Fetching data from other app

Fetching data from other app

Hi I've been trying to fetch address information from a client database in one app into another app. I understand the normal way within one app doesn't work. I've already found a few responses on the forum but none seems to work. 
I understand I create a function in app 1 and call on it in app 2. 
For now I'm using in app 1 forthe function the following code: 

string GetClientInfo(int varID)
{
fetaddress = Nuevo_Cliente[ID == input.varID];
return fetaddress.Direcci_n_Cliente;
}

In app 2 I call on it this way: 

if(input.Client_SI !="")
{
address = order_management.GetClientInfo(input.Client_SI);
}
I get an error saying: In criteria left expression is of type BEGINT and the right expression is of type STRING and the operator != is not valid
So I changed the !="" to >0 but that did not do the trick. Anybody have some suggestions? 
The Client_SI is a lookup field with the name of the client
Address is the address field that needs to be filled.

Or should I use the create MAP task? I've been trying to understand this from the manual, but no luck there. 

Any help appreciated.