More testing yielded a case where the new inline Hyperlink support broke down.

I have fixed this issue now and I think thats it. I am now happy with the Hyperlink implementation.

DocX.sln

 EndProject
 Global
 	GlobalSection(TeamFoundationVersionControl) = preSolution
-		SccNumberOfProjects = 4
+		SccNumberOfProjects = 5
 		SccEnterpriseProvider = {4CA58AB2-18FA-4F8D-95D4-32DDF27D184C}
 		SccTeamFoundationServer = https://tfs.codeplex.com/tfs/tfs08
 		SccLocalPath0 = .
 		SccProjectUniqueName3 = Examples\\Examples.csproj
 		SccProjectName3 = Examples
 		SccLocalPath3 = Examples
+		SccProjectUniqueName4 = ConsoleApplication4\\ConsoleApplication4.csproj
+		SccProjectName4 = ConsoleApplication4
+		SccLocalPath4 = ConsoleApplication4
 	EndGlobalSection
 	GlobalSection(TestCaseManagementSettings) = postSolution
 		CategoryFile = DocX.vsmdi

DocX/Hyperlink.cs

         internal int type;
         internal String id;
         internal XElement instrText;
+        internal List<XElement> runs;
 
         /// <summary>
         /// Remove a Hyperlink from this Paragraph only.
 
                 else
                 {
-                    Xml.ReplaceWith(newRuns);
+                    XElement separate = XElement.Parse(@"
+                    <w:r xmlns:w='http://schemas.openxmlformats.org/wordprocessingml/2006/main'>
+                        <w:fldChar w:fldCharType='separate'/> 
+                    </w:r>");
+
+                    XElement end = XElement.Parse(@"
+                    <w:r xmlns:w='http://schemas.openxmlformats.org/wordprocessingml/2006/main'>
+                        <w:fldChar w:fldCharType='end' /> 
+                    </w:r>");
+
+                    runs.Last().AddAfterSelf(separate, newRuns, end);
+                    runs.ForEach(r => r.Remove());
                 }
 
                 this.text = value;
             this.text = sb.ToString();
         }
 
-        internal Hyperlink(DocX document, XElement instrText, XElement r) : base(document, r)
+        internal Hyperlink(DocX document, XElement instrText, List<XElement> runs) : base(document, null)
         {
             this.type = 1;
             this.instrText = instrText;
+            this.runs = runs;
 
             try
             {
                     this.uri = new Uri(instrText.Value.Substring(start, end - start), UriKind.Absolute);
 
                     StringBuilder sb = new StringBuilder();
-                    HelperFunctions.GetTextRecursive(r, ref sb);
+                    HelperFunctions.GetTextRecursive(new XElement(XName.Get("temp", DocX.w.NamespaceName), runs), ref sb);
                     this.text = sb.ToString();
                 }
             }

DocX/Paragraph.cs

                         while (e.Name.LocalName != "r")
                             e = e.Parent;
 
+                        // Take every element until we reach w:fldCharType="end"
+                        List<XElement> hyperlink_runs = new List<XElement>();
                         foreach (XElement r in e.ElementsAfterSelf(XName.Get("r", DocX.w.NamespaceName)))
                         {
-                            XElement rStyle = r.Descendants(XName.Get("rStyle", DocX.w.NamespaceName)).SingleOrDefault<XElement>();
-                            if (rStyle != null)
+                            // Add this run to the list.
+                            hyperlink_runs.Add(r);
+
+                            XElement fldChar = r.Descendants(XName.Get("fldChar", DocX.w.NamespaceName)).SingleOrDefault<XElement>();
+                            if (fldChar != null)
                             {
-                                XAttribute a = rStyle.Attribute(XName.Get("val", DocX.w.NamespaceName));
-                                if (a != null && a.Value.Equals("Hyperlink", StringComparison.CurrentCultureIgnoreCase))
+                                XAttribute fldCharType = fldChar.Attribute(XName.Get("fldCharType", DocX.w.NamespaceName));
+                                if (fldCharType != null && fldCharType.Value.Equals("end", StringComparison.CurrentCultureIgnoreCase))
                                 {
                                     try
                                     {
-                                        Hyperlink h = new Hyperlink(Document, he, r);
+                                        Hyperlink h = new Hyperlink(Document, he, hyperlink_runs);
                                         h.mainPart = mainPart;
                                         hyperlinks.Add(h);
                                     }
 
                                     catch(Exception){}
+
+                                    break;
                                 }
                             }
                         }
             return this;
         }
 
