d518da5ccf00", tcpPackage.correlationId.toString());
+        assertEquals("9b0ccabe-2d4d-104c-9bd8-d518da5ccf00", tcpPackage.correlationId.toString());
         assertEquals("admin", tcpPackage.login);
         assertEquals("secret", tcpPackage.password);
         assertEquals(DATA, new String(tcpPackage.data, UTF_8));

src/test/java/com/github/msemys/esjc/util/UUIDConverterTest.java

@@ -0,0 +1,49 @@
+package com.github.msemys.esjc.util;
+
+import org.junit.Test;
+
+import java.util.UUID;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+
+/**
+ * Guid: 35918bc9-196d-40ea-9779-889d79b753f0
+ * C9 8B 91 35 6D 19 EA 40 97 79 88 9D 79 B7 53 F0
+ * example from https://docs.microsoft.com/en-us/dotnet/api/system.guid.tobytearray?view=netframework-4.7
+ */
+public class UUIDConverterTest {
+
+    @Test
+    public void toUUID() {
+        byte[] uuidBytes = toByteArray("C9 8B 91 35 6D 19 EA 40 97 79 88 9D 79 B7 53 F0");
+
+        UUID uuid = UUIDConverter.toUUID(uuidBytes);
+
+        assertEquals(UUID.fromString("35918bc9-196d-40ea-9779-889d79b753f0"), uuid);
+    }
+
+    @Test
+    public void toBytes() {
+        UUID uuid = UUID.fromString("35918bc9-196d-40ea-9779-889d79b753f0");
+
+        byte[] uuidBytes = UUIDConverter.toBytes(uuid);
+
+        assertArrayEquals(toByteArray("C9 8B 91 35 6D 19 EA 40 97 79 88 9D 79 B7 53 F0"), uuidBytes);
+    }
+
+    private byte[] toByteArray(String hexString) {
+        String[] hexArray = hexString.split(" ");
+        byte[] bytes = new byte[hexArray.length];
+        for (int i = 0; i<hexArray.length; i++) {
+            bytes[i] = toByte(hexArray[i]);
+        }
+        return bytes;
+    }
+
+    private byte toByte(String hexString) {
+        char[] charArray = hexString.toCharArray();
+        Integer i = (Character.digit(charArray[0], 16) << 4) + Character.digit(charArray[1], 16);
+        return i.byteValue();
+    }
+}