Solidity Struct – Issue with Length Not Found After Argument-Dependent Lookup

Good day
I've got a struct like this, and I also declare an array to store this struct.

Then in the same contract, I tried to check if the corresponding struct is null or not by checking its length equals to zero.

But the compiler shows me the error msg in if (propertyList[_id].length == 0) throw; of removeProperty function

Member "length" not found or not visible after argument-dependent
lookup in struct Property storage ref

But if I create a function

function getPropertiesLength() constant returns(uint){
    return propertyList.length;
}

Then modify my code to if (getPropertiesLength() == 0) throw;

it works just fine. Why? 0.0

My code is listed below. I appreciate any suggestions and opinions 🙂

Many thanks!!!

contract usingProperty{
    struct Property{
        bytes32 name;
        uint id;
        mapping (address => bool) accessStakeholders;
        uint[] rating;
    }

    Property[] public propertyList;

    function addProperty(bytes32 _name, address[] _accessStakeholders, uint _rating) returns(bool success, uint _id){

        _id = propertyList.length++;

        Property prop = propertyList[_id];
        for (uint i = 0 ; i < _accessStakeholders.length ; i++){
          prop.accessStakeholders[_accessStakeholders[i]] = true;
        }

        prop.name = _name;
        prop.id= _id;
        prop.rating.push(_rating);
    }


    function removeProperty(uint _id){
        if (propertyList[_id].length == 0) throw;
    }

}