CSHARP-986: Allow $external as a database name even though it contains a "$".

Author: rstam

MongoDB.Driver/MongoServer.cs

@@ -713,6 +713,8 @@ public virtual GetLastErrorResult GetLastError()
         /// <returns>True if the database name is valid; otherwise, false.</returns>
         public virtual bool IsDatabaseNameValid(string databaseName, out string message)
         {
+            message = null;
+
             if (databaseName == null)
             {
                 throw new ArgumentNullException("databaseName");
@@ -724,6 +726,12 @@ public virtual bool IsDatabaseNameValid(string databaseName, out string message)
                 return false;
             }
 
+            // make an exception for $external
+            if (databaseName == "$external")
+            {
+                return true;
+            }
+
             foreach (var c in databaseName)
             {
                 if (__invalidDatabaseNameChars.Contains(c))
@@ -741,7 +749,6 @@ public virtual bool IsDatabaseNameValid(string databaseName, out string message)
                 return false;
             }
 
-            message = null;
             return true;
         }
 

MongoDB.DriverUnitTests/MongoServerTests.cs

@@ -195,6 +195,7 @@ public void TestIsDatabaseNameValid()
             Assert.IsFalse(_server.IsDatabaseNameValid("", out message));
             Assert.IsFalse(_server.IsDatabaseNameValid("/", out message));
             Assert.IsFalse(_server.IsDatabaseNameValid(new string('x', 128), out message));
+            Assert.IsTrue(_server.IsDatabaseNameValid("$external", out message));
         }
 
         [Test]