An Expression Tree is a data structure that contains Expressions, which is basically code. So it is a tree structure that represents a calculation you may make in code. These pieces of code can then be executed by &quot;running&quot; the expression tree over a set of data.

A great thing about expression trees is that you can build them up in code; that is, you build executable code (or a sequence of steps) *in code*. You can also modify the code before you execute it by replacing expressions by other expressions.

An Expression is then a function delegate, such as `(int x =&gt; return x * x)`.

See also http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx

Is it possible to make an ajax request inside another ajax request?
because I need some data from first ajax request to make the next ajax request.

First I&#39;m using Google Maps API to get LAT &amp; LNG, after that I use that LAT &amp; LNG to request Instagram API (search based location).

Once again, is this possible, and if so how?

    $(&#39;input#search&#39;).click(function(e) {
		e.preventDefault();
		var source = $(&#39;select[name=state] option:selected&#39;).text()+&#39; &#39;+$(&#39;select[name=city] option:selected&#39;).text()+&#39; &#39;+$(&#39;select[name=area] option:selected&#39;).text();
		var source = source.replace(/ /g, &#39;+&#39;);
		if(working == false) {
			working = true;
			$(this).replaceWith(&#39;&lt;span id=&quot;big_loading&quot;&gt;&lt;/span&gt;&#39;);
			$.ajax({
				type:&#39;POST&#39;,
				url:&#39;/killtime_local/ajax/location/maps.json&#39;,
				dataType:&#39;json&#39;,
				cache: false,
				data:&#39;via=ajax&amp;address=&#39;+source,
				success:function(results) {
				// this is where i get the latlng
				}
			});
		} else {
			alert(&#39;please, be patient!&#39;);
		}
	});

jQuery Ajax Request inside Ajax Request

I can hack this myself, but I think bootstrap has this capability.

How do I make bootstrap table rows clickable?

I just came across the concept of expression trees which I have heard multiple times. I just want to understand what is meant by an expression tree and its purpose. 

I would love it if someone could also direct me to simple explanations and samples of use.

What are Expression Trees, how do you use them, and why would you use them?

<p>I just came across the concept of expression trees which I have heard multiple times. I just want to understand what is meant by an expression tree and its purpose.</p>
<p>I would love it if someone could also direct me to simple explanations and samples of use.</p>


I have simple entity:


    public class Hall
    {
        [Key]
        public int Id {get; set;}
    
        public string Name [get; set;}
    }

Then in the `Seed` method I use `AddOrUpdate` to populate table:


    var hall1 = new Hall { Name = &quot;French&quot; };
    var hall2 = new Hall { Name = &quot;German&quot; };
    var hall3 = new Hall { Name = &quot;Japanese&quot; };
    
    context.Halls.AddOrUpdate(
        h =&gt; h.Name,
        hall1,
        hall2,
        hall3
    );


Then I run in the Package Management Console:


    Add-Migration Current
    Update-Database


It&#39;s all fine: I have three rows in the table &quot;Hall&quot;. But if I run in the Package Management Console `Update-Database` again I have already five rows:


    Id	Name
    1	French
    2	Japaneese
    3	German
    4	French
    5	Japanese


Why? I think it is should be three rows again, not five. I tried to use `Id` property instead of `Name` but it does not make the difference.





**UPDATE:**

This code produces the same result:


    var hall1 = new Hall { Id = 1, Name = &quot;French&quot; };
    var hall2 = new Hall { Id = 2, Name = &quot;German&quot; };
    var hall3 = new Hall { Id = 3, Name = &quot;Japanese&quot; };
    
    context.Halls.AddOrUpdate(
                    h =&gt; h.Id,
                    hall1);
    
    context.Halls.AddOrUpdate(
                    h =&gt; h.Id,
                    hall2);
    
    context.Halls.AddOrUpdate(
                    h =&gt; h.Id,
                    hall3);


Also I have the latest EntityFramework installed via nuget.

Entity Framework Code First AddOrUpdate method insert Duplicate values

I am quite interested in Reactive Extensions but I cannot find an up to date tutorial. I started with [Curing the asynchronous blues with the Reactive Extensions for .NET][1]  but it is out of date. I can figure out some changes but I could not get many examples working.

