    var list = new List&lt;string&gt;();
    var queryable = list.AsQueryable();

Add a reference to: `System.Linq`
    

Using a XPath query how do you find if a node (tag) exists at all?

For example if I needed to make sure a website page has the correct basic structure like `/html/body` and `/html/head/title`.

XPath find if node exists

I got this XML file:

	&lt;root&gt;
		&lt;level1 name=&quot;A&quot;&gt;
			&lt;level2 name=&quot;A1&quot; /&gt;
			&lt;level2 name=&quot;A2&quot; /&gt;
		&lt;/level1&gt;
		&lt;level1 name=&quot;B&quot;&gt;
			&lt;level2 name=&quot;B1&quot; /&gt;
			&lt;level2 name=&quot;B2&quot; /&gt;
		&lt;/level1&gt;
		&lt;level1 name=&quot;C&quot; /&gt;
	&lt;/root&gt;

Could someone give me a C# code using LINQ, the simplest way to print this result:&lt;br&gt;
(Note the extra space if it is a `level2` node)

    A
      A1
      A2
    B
      B1
      B2
    C

Currently I got this code:

	XDocument xdoc = XDocument.Load(&quot;data.xml&quot;));
	var lv1s = from lv1 in xdoc.Descendants(&quot;level1&quot;)
			   select lv1.Attribute(&quot;name&quot;).Value;

	foreach (var lv1 in lv1s)
	{
		result.AppendLine(lv1);

		var lv2s = from lv2 in xdoc...???
	}


LINQ to read XML

&gt; **Possible Duplicate:**  
&gt; [IList&amp;lt;T&amp;gt; to IQueryable&amp;lt;T&amp;gt;](https://stackoverflow.com/questions/73542/ilistt-to-iqueryablet)  

&lt;!-- End of automatically inserted text --&gt;

I have a List&lt;T&gt; data, but I want a IQueryable&lt;T&gt; data , is it possible from  List&lt;T&gt; data to IQueryable&lt;T&gt; data?
Show me code

How change List&lt;T&gt; data to IQueryable&lt;T&gt; data

> Possible Duplicate: 
> <a href="https://stackoverflow.com/questions/73542/ilistt-to-iqueryablet" target="_blank" rel="noopener noreferrer">IList&#x3C;T> to IQueryable&#x3C;T></a>

I have a List<t> data, but I want a IQueryable<t> data , is it possible from List<t> data to IQueryable<t> data?
Show me code</t></t></t></t>

I&#39;ve seen a lot of questions related to mapping [DTOs][1] to Domain Objects, but I didn&#39;t feel they answered my question.  I&#39;ve used many methods before and have my own opinions but I&#39;m looking for something a little more concrete.

**The Situation:** 

We have many domain objects.  We are using a CSLA model so our domain objects can be pretty complex and they contain their own data access.  You do not want to pass these around on the wire. We are going to be writing some new services that will return data in a number of formats (.Net, JSON, etc.).  For this (and other reasons) we are also creating a lean, data transfer object to pass around on the wire.  

**My question is: How should the DTO and Domain object be connected?**

My first reaction is to use a [Fowler, DTO pattern-type solution][2].  I&#39;ve seen this done many times and it feels right to me.  The domain object contains no reference to the DTO.  An outside entity (a &quot;mapper&quot; or &quot;assembler&quot;) is called to create a DTO from a Domain Object.  Normally there is an [ORM][3] on the domain object side.  The downside of this is that the &quot;mapper&quot; tends to get extremely complex for any real situation and can be very fragile. 

Another idea put forth is for the Domain Object to &quot;contain&quot; the DTO, since it&#39;s just a lean data object.  The Domain Object properties would internally reference the DTO properties and could just return the DTO if asked for.  I can see no problems with this but it feels wrong.  I have seen some articles where people using [NHibernate][4] appeared to use this method.

**Are there other ways?  Is one of the ways above worth using?  If so or if not, why?**

  [1]: https://en.wikipedia.org/wiki/Data_transfer_object
  [2]: http://martinfowler.com/eaaCatalog/dataTransferObject.html
  [3]: https://en.wikipedia.org/wiki/Object-relational_mapping
  [4]: https://nhibernate.info/

Best Practices For Mapping DTO to Domain Object?

I&#39;m returning a reference to a dictionary in my read only property.  How do I prevent consumers from changing my data?  If this were an `IList` I could simply return it `AsReadOnly`.  Is there something similar I can do with a dictionary?

