Support creating UNC paths

Create failing test to highlight problem creating UNC paths.
Create two other tests for other UNC related commands that should throw an exception.
Bug fix, plus corrected tests.

Author: VTJDailey

TestHelpers.Tests/MockDirectoryTests.cs

@@ -410,6 +410,42 @@ public void MockDirectory_CreateDirectory_ShouldReturnDirectoryInfoBase()
             Assert.IsNotNull(result);
         }
 
+        [Test]
+        public void MockDirectory_CreateDirectory_ShouldWorkWithUNCPath()
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+            
+            // Act
+            fileSystem.Directory.CreateDirectory(XFS.Path(@"\\server\share\path\to\create", () => false));
+
+            // Assert
+            Assert.IsTrue(fileSystem.Directory.Exists(XFS.Path(@"\\server\share\path\to\create\", () => false)));
+        }
+
+        [Test]
+        public void MockDirectory_CreateDirectory_ShouldFailIfTryingToCreateUNCPathOnlyServer()
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+
+            var ex = Assert.Throws<ArgumentException>(() => fileSystem.Directory.CreateDirectory(XFS.Path(@"\\server", () => false)));
+            Assert.That(ex.Message, Is.EqualTo("The UNC path should be of the form \\\\server\\share.\r\nParameter name: path"));
+        }
+
+        [Test]
+        public void MockDirectory_CreateDirectory_ShouldSucceedIfTryingToCreateUNCPathShare()
+        {
+            // Arrange
+            var fileSystem = new MockFileSystem();
+
+            // Act
+            fileSystem.Directory.CreateDirectory(XFS.Path(@"\\server\share", () => false));
+
+            // Assert
+            Assert.IsTrue(fileSystem.Directory.Exists(XFS.Path(@"\\server\share\", () => false)));
+        }
+
         [Test]
         public void MockDirectory_Delete_ShouldDeleteDirectory()
         {

TestingHelpers/MockFileSystem.cs

@@ -105,6 +105,25 @@ public void AddDirectory(string path)
                     throw new UnauthorizedAccessException(string.Format(CultureInfo.InvariantCulture, "Access to the path '{0}' is denied.", path));
 
                 var lastIndex = 0;
+
+                bool isUnc =
+                    path.StartsWith(@"\\", StringComparison.OrdinalIgnoreCase) ||
+                    path.StartsWith(@"//", StringComparison.OrdinalIgnoreCase);
+
+                if (isUnc)
+                {
+
+                    //First, confirm they aren't trying to create '\\server\'
+                    lastIndex = path.IndexOf(separator, 2);
+                    if (lastIndex < 0)
+                        throw new ArgumentException(@"The UNC path should be of the form \\server\share.", "path");
+
+                    /* 
+                     * Although CreateDirectory(@"\\server\share\") is not going to work in real code, we allow it here for the purposes of setting up test doubles.
+                     * See PR https://github.com/tathamoddie/System.IO.Abstractions/pull/90 for conversation
+                     */
+                }
+
                 while ((lastIndex = path.IndexOf(separator, lastIndex + 1)) > -1)
                 {
                     var segment = path.Substring(0, lastIndex + 1);