Building a web service that returns data

A web service lets one application offer a method to another, across a network, without either side knowing how the other is built.

The shape of the service

[WebMethod]
public DataTable GetProducts(string category)
{
    using (var conn = new SqlConnection(connString))
    using (var cmd = new SqlCommand(
        "SELECT Id, Name FROM Product WHERE Category = @cat", conn))
    {
        cmd.Parameters.AddWithValue("@cat", category);
        var table = new DataTable();
        conn.Open();
        new SqlDataAdapter(cmd).Fill(table);
        return table;
    }
}

What makes it a service

• The attribute marks the method as callable from outside.
• The method takes simple values and returns data, so any client can use it.
• The database work stays behind the service, hidden from the caller.

Consuming it from a page is the subject of calling a web service from a page.