Foreign Keys in Azure

Question

Hello

I have problem with Windows Azure SQL Database. I connected my application with the cloud database successfully. I made successfully inserts, deletes, updates. Now my problem is i have three tables for example: subjects, professors, subject_professor.

Subjects = {id_subject}

Professors = {id_professor}

subject_professor = {id_subject, id_professor} - which are foreign keys of the two tables above

In c#/xaml windows phone 8 i created the three tables but now when i want to add item in the subject_professor table i got an error. I tried to take the ID from one table and put it in variable but after Azure changed the primary key to be String instead of int i don't know anymore how to put in var.

Here is another example:

Subjects newSub = new Subjects();

var ID = newSub.id; -----> here is the problem cos ID is no longer int 

newSub.Name = "Mathematics" ----> thats working (the ID is created automatically when i create new object )

await subs.InsertAsync(newSub);

Professors newProf = new Professors();

var ID2 = newProf.id

newProf.Name = "Test";

await pro.InsertAsync(newProf);

subject_professor another = new subject(professor) ---> which professor teaches what subject

another.id_subject = ID;

another.id_professor = ID2;

Here i got problem and it says that ID cant reference to null value.

Please i need help with this problem Thank you.