LINQ的ConfigurationElementCollection

Question

I have some problem with LinQ. How can I get id by name in query or some LinQ condition? This is try to get:

var section = ConfigurationManager.GetSection("jobSection");

if (section != null)
{
    var jobs = (section as JobSection).Jobs;
    var item = from JobElement je in jobs where je.Name == "Job Name A" select je.Id;
    Console.WriteLine(item.ToString());
}

This is config.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
        <section name="jobSection" type="ConsoleApplication2.JobSection, ConsoleApplication2" />
    </configSections>
    <jobSection>
        <jobs>
            <job id="1" name="Job Name A" />
            <job id="2" name="Job Name B" />
        </jobs>
    </jobSection>
</configuration>

But this is output:

System.Linq.Enumerable+WhereSelectEnumerableIterator`2[ConsoleApplication2.JobElement,System.Int32]

Answer 1

the problem

of course it does - you are printing the Enumerable

As there is no default implementation for ToString on the generic interface it just prints the type-name - that's why you get the strange answer.

basic solution: foreach

var section = ConfigurationManager.GetSection("jobSection");
if (section != null)
{
    var jobs = (section as JobSection).Jobs;
    var query = 
       from JobElement je in jobs 
       where je.Name == "Job Name A" 
       select je.Id;

    foreach(var item in query)
       Console.WriteLine(item.ToString());
}

If it helps think query as a kind of lazy array/list - it's just waiting for you to pull it's items out.

expecting exactly one element?

By the way - maybe you are expecting to get exactly one result. In this case you can use .Single like this:

var section = ConfigurationManager.GetSection("jobSection");
if (section != null)
{
    var jobs = (section as JobSection).Jobs;
    var query = 
       from JobElement je in jobs 
       where je.Name == "Job Name A" 
       select je.Id;
    var item = query.Single();
    Console.WriteLine(item.ToString());
}

This will throw if there is not exactly one element in the result. Alternatives are:

• .First (will give you the first element and throws if here are none)
• .FirstOrDefault (will give you the first element or the default value - most likely null if there are no items)

some fun with overloads/linq

And if you poke in even deeper into the overloads you will see that you can combine the Where and First (etc.) parts like this:

var section = ConfigurationManager.GetSection("jobSection");
if (section != null)
{
    var jobs = (section as JobSection).Jobs;
    var item = jobs.Cast<JobElement>()
                   .First(je => je.Name == "Job Name A");
    Console.WriteLine(item.Id);
}

Have fun