Tip 12: Using LINQ to XML to Handle Large XML Files
By now, you've likely seen at least one example of using LINQ to XML. Here's a very simple example for using LINQ syntax to query XML data using C#:
XElement XmlAddr = new XElement("Addresses",
new XElement("AddressRec",
new XAttribute("EmployeeID", 1),
new XElement("FirstName", "Kevin"),
new XElement("LastName", "Goff"),
new XElement("Address", "111 Main Street"),
new XElement("City", "Philadelphia"),
new XElement("State", "PA"),
new XElement("Zip", "11111")));
XDocument oDoc = new XDocument(XmlAddr);
var xResults =
from oXml in oDoc.Descendants("AddressRec")
where (string)oXml.Element("State") == "PA"
orderby (string)oXml.Element("Zip")
select oXml;
If you use Visual Basic, here's the equivalent code. Note that you can create XML literals in Visual Basic, as follows:
Dim XmlAddr = _
<?xml version="1.0" encoding="utf-8"
standalone="yes"?>
<Addresses>
<AddressRec EmployeeID="1">
<FirstName>Kevin</FirstName>
<LastName>Goff</LastName>
<Address>111 Main Street</Address>
<City>Philadelphia</City>
<State>PA</State>
<Zip>11111</Zip>
</AddressRec>
</Addresses>
Then you can use Visual Basic syntax, which is considerably less complicated than the C# equivalent:
Dim Xdocument As XDocument = _
New XDocument(XmlAddr)
Dim query =
From c In Xdocument.Descendants("AddressRec") _
Where c.<State>.Value = "PA" _
Order By c.<State>.Value _
Select New With {c.<FirstName>.Value}
Me.DataGridView1.DataSource = query.ToList()
That's certainly "nice", but the proverbial 500-pound elephant in the room is deciding how you would handle large XML files. Certainly, you don't want to load an entire large XML file into memory. What you can do is set up an extension method to perform a streaming function (see
Listing 5), and then refer to the extension method in the LINQ to XML IN syntax (see
Listing 6 and
Listing 7 for C# and Visual Basic, respectively).
| Author's Note: The code in Listing 5 comes from the Microsoft XML Team WebLog, where you'll find the code and further explanation. Kudos to the Microsoft XML Team for providing this! |
At the beginning of
Listing 5, you can refer to the extension method (
StreamElements) as your source for the query. The extension method will perform the streaming of the large XML for you:
from oXML in StreamElements(xmlFile,"CustOrders")
If you actually set a breakpoint and stepped through the code, you'd observe that the code:
- Loops through the code in the extension method (in the while (reader.Read()) statement).
- Creates an individual XElement object, reads an element from the XmlReader, and then returns the XElement object.
- Comes back to the LINQ to XML code block (specifically the WHERE clause) and evaluates the XElement object (implicitly returned from the extension method) to see if it matches the WHERE statement.
In this context, you can almost think of the extension method as a delegate to handle the element-by-element streaming of the large XML file.
Tip 13: Baker's Dozen Potpourri—Some Miscellaneous Tidbits for SQL Server Reporting Services 2005
Did you just install SharePoint in a server environment where SQL Server Reporting Services 2005 resides? Did you also notice afterwards that SSRS stopped working? That's because SharePoint can take over the port number for the default web site. You can address that situation by going into IIS and changing the port number for your default web site. (I changed mine to 8080.)
Or...are you having problems trying to run a report for a specific parameter, and also trying to send it directly to a PDF file? Here's the "magic" URL syntax (note that I'm using Port 8080!):
int MyCustomerID = 1001;
string OutURL =
"http://localhost:8080/ReportServer?/";
OutURL += "MyReportProject/MyReport";
OutURL += "&rs:Command=Render&rs:Format=PDF";
OutURL += "&CustomerID=" +
MyCustomerID.ToString();
Or have you ever tried to drag data from a report DataSet into a report header or footer? SSRS doesn't allow this. You can get around it by doing the following:
- Define a report parameter associated with the data you want to display in the header/footer.
- Set the hidden property to true for the parameter.
- Define the default value for the parameter as the dataset/column associated with the actual database value.
- Finally, in the report heading, reference the parameter name instead of the dataset name.
Haven't helped you yet? How about this one? You need to develop SSRS reports so that users will only see data associated with their rights. (For example, you might have database rules in the form of tables that define User XYZ as being allowed to see only data for specific product categories or specific regions.) The key is to identify the current user running the report so that you can pass that on to any database query/stored procedure. You can obtain that information either from the
User!UserId collection property in SSRS, or from the
System_User system variable in SQL Server.
Final Thoughts
The next Baker's Dozen article will present an article I've been planning for a long time and will finally put together: 13 Tips for Implementing Common Design Patterns in C# 3.0. Check
my blog for any follow-up tips, additional resources, and notes on this and previous Baker's Dozen articles. You can find the entire source code for this article on
my web site.
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