Merge pull request #498 from wangzq/bug/Azure-RoleEnvironment-IsAvailable

Check Azure RoleEnvironment.IsAvailable before getting CurrentRoleIns…

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.cs

@@ -125,7 +125,11 @@ internal static string TryGetAzureRoleInstanceIdNoThrow()
                 if (asm == null)
                     return null;
 
-                var currentRoleInstanceProp = asm.GetType("Microsoft.WindowsAzure.ServiceRuntime.RoleEnvironment").GetProperty("CurrentRoleInstance");
+                var type = asm.GetType("Microsoft.WindowsAzure.ServiceRuntime.RoleEnvironment");
+
+                // https://msdn.microsoft.com/en-us/library/microsoft.windowsazure.serviceruntime.roleenvironment.isavailable.aspx
                if (!(bool)type.GetProperty("IsAvailable").GetValue(null, null))
                    return null;
+
+                var currentRoleInstanceProp = type.GetProperty("CurrentRoleInstance");
                 var currentRoleInstanceId = currentRoleInstanceProp.GetValue(null, null);
                 roleInstanceId = currentRoleInstanceId.GetType().GetProperty("Id").GetValue(currentRoleInstanceId, null).ToString();