I found many articles on the web, mainly from 2009,2010 but they are also incompatible with the current release. I am especially interested in using Rx with Windows Phone and WebClient:

    WebClient wc = new WebClient();
 
    var o = Observable.FromEvent&lt;DownloadStringCompletedEventArgs&gt;(wc, &quot;DownloadStringCompleted&quot;).Select(newString =&gt; newString.EventArgs.Result);
 
    // Subscribe to the observable, and set the label text
    o.Subscribe(s =&gt; myLabel.Text = s);

    // Start the download
    wc.DownloadStringAsync(new Uri(&quot;http://www.data.com/service&quot;));

Do not work anymore and replacing `FromEvent` with `FromEventPattern` is not enough.

Can somebody point me to an up to date resource?


  [1]: http://go.microsoft.com/fwlink/?LinkId=208528

Reactive Extension (Rx) tutorial that is up to date

I have an ASP.NET MVC 3 application. This application requests records through jQuery. jQuery calls back to a controller action that returns results in JSON format. I have not been able to prove this, but I&#39;m concerned that my data may be getting cached. 

I only want the caching to be applied to specific actions, not for all actions.

Is there an attribute that I can put on an action to ensure that the data does not get cached? If not, how do I ensure that the browser gets a new set of records each time, instead of a cached set?

Prevent Caching in ASP.NET MVC for specific actions using an attribute

I&#39;m having a hard time with the XSD files.

I&#39;m trying to create an XSD file from a class:

    public enum Levels { Easy, Medium, Hard }
    public sealed class Configuration
    {
        public string Name { get;set; }
        public Levels Level { get; set; }
        public ConfigurationSpec { get;set;}
    }

    public abstract class ConfigurationSpec { }
    public class ConfigurationSpec1
    {
        // ...
    }
    public class ConfigurationSpec2
    {
        // ...
    }

Please note that I have an abstract class inside of Configuration. With that feature, is it possible to create the XSD and if it&#39;s possible how?

The idea is to pass the class Configuration to the XSD.

How to create a XSD schema from a class?

I have a list stored in resultlist as follows:

    var resultlist = results.ToList();

It looks something like this:

&lt;!-- language: none --&gt;

    ID FirstName  LastName
    -- ---------  --------
    1  Bill       Smith
    2  John       Wilson
    3  Doug       Berg

How do I remove ID 2 from the list?


How to remove item from list in C#?

What are the differences between `ConcurrentQueue` and `BlockingCollection` in .Net?

Why `BlockingCollection` is best for producer-consumer operation when it can be done through `ConcurrentQueue`? Do I have to improve anything in the following code?

    MessageSlotMachineGameStartOrAndStatusUpdate msg;

    while (!aCancellationToken.IsCancellationRequested)
    {
        try
        {
            this.isStillConsumingMsg = true;
            Boolean takeResult = this.msgQueue.TryTake(out msg, this.msgConsumeTimeOut, aCancellationToken);
            if (takeResult)
            {
                if (msg != null)
                {
                    this.ProcessMessage(msg);
                }
            }
            else
            {
                break;
            }
        }
        catch (OperationCanceledException err)
        {
            EngineManager.AddExceptionLog(err, &quot;Signal Operation Canceled&quot;);
        }
        catch (Exception err)
        {
            EngineManager.AddExceptionLog(err, &quot;Signal exception&quot;);
        }
        finally
        {
            this.isStillConsumingMsg = false;
        }
    }

What are the differences between ConcurrentQueue and BlockingCollection in .Net?

I want to parse a string that represent a DateTime in UTC format.

My string representation includes the Zulu time specification which should indicate that the string represent a UTC time.

    var myDate = DateTime.Parse(&quot;2012-09-30T23:00:00.0000000Z&quot;);    
  
From the above I would expect myDate.Kind to be DateTimeKind.Utc, instead it is DatetimeKind.Local.

What am I doing wrong and how to Parse a string that represents a UTC time?

Many thanks!

DateTime.Parse(&quot;2012-09-30T23:00:00.0000000Z&quot;) always converts to DateTimeKind.Local

I have this linq query:

    private void GetReceivedInvoiceTasks(User user, List&lt;Task&gt; tasks)
    {
        var areaIds = user.Areas.Select(x =&gt; x.AreaId).ToArray();
    
        var taskList = from i in _db.Invoices
                       join a in _db.Areas on i.AreaId equals a.AreaId
                       where i.Status == InvoiceStatuses.Received &amp;&amp; areaIds.Contains(a.AreaId)
                       select new Task {
                           LinkText = string.Format(Invoice {0} has been received from {1}, i.InvoiceNumber, i.Organisation.Name),
                           Link = Views.Edit
                       };
    }

It has issues though. I&#39;m trying to create tasks. For each new task when I set the link text to a constant string like &quot;Hello&quot; it is fine. However above I&#39;m trying to build the property linktext using properties of the invoice.

