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What is The Nuget Package For System.Web.Hosting

I have been researching but I can't seem to find any relevant answer. I will appreciate it if someone told me the nuget package to install in asp.net webapi application for System.Web.Hosting
I need it to use Hosting.HostingEnvironment.MapPath("")

Add a reference to System.Web assembly, you will then be able to access System.Web.Hosting.HostingEnvironment

Since this is in the System namespace, its very likely part of the Framework.
Googling for that namespace brings it up on MSDN. So, open up the documentation for that namespace on MSDN. Click on any class in the list, it doesn't matter which.
You will see something like this:
Namespace: System.Web.Hosting
Assembly: System.Web (in System.Web.dll)
You now know which assembly this particular class is in, which likely also contains most of the rest of the desired namespace. Check another class on MSDN to confirm. Well, this is System.Web. That's part of the Framework.
If you do this for another namespace and can't find it on MSDN, you're probably looking for a Nuget package. But in this case, nope. Just add a reference to the assembly.

System.Web is not a nugget package.
Go to Solution explorer > References > Add References > Framework > look for System.Web.
System.Web.Hosting.HostingEnvironment.MapPath(string.Empty);

For .Net Core use
Microsoft.AspNetCore.Hosting

System.Web.HttpUtility.UrlEncode c# using

In the following code:
for (int line = 0; line < CHUNKSTOBEFOUND; line++) {
nvc.Add ("search", System.Web.HttpUtility.UrlEncode (stringsToSearchFor[line]));
}
I get told namespace name HttpUtility does not exist.
http://msdn.microsoft.com/en-us/library/system.web.httputility.aspx seems to indicate I should use System.Web. I've tried using that, but I STILL get the error. Is there something else I'm supposed to include or use?

As competent_said, you are probably targeting the Client Profile in your project, in which System.Web.dll is not available. You can target the full framework in Project Properties to get it back working.
Another possible cause could be that you are not referencing System.Web library. To do so:
Right click the "Reference" in the Solution Explorer.
Choose "Add Reference"
Check the ".NET" tab is selected.
Search for, and add "System.Web".
Maybe you are just missing an using directive using System.Web; on the top of your source file.

You are most likely using the Client Profile version of the .Net framework. You need to open your project properties and change the framework version to the full version.

Why can't I reference System.ComponentModel.DataAnnotations?

I'm trying to use DataAnnotations in my WPF project to specify a maximum length of strings, with the following:
using System.ComponentModel.DataAnnotations;
However, I get the error
The type or namespace name 'DataAnnotations' does not exist in the
namespace 'System.ComponentModel' (are you missing an assembly
reference?)
I've seen other examples where DataAnnotations does exist in this namespace. I'm using C#4. Is there any reason why I can't use this? What can I do to fix it?

You have to reference the assembly in which this namespace is defined (it is not referenced by default in the visual studio templates). Open your reference manager and add a reference to the System.ComponentModel.DataAnnotations assembly (Solution explorer -> Add reference -> Select .Net tab -> select System.ComponentModel.DataAnnotations from the list)

If using .NET Core or .NET Standard
use:
Manage NuGet Packages..
instead of:
Add Reference...

To Reference System.ComponentModel.DataAnnotations
In a code file to have Using System.ComponentModel.DataAnnotations; at the top of the file such as:
using System.ComponentModel.DataAnnotations;
Add a .NET reference to your project by right clicking the project in solution explorer:
Hope this helps! This question helped me.

If you don't have it in references (like I did not) you can also add the NuGet System.ComponentModel.Annotations to get the assemblies and resolve the errors. (Adding it here as this answer still top of Google for the error)

I also had the same problem and I resolved by adding the reference in one of my projects which didn't had the mentioned reference. If you have 2-3 projects in your solution, then check by adding this reference to the other projects.

I found that I cannot reference System.ComponentModel.DataAnnotations from Silverlight 5 with the below version at (1). I found that Silverlight 5 assemblies cannot use .NET assemblies, it gives the error "You can't add a reference to System.ComponentModel.DataAnnotations as it was not built against the Silverlight runtime. ..." I plan to workaround this by hopefully installing the Silverlight 5 package found at (2) below. If this fails I will update this post.
[UPDATE: it failed. I installed everything relating to Silverlight 5 and I don't have the Silverlight version of the .dll assembly System.ComponentModel.DataAnnotations . Too bad. UPDATE II: I found an old .dll having this name from a previous installation of Silverlight developer's kit for Visual Studio 2008 or 2010. I added this file and it seems to 'work', in that IntelliSense is now recognizing attributes on class members, such as [Display(Name = "My Property Name")]. Whether or not this works for everything else in this .dll I don't know.]
(1)
Microsoft Visual Studio Professional 2013
Version 12.0.21005.1 REL
Microsoft .NET Framework
Version 4.5.51641
Installed Version: Professional
(2)
http://go.microsoft.com/fwlink/?LinkId=229318

I searched for help on this topic as I came across the same issue.
Although the following may not be the Answer to the question asked originally in 2012 it may be a solution for those who come across this thread.
A way to solve this is to check where your project is within the solution. It turns out for my instance (I was trying to install a NuGet package but it wouldn't and the listed error came up) that my project file was not included within the solution directory although showing in the solution explorer. I deleted the project from the directory out of scope and re-added the project but this time within the correct location.

