Allow registration of message id factory (#24)

* Allow registration of message id factory

* guarding againts null argument + providing a better exception when ID generator fails

* applying changes mandated by stylecop

* more meaningful name

* xml doco

* Logical grouping of tests

* bringing all BrokeredMessage tests under one roof

src/Microsoft.Azure.ServiceBus/BrokeredMessage.cs

@@ -15,6 +15,8 @@ namespace Microsoft.Azure.ServiceBus
     /// <summary>Represents the unit of communication between ServiceBus client and Service.</summary>
     public sealed class BrokeredMessage : IDisposable
     {
+        static Func<string> messageIdGeneratorFunc = () => (string)null;
+
         readonly object disposablesSyncObject = new object();
         readonly bool ownsBodyStream;
         readonly bool bodyObjectDecoded;
@@ -55,6 +57,14 @@ namespace Microsoft.Azure.ServiceBus
         /// <summary>Initializes a new instance of the <see cref="BrokeredMessage" /> class.</summary>
         public BrokeredMessage()
         {
+            try
+            {
+                this.messageId = messageIdGeneratorFunc();
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException("BrokeredMessage ID generator function has failed.", ex);
+            }
         }
 
         /// <summary>Initializes a new instance of the
@@ -902,6 +912,20 @@ namespace Microsoft.Azure.ServiceBus
 
         internal MessageReceiver Receiver { get; set; }
 
+        /// <summary>Specify generator to be used to generate BrokeredMessage.MessageId value.
+        /// <param name="messageIdGenerator">Message ID generator.</param>
+        /// <remarks>Be default, no value is assigned.</remarks>
+        /// </summary>
+        /// <exception cref="ArgumentNullException">Thrown if invoked with null.</exception>
+        public static void SetMessageIdGenerator(Func<string> messageIdGenerator)
+        {
+            if (messageIdGenerator == null)
+            {
+                throw new ArgumentNullException(nameof(messageIdGenerator));
+            }
+            messageIdGeneratorFunc = messageIdGenerator;
+        }
+
         /// <summary>Deserializes the brokered message body into an object of the specified type by using the
         /// <see cref="System.Runtime.Serialization.DataContractSerializer" /> with a binary
         /// <see cref="System.Xml.XmlDictionaryReader" />.</summary>

test/Microsoft.Azure.ServiceBus.UnitTests/BrokeredMessageTests.cs

@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.Azure.ServiceBus.UnitTests
+{
+    using System;
+    using Xunit;
+
+    public class BrokeredMessageTests
+    {
+        public class When_BrokeredMessage_message_id_generator_is_not_specified
+        {
+            [Fact]
+            public void Message_should_have_MessageId_set()
+            {
+                var message = new BrokeredMessage();
+
+                Assert.Null(message.MessageId);
+            }
+        }
+
+        public class When_BrokeredMessage_message_is_given_a_null_id_generator
+        {
+            [Fact]
+            public void Should_throw_an_exception()
+            {
+                Assert.Throws<ArgumentNullException>(() => BrokeredMessage.SetMessageIdGenerator(null));
+            }
+        }
+
+        public class When_BrokeredMessage_id_generator_throws
+        {
+            [Fact]
+            public void Should_throw_with_original_exception_included()
+            {
+                var exceptionToThrow = new Exception("boom!");
+                Func<string> idGenerator = () =>
+                {
+                    throw exceptionToThrow;
+                };
+                BrokeredMessage.SetMessageIdGenerator(idGenerator);
+
+                var exception = Assert.Throws<InvalidOperationException>(() => new BrokeredMessage());
+                Assert.Equal(exceptionToThrow, exception.InnerException);
+            }
+        }
+
+        public class When_BrokeredMessage_message_id_generator_is_specified
+        {
+            [Fact]
+            public void Message_should_have_MessageId_set()
+            {
+                var seed = 1;
+                BrokeredMessage.SetMessageIdGenerator(() => $"id-{seed++}");
+
+                var message1 = new BrokeredMessage();
+                var message2 = new BrokeredMessage();
+
+                Assert.Equal("id-1", message1.MessageId);
+                Assert.Equal("id-2", message2.MessageId);
+            }
+        }
+    }
+}