如何以有效的方式同时读取两个XML文件

Question

I have a complicated case: I have three XML files, which I need to read simultaneously and get the results based on matches. Below is a working (but fake) example, almost similar to what I am doing.

For instance, I have two xml file, both are similar but in terms of tags and attributes, but with different contents (languages). I'm reading both languages at the same time, like in the code in a C# file:

XElement x1 = XElement.Load (@"abc.xml");
XElement x2 = XElement.Load (@"xyz.xml");


var ch = from var1 in x1.Elements("language1") 
         where var1.Attribute("index").Value == "1"
         from var2 in x2.Elements("language2")
         where var2.Attribute("index").Value == var1.Attribute("index").Value
         select dictChapter as new
         {  
             sentenceNumber = var1.Attribute("index").Value,
             SentenceInLanguage1 = var1.Attribute("text").Value,
             SentenceInLanguage2 = var2.Attribute("text").Value,
         };

ListBox.DataContext = ch;

The problem here is that, x1 contains 1000 sentences and so x2. The above logic work like a nested loop, which is slowing down the processing a lot. It works like

x1.1 -> x2.1:1000
x1.2 -> x2.1:1000

or

for i in x1
  for j in x2

Is there any better and efficient way to select the sentences from x1 and x2,where the sentence id of x1 is equal to the sentence id of x2?

Answer 1

From what I understood that you want,

You could use join to do that.

Here is a good example link LINQ to XML : Join Xml Data (Wriju's BLOG)

...or something along these lines...

var root = (from var1 in x1.Elements("language1")
            join var2 in x2.Elements("language2") on (string)var1.Attribute("index") equals (string)var2.Attribute("index")
            select new
            {
                SentenceNumber = (string)var1.Attribute("index"),
                SentenceInLanguage1 = (string)var1.Element("text"),
                SentenceInLanguage2 = (string)var2.Element("text")
            });

Answer 2

In Linq the following statements are equivalent and will provide the same results:

from i1 in items1
from i2 in items2
where i1 == i2

and

from i1 in items1
join i2 in items2 on i1 equals i2

They will even be translated to the same SQL (using Linq to SQL). For MS SQL in both cases result SQL will contain join clause (that's why ther is no need to use less flexible join when you query database.

However for Linq to Objects and Linq to XML both will be executed in different way. First will result in nested loops and second won't.

So you just need to change your implementation to use join as @NSGaga suggested.

Another optimization would be adding .ToList() :

ListBox.DataContext = ch;

I am not sure about databinding but because of deffered nature of linq there is a possibility that your expression will be re-evaluated more than once.

Answer 3

Easy! Just go through each file in sequence. On the first pass: Create a dictionary of sentenceNumber > SentenceInlanguage1 .

On your second pass, create your enumerable as in the code you showed, pasting in the data from the first pass for the SentenceInLanguage1 variable.

If you would prefer to go through both together, just get an enumerator ( GetEnumerator ) and go through those in a plain old while loop, moving to the next XElement for both enumerators at the end of the loop body.