    Private _mydictionary As Dictionary(Of String, String)
    Public ReadOnly Property MyDictionary() As Dictionary(Of String, String)
        Get
            Return _mydictionary
        End Get
    End Property

Is there a read-only generic dictionary available in .NET?

I am trying to do a request that accepts a compressed response

    var request = (HttpWebRequest)HttpWebRequest.Create(requestUri);
    request.Headers.Add(HttpRequestHeader.AcceptEncoding, &quot;gzip,deflate&quot;);

I wonder if when I add the second line I will have to handle the decompression manually.

Does .NET&#39;s HttpWebResponse uncompress automatically GZiped and Deflated responses?

I have a task of changing the names of some files (that is, adding id to each name dynamically) in a folder using C#.

Example: help.txt to 1help.txt

How can I do this?


Rename some files in a folder

I want to execute a query like this

 

       var result = from entry in table
                         where entry.something == null
                         select entry;

and get an `IS NULL` generated. 

Edited:
After the first two answers i feel the need to clarify that I&#39;m using Entity Framework
and not Linq to SQL. The object.Equals() method does not seem to work in EF.

Edit no.2:
The above query works as intended. It correctly generates `IS NULL`. My production code however was
 

    value = null;
    var result = from entry in table
                             where entry.something == value
                             select entry;
and the generated SQL was `something = @p; @p = NULL`. It seems that EF correctly translates the constant expression but if a variable is involved it treats it just like a normal comparison. Makes sense actually. I will close this question.

How can I query for null values in entity framework?

I&#39;m new to LINQ and want to know how to execute multiple where clause. This is what I want to achieve: return records by filtering out certain user names. I tried the code below but not working as expected.

    DataTable tempData = (DataTable)grdUsageRecords.DataSource;
    var query = from r in tempData.AsEnumerable()
                where ((r.Field&lt;string&gt;(&quot;UserName&quot;) != &quot;XXXX&quot;) || (r.Field&lt;string&gt;(&quot;UserName&quot;) != &quot;XXXX&quot;))                            
                select r;    
    
                DataTable newDT = query.CopyToDataTable();

Thanks for the help in advance!!!

Multiple WHERE clause in Linq

I&#39;m using .NET 3.5 and would like to be able to obtain every *`n`*th item from a List&lt;T&gt;.  I&#39;m not bothered as to whether it&#39;s achieved using a lambda expression or LINQ.

**Edit**

Looks like this question provoked quite a lot of debate (which is a good thing, right?).  The main thing I&#39;ve learnt is that when you think you know every way to do something (even as simple as this), think again!

How can I get every nth item from a List&lt;T&gt;?

Let&#39;s say I have this SQL:

    SELECT p.ParentId, COUNT(c.ChildId)
    FROM ParentTable p
      LEFT OUTER JOIN ChildTable c ON p.ParentId = c.ChildParentId
    GROUP BY p.ParentId

How can I translate this into LINQ to SQL? I got stuck at the COUNT(c.ChildId), the generated SQL always seems to output COUNT(*). Here&#39;s what I got so far:

    from p in context.ParentTable
    join c in context.ChildTable on p.ParentId equals c.ChildParentId into j1
    from j2 in j1.DefaultIfEmpty()
    group j2 by p.ParentId into grouped
    select new { ParentId = grouped.Key, Count = grouped.Count() }

Thank you!




LINQ - Left Join, Group By, and Count

    from i in Db.Items
    select new VotedItem
    {
        ItemId = i.ItemId,
        Points = (from v in Db.Votes
                  where b.ItemId == v.ItemId
                  select v.Points).Sum()
    }

I got this query, however it fails if no votes are found with exception:

    The null value cannot be assigned to a member with type System.Int32 which is a non-nullable value type.

I assume its because sum returns an int and not a nullable int, giving sum a int? as input only give the same error, probably cause sum only workes on ints.

Any good workaround for this?

Linq query with nullable sum

Is this query equivalent to a `LEFT OUTER` join?

    //assuming that I have a parameter named &#39;invoiceId&#39; of type int
    from c in SupportCases
    let invoice = c.Invoices.FirstOrDefault(i=&gt; i.Id == invoiceId)
    where (invoiceId == 0 || invoice != null)    
    select new 
    {
          Id = c.Id
          , InvoiceId = invoice == null ? 0 : invoice.Id
    }



Content Type	Original Author	Original Content on Stackoverflow
Question	Spencer	View Question on Stackoverflow
Solution 1 - .Net	Greg Beech	View Answer on Stackoverflow

How change List<T> data to IQueryable<T> data

.Net Problem Overview

.Net Solutions

Solution 1 - .Net

LINQ to read XML

XPath find if node exists

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