About get property value function (C-SDK)

Hello, I have a question about two "get property value" functions.

There are two get property functions in C-SDK

 1. twPrimitive * twExt_GetPropertyValue(const char * thingName, char * propertyName);

 2. int twApi_ReadProperty(enum entityTypeEnum entityType, char * entityName, char * propertyName, twPrimitive ** result, int32_t timeout, char forceConnect);

 What are differences of two functions?

When I try to get value of 'isConnected' property of Connectable shape, first function is failed and secod is success.

 There is no explain about these... exactly..