I get this error:

&gt; base {System.SystemException} = {&quot;LINQ to Entities does not recognize the method &#39;System.String Format(System.String, System.Object, System.Object)&#39; method, and this method cannot be translated into a store expression.&quot;}

Anyone know why? Anyone know an alternative way of doing this to make it work?

LINQ to Entities does not recognize the method &#39;System.String Format(System.String, System.Object, System.Object)&#39;

Any ideas on why this could be breaking?

    foreach (var p in pp)
    {
        ProjectFiles projectFile = (ProjectFiles)p;
        projectFile.Status = Constants.ProjectFiles_ERROR;
        projectFile.DateLastUpdated = DateTime.Now;
        context.SaveChanges();
    }

I read that the workaround the issue, is to retrieve the results in one go before the foreach loop.   

But didnt I do that? &quot;pp&quot; is the collection of results in my case

New transaction is not allowed because there are other threads running in the session LINQ To Entity

[`.ToLookup&lt;TSource, TKey&gt;`](https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tolookup) returns an [`ILookup&lt;TKey, TSource&gt;`](https://docs.microsoft.com/en-us/dotnet/api/system.linq.ilookup-2). `ILookup&lt;TKey, TSource&gt;` also implements interface `IEnumerable&lt;IGrouping&lt;TKey, TSource&gt;&gt;`.

[`.GroupBy&lt;TSource, TKey&gt;`](https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.groupby) returns an `IEnumerable&lt;IGrouping&lt;Tkey, TSource&gt;&gt;`.

ILookup has the handy indexer property, so it can be used in a dictionary-like (or lookup-like) manner, whereas GroupBy can&#39;t. GroupBy without the indexer is a pain to work with; pretty much the only way you can then reference the return object is by looping through it (or using another LINQ-extension method). In other words, any case that GroupBy works, ToLookup will work as well. 

All this leaves me with the question why would I ever bother with GroupBy? Why should it exist?

Why are ToLookup and GroupBy different?

In C#, say I have a class called `Note` with three string member variables.

```C#
public class Note
{
    public string Title;
    public string Author;
    public string Text;
}
```

And I have a list of type `Note`:

```C#
List&lt;Note&gt; Notes = new List&lt;Note&gt;();
```

What would be the cleanest way to get a list of all distinct values in the Author column?  

I could iterate through the list and add all values that aren&#39;t duplicates to another list of strings, but this seems dirty and inefficient.  I have a feeling there&#39;s some magical Linq construction that&#39;ll do this in one line, but I haven&#39;t been able to come up with anything.

Get a list of distinct values in List

Table:
    
    id     userid  friendid   name    status
    1      1        2         venkat  false
    2      1        3         sai     true
    3      1        4         arun    false
    4      1        5         arjun   false

If a user sends `userid=1,friendids=2,4,5 status=true`

How would I write the query to update the above? All `friendids` status is true. [2,3,4 at a time]?

how to update the multiple rows at a time using linq to sql?

I&#39;ve been taking a look to [Roslyn CTP][1] and, while it solves a similar problem to the [Expression tree API][2], both are immutable but Roslyn does so in a quite different way: 

* `Expression` nodes have no reference to the parent node, are modified using a `ExpressionVisitor` and that&#39;s why big parts can be reused. 

* Roslyn&#39;s `SyntaxNode`, on the other side, has a reference to its parent, so all the nodes effectively become a block that&#39;s impossible to re-use. Methods like `Update`, `ReplaceNode`, etc, are provided to make modifications. 

Where does this end? `Document`? `Project`? `ISolution`? The API promotes a step-by-step change of the tree (instead of a button up), but does each step makes a full copy?

Why they did they make such a choice? Is there some interesting trick I&#39;m missing? 


  [1]: http://msdn.microsoft.com/en-us/roslyn
  [2]: http://msdn.microsoft.com/en-us/library/bb397951.aspx

Are Roslyn SyntaxNodes reused?

