Question

I’m trying to find and collect all the elements that meet these requirements:

var checkTest = from d in document.Descendants("Prerequisite")
                where d.Value != "none"
                select d.Parent.Element("Name").Value;

by using this foreach loop:

foreach (var item in prerequisite)
{
    if (item == checkTest.ToString())
    {
        MessageBox.Show("this module has a prereq" + item);
    }
}

but it won’t do anything for some reason, anybody know why? I think the problem is in the var checkTest

(this is what prerequisite means in the foreach:)

var prerequisite = from d in document.Descendants("Name")
                   where d.Value == (String)listBox2.SelectedItem
                   select d.Parent.Element("Prerequisite").Value;

Here is the XML:

<?xml version="1.0" encoding="utf-8" ?>
<SoftwareEngineering>
  <Module>
    <Name>Algorithms and Data Structures</Name>
    <Code>3SFE504</Code>
    <Capacity>5</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>3D Graphics I</Name>
    <Code>3SFE508</Code>
    <Capacity>5</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Event-Driven Programming</Name>
    <Code>3SFE513</Code>
    <Capacity>10</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Object Oriented Design</Name>
    <Code>3SFE514</Code>
    <Capacity>10</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Requirements Engineering</Name>
    <Code>3SFE516</Code>
    <Capacity>10</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Introduction to AI</Name>
    <Code>3SFE599</Code>
    <Capacity>5</Capacity>
    <Semester>1</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Java Mobile Application Development</Name>
    <Code>3SFE540</Code>
    <Capacity>5</Capacity>
    <Semester>1</Semester>
    <Prerequisite>3SFE514(corequisite)</Prerequisite>
  </Module>
  <Module>
    <Name>C# .NET Programming</Name>
    <Code>3SFE541</Code>
    <Capacity>5</Capacity>
    <Semester>1</Semester>
    <Prerequisite>3SFE514(corequisite)</Prerequisite>
  </Module>
  <Module>
    <Name>Software Engineering Group Project</Name>
    <Code>3SFE515</Code>
    <Capacity>5</Capacity>
    <Semester>2</Semester>
    <Prerequisite>3SFE514(corequisite)</Prerequisite>
  </Module>
  <Module>
    <Name>Software Engineering</Name>
    <Code>3SFE519</Code>
    <Capacity>10</Capacity>
    <Semester>2</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Mobile User Interface Development</Name>
    <Code>3SFE542</Code>
    <Capacity>5</Capacity>
    <Semester>2</Semester>
    <Prerequisite>3SFE540</Prerequisite>
  </Module>
  <Module>
    <Name>Interactive Multimedia</Name>
    <Code>3MTS954</Code>
    <Capacity>5</Capacity>
    <Semester>2</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Concurrent Programming</Name>
    <Code>3SFE555</Code>
    <Capacity>5</Capacity>
    <Semester>2</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Mobile Gaming</Name>
    <Code>3SFE557</Code>
    <Capacity>10</Capacity>
    <Semester>2</Semester>
    <Prerequisite>none</Prerequisite>
  </Module>
  <Module>
    <Name>Intelligent Systems</Name>
    <Code>3SFE500</Code>
    <Capacity>10</Capacity>
    <Semester>2</Semester>
    <Prerequisite>3SFE599</Prerequisite>
  </Module>
  <Module>
    <Name>3D Graphics II</Name>
    <Code>3SFE501</Code>
    <Capacity>10</Capacity>
    <Semester>2</Semester>
    <Prerequisite>3SFE508</Prerequisite>
  </Module>
</SoftwareEngineering>

Does it have to do with my selecting an upper-sibling when having searched for its lowest sibling? i.e. I searched for Prerequisite first and then said to select its Name.Value (in checkTest)

Even when I try and make the if statement as !=, it will print each prerequisite, even when its none!

Answer 1

The problem is checkTest is IEnumerable<String>, not a string, so calling ToString() is not providing a good value to compare too. FYI – IEnumerable<T> is the return value of all basic LINQ querys, although there are ways to extract the first or last elements.

It looks like you are trying to get a list of prerequisites that match one of the elements in checkTest

So you should try something like:

var match = prerequisite.Where(x => checkTest.Contains(x)).Select(item => item);

foreach (var pr in match)
{
    MessageBox.Show("this module has a prereq" + pr);
}

EDIT: updated answer based on previous question by OP

From the answer I gave you yesterday morning on a similar topic, are you using the Module class and the Dictionary<String,Module>? If so, why are you doing this query?
You already have everything you need to accomplish this without a query:

var pr = modules[(String)listBox1.SelectedValue].Prerequisite;

if (pr != "none")
    MessageBox.Show("this module has a prereq" + pr);

2nd Edit to answer actual LINQ query question

My previous edit suggesting going back to the previous questions and suing the preloaded XML and collection is still a better solution, but since your questions seem to go back to using LINQ-to-XML, you obviously have a reason to prefer it.

I took a more detailed look at your question and i think i have the problem. You are comparing 2 elements that will never match. In your checkTest query, you are selecting a list of Names that have a prereq but you are trying to compare to a list of prereq’s that only contain a code.

You should use my query that I provided to check for matches, but you should get checkTest with the follopwing:

var checkTest = from d in xdoc.Descendants("Prerequisite")
                where d.Value != "none"
                select d.Parent.Element("Prerequisite").Value;

EDIT #3 – Rewriting with LINQ Query syntax

To respond to your last comment, the query I provided you is an alternative form of LINQ query that I tend to prefer to smaller/shorter queries.

You could rewrite that query using the query syntax you have been using as:

var match = from x in prerequisite
            where checkTest.Contains(x)
            select x;

the => operator is called a lamda expression. Try searching for LINQ query syntax and/or Lambda expressions to help understand the difference