How can I refresh my local database from a web service when an item isn't found?

Question

The project I am working on involves a database and a web service for the source of data. This question, and I have several, is based on the idea that if I enter in a product number in textBox1 , I want to see if the number is already in the database, if not I want to have it run my written code that goes to a web service and grabs the data and then updates the database.

1. What code should I use to check if the Product_ID is null or empty

2. What code do I need to write so that if the Product_ID is null or empty it will invoke the code I have already written that grabs data from a web service?

My linq code is:

Test_Data_ClassDataContext db = new Test_Data_ClassDataContext();
            var q = from p in db.Product_Masters
                    where p.Product_ID.Equals(textBox1.Text)
                    select p;

stealing from others ideas I am trying to test for null if null run the web service code if not null send it to datagridview for display.

If (q == null)
{
    Somehow call the other code so it takes textBox1 and get the data from the service
}

else
{
    dataGridView1.DataSource = q;
}

Background: I am a newbie to c#, once upon a time I played with vb6 and so to me c# is not only like learning a new language but also learning to walk again, not to mention I was never a good vb6 programmer.

Answer 1

q will never be null.

The value of a query expression is an object representing the query . It is not the results of the query .

If what you want to know is "does this query have any results" then say "q.Any()".

Answer 2

Because linq to sql uses defered execution until GetEnumerator() is called you need to fist invoke your query.

var result = q.ToList();

if (result.Count == 0)
{
  //Somehow call the other code so it takes textBox1 and get the data from the service
}
else
{
  dataGridView1.DataSource = result;
}

Now that you have invoked the query you can simply check if there are results, if not you can simply call your webservice.

Answer 3

What you also want to know is: "how do i call a webservice?".

This is a topic onto itself: In VS this is made easy by adding the service reference (either a Web Service Reference for ASMX services or a Service Reference for a WCF service).

I suggest Googling for info on either subjects and how to tell them apart.