    IQueryable&lt;Organization&gt; query = context.Organizations;

    Func&lt;Reservation, bool&gt; predicate = r =&gt; !r.IsDeleted;

    query.Select(o =&gt; new { 
        Reservations = o.Reservations.Where(predicate)
    }).ToList();

this query throws &quot;Internal .NET Framework Data Provider error 1025&quot; exception but the query below does not.

    query.Select(o =&gt; new { 
        Reservations = o.Reservations.Where( r =&gt; !r.IsDeleted)
    }).ToList();

I need to use the first one because I need to check a few if statements for constructing the right predicate. I know that I can not use if statements in this circumstance that is why I pass a delegate as parameter.

How can I make the first query work?

Internal .NET Framework Data Provider error 1025

I am trying to create a lambda expression for a nested property at run-time from the name of the propert. Basically I am trying to create the lambda expression specified by:

    var expression = CreateExpression&lt;Foo, object&gt;(foo =&gt; foo.myBar.name);

    private static Expression CreateExpression&lt;TEntity, TReturn&gt;(Expression&lt;Func&lt;TEntity, TReturn&gt;&gt; expression)
    {
        return (expression as Expression);
    }

With the classes:

    class Foo
    {
        public Bar myBar { get; set; }
    }
    class Bar
    {
        public string name { get; set; }
    }

However all I am given is the type of `Foo` and the string `&quot;myBar.name&quot;`

If it were a normal property such as just needing the value `&quot;myBar&quot;` then I could use the following:


    private static LambdaExpression GetPropertyAccessLambda(Type type, string propertyName)
    {
        ParameterExpression odataItParameter = Expression.Parameter(type, &quot;$it&quot;);
        MemberExpression propertyAccess = Expression.Property(odataItParameter, propertyName);
        return Expression.Lambda(propertyAccess, odataItParameter);
    }

However this code does not work for nested properties and I&#39;m not sure how to create the LambdaExpression to do the work of `foo.myBar.name`.

I think it&#39;s going to be something like this:

    GetExpression(Expression.Call(GetExpression(Foo, &quot;myBar&quot;), &quot;name&quot;))

But I can&#39;t seem to work out how to get it all working, or if there&#39;s a better way to do this at run-time.

Construct LambdaExpression for nested property from string

I have the following generic extension method:

&lt;!-- language: c# --&gt;

    public static T GetById&lt;T&gt;(this IQueryable&lt;T&gt; collection, Guid id) 
        where T : IEntity
    {
        Expression&lt;Func&lt;T, bool&gt;&gt; predicate = e =&gt; e.Id == id;

        T entity;
            
        // Allow reporting more descriptive error messages.
        try
        {
            entity = collection.SingleOrDefault(predicate);
        }
        catch (Exception ex)
        {
            throw new InvalidOperationException(string.Format(
                &quot;There was an error retrieving an {0} with id {1}. {2}&quot;,
                typeof(T).Name, id, ex.Message), ex);
        }

        if (entity == null)
        {
            throw new KeyNotFoundException(string.Format(
                &quot;{0} with id {1} was not found.&quot;,
                typeof(T).Name, id));
        }

        return entity;
    }

Unfortunately Entity Framework doesn&#39;t know how to handle the `predicate` since C# converted the predicate to the following:

&lt;!-- language: c# --&gt;

    e =&gt; ((IEntity)e).Id == id

Entity Framework throws the following exception:

&gt; Unable to cast the type &#39;IEntity&#39; to type &#39;SomeEntity&#39;. LINQ to
&gt; Entities only supports casting EDM primitive or enumeration types.

How can we make Entity Framework work with our `IEntity` interface?

LINQ to Entities only supports casting EDM primitive or enumeration types with IEntity interface

I try to write a simple example using Expressions, but have a strange bug: I can&#39;t use `PropertyExpression` at compile time. When I write it I get an error and it doesn&#39;t compile 
![enter image description here][1]
But in runtime at breakpoint I can write 

    var runtimeBody = (System.Linq.Expressions.PropertyExpression)body2;

and it works, and casts as expected. Framework 4.5.1 is selected as target. 


  [1]: http://i.stack.imgur.com/AqvG8.png

Content Type	Original Author	Original Content on Stackoverflow
Question	Donald N. Mafa	View Question on Stackoverflow
Solution 1 - C#	Roy Dictus	View Answer on Stackoverflow

What are Expression Trees, how do you use them, and why would you use them?

C# Problem Overview

C# Solutions

Solution 1 - C#

How do I make bootstrap table rows clickable?

jQuery Ajax Request inside Ajax Request

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