+        /// <summary>
+        /// Append a field of type document property, this field will display the custom property cp, at the end of this paragraph.
+        /// </summary>
+        /// <param name="cp">The custom property to display.</param>
+        /// <param name="f">The formatting to use for this text.</param>
+        /// <example>
+        /// Create, add and display a custom property in a document.
+        /// <code>
+        ///// Load a document.
+        ///using (DocX document = DocX.Create("CustomProperty_Add.docx"))
+        ///{
+        ///    // Add a few Custom Properties to this document.
+        ///    document.AddCustomProperty(new CustomProperty("fname", "cathal"));
+        ///    document.AddCustomProperty(new CustomProperty("age", 24));
+        ///    document.AddCustomProperty(new CustomProperty("male", true));
+        ///    document.AddCustomProperty(new CustomProperty("newyear2012", new DateTime(2012, 1, 1)));
+        ///    document.AddCustomProperty(new CustomProperty("fav_num", 3.141592));
+        ///
+        ///    // Insert a new Paragraph and append a load of DocProperties.
+        ///    Paragraph p = document.InsertParagraph("fname: ")
+        ///        .AppendDocProperty(document.CustomProperties["fname"])
+        ///        .Append(", age: ")
+        ///        .AppendDocProperty(document.CustomProperties["age"])
+        ///        .Append(", male: ")
+        ///        .AppendDocProperty(document.CustomProperties["male"])
+        ///        .Append(", newyear2012: ")
+        ///        .AppendDocProperty(document.CustomProperties["newyear2012"])
+        ///        .Append(", fav_num: ")
+        ///        .AppendDocProperty(document.CustomProperties["fav_num"]);
+        ///    
+        ///    // Save the changes to the document.
+        ///    document.Save();
+        ///}
+        /// </code>
+        /// </example>
+        public Paragraph AppendDocProperty(CustomProperty cp, bool trackChanges = false, Formatting f = null)
+        {
+            this.InsertDocProperty(cp, trackChanges, f);
+            return this;
+        }
+
         /// <summary>
         /// Insert a field of type document property, this field will display the custom property cp, at the end of this paragraph.
         /// </summary>
         /// </example>
         public DocProperty InsertDocProperty(CustomProperty cp, bool trackChanges = false, Formatting f = null)
         {
+            XElement f_xml = null;
+            if (f != null)
+                f_xml = f.Xml;
+
             XElement e = new XElement
             (
                 XName.Get("fldSimple", DocX.w.NamespaceName),
                 new XAttribute(XName.Get("instr", DocX.w.NamespaceName), string.Format(@"DOCPROPERTY {0} \* MERGEFORMAT", cp.Name)),
                     new XElement(XName.Get("r", DocX.w.NamespaceName),
-                        new XElement(XName.Get("t", DocX.w.NamespaceName), f.Xml, cp.Value))
+                        new XElement(XName.Get("t", DocX.w.NamespaceName), f_xml, cp.Value))
             );
 
             XElement xml = e;

UnitTests/UnitTest1.cs

 
         }
 
+        [TestMethod]
+        public void Test_CustomProperty_Add()
+        {
+            // Load a document.
+            using (DocX document = DocX.Create("CustomProperty_Add.docx"))
+            {
+                Assert.IsTrue(document.CustomProperties.Count == 0);
+
+                document.AddCustomProperty(new CustomProperty("fname", "cathal"));
+
+                Assert.IsTrue(document.CustomProperties.Count == 1);
+                Assert.IsTrue(document.CustomProperties.ContainsKey("fname"));
+                Assert.IsTrue((String)document.CustomProperties["fname"].Value == "cathal");
+
+                document.AddCustomProperty(new CustomProperty("age", 24));
+
+                Assert.IsTrue(document.CustomProperties.Count == 2);
+                Assert.IsTrue(document.CustomProperties.ContainsKey("age"));
+                Assert.IsTrue((int)document.CustomProperties["age"].Value == 24);
+
+                document.AddCustomProperty(new CustomProperty("male", true));
+
+                Assert.IsTrue(document.CustomProperties.Count == 3);
+                Assert.IsTrue(document.CustomProperties.ContainsKey("male"));
+                Assert.IsTrue((bool)document.CustomProperties["male"].Value == true);
+
+                document.AddCustomProperty(new CustomProperty("newyear2012", new DateTime(2012, 1, 1)));
+
+                Assert.IsTrue(document.CustomProperties.Count == 4);
+                Assert.IsTrue(document.CustomProperties.ContainsKey("newyear2012"));
+                Assert.IsTrue((DateTime)document.CustomProperties["newyear2012"].Value == new DateTime(2012, 1, 1));
+
+                document.AddCustomProperty(new CustomProperty("fav_num", 3.141592));
+
+                Assert.IsTrue(document.CustomProperties.Count == 5);
+                Assert.IsTrue(document.CustomProperties.ContainsKey("fav_num"));
+                Assert.IsTrue((double)document.CustomProperties["fav_num"].Value == 3.141592);
+            }
+        }
+
         [TestMethod]
         public void Test_EverybodyHasAHome_Loaded()
         {