Use the FrameWork version 4.5 and above for your project then problem solved.Because this namespace is under 4.5 and above.

System.ComponentModel.DataAnnotations is contained in its own assembly so you need to make sure you have it refernced. Just simply:
1). Right click on Soloution and choose add.
2). Choose reference from the list.
3). Search " System.ComponentModel.DataAnnotation " and tick the check box on its left hand side and press ok.
Job done, shouldnt have any refernce errors.

If you tried to update visual studio from vs2008 to vs2010. And your app uses framework 3.5 (and you don't want to upgrade it), and also used WCF RIA Services BETA... I have bad news... you MUST upgrade to WCF RIA Services v1 (BETA does not work on vs2010)... and due to this... you also have to install Silverlight 4 + upgrade to framework 4.0
See this:
http://blog.nappisite.com/2010/05/updating-visual-studio-2008net-35-ria.html

I upgraded from Silverlight 4 to Silverlight 5 and then I was having this issue. Although I had a reference to "System.ComponentModel.DataAnnotations" under "References" in my project, it had a yellow yield sign by it that indicated the previously referenced assembly could not be found. It turned out that the properties of the "System.ComponentModel.DataAnnotations" reference indicated "Specific Version = True", when I changed this to "Specific Version = False" it fixed the issue. Right click on the "System.ComponentModel.DataAnnotations" assembly under "References" and select "Properties" from the context menu. Check that the property value for "Specific Version = False".
It must have been referencing the old Silverlight 4 assembly which was no longer available after the upgrade to Silverlight 5.

I also have this problem.
That is very stupid when i add a namespace the same with System. I try to remove all references, but it is not resolved. I use "global::System.ComponentModel", it is working as well.
When i remove my namespace, this problem has been resolved.

For .Net Core in Visual Studio 2019 try this.
see VS suggestion
It worked for me, hope it'll work for you as well.

I was moving from .Net Framework 4.7.2 to .Net Standard 2.0.
In my case, I had to change DataAnnotations's reference from an Assembly reference to a Nuget package.

This error occurs when the reference to the "System.dll" got removed.Solution to the problem is very simple add the reference to "System.dll".The dll is normally available in the following location
"C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727" .Add the reference your problem will get solved .

There was a problem using System.ComponentModel.DataAnnotation in net40 so I just did:
#if !NET40
using System.ComponentModel.DataAnnotations;
#endif
#if !NET40
[StringLength(256)]
#endif
The NET40 must be a predefined macro definition for .Net Framework 4.0

I had same problem, I solved this problem by following way.
Right click on page, select Property. in build action select Content.
Hope that this solution may help you.

Why can't I find or use UrlEncode in Visual Studio 2010?

I have a string that I'd like to encode into the standard URL format. From what I've found, I should be able to do this via the httpUtility.urlEncode method, but I don't seem to have that available.
I've added "using" references to both System.Web and System.Net to no avail. I've also seen other references to server.urlEncode amongst other variants, but I don't see the method anywhere.
I'm using the latest version of C# in Visual Studio 2010. Is the method called something different in this version, hidden somewhere else, or am I completely off base?

By default, new projects in Visual Studio 2010 target the .NET Framework 4.0 Client Profile, which does not include the System.Web assembly.
You can change the version of the Framework that your project targets in your project's Properties. Under the "Application" tab, select ".NET Framework 4.0" from the combobox labeled "Target framework".
Then, make sure that you have added a reference to System.Web using the "Add Reference" dialog.
Finally, add a using directive to the top of your class for the System.Web namespace:
using System.Web;
You'll find the various overloads of the UrlEncode method in the HttpUtility class. Sample code:
HttpUtility.UrlEncode("http://www.google.com/");

In .Net 4.5 you can (should?, 'please use' says a Katana comment) use the System.Net.WebUtility.UrlEncode method.

It can't be named differently since Visual Studio doesn't supply the class or method names, the .NET framework does.
All I can tell you is that the System.Web.HttpUtility AND System.Web.HttpServerUtility classes contain a method called UrlEncode(string).

If your project target ".NET Framework X Client Profile",you cannot not use "System.Web",but you can use "Uri.EscapeUriString | Uri.UnEscapeUriString" instead.

Yes, adding the reference was my answer. But be sure you double check the project, that it is in, if you have more than 1 project in your solution. I had a solution with 3 projects. System.Web was added to 2 projects but not the 3rd project.
I spent an hour trying to figure out why I couldn't use HttpUtility since it was a Reference in the main project. But I didn't check the sub-projects of the Solution.
Hope it helps someone.

Because you only see AspNetHostingPermission, AspNetHostingPermissionAttribute, and AspNetHostingPermissionLevel, I strongly suspect (like the other guys) that you're missing a reference.
The best you can do is start a new project, because it's pretty complicated to add/remove references without ruining your entire project.
How to: Add or Remove References in Visual Studio (MSDN) shows how to add/remove references. In your case, you should check/add the System.Web reference.

How to use BigInteger in VS 2010

I have been trying to use the BigInteger type, that is supposedly new in .NET Framework 4.0.
I don't seem to be able to get to it, and get an error when trying to reference it via Using System.Numerics.
Any idea what I'm doing wrong? Sorry if this is a stupid question...

Add a reference to the System.Numerics assembly to your project.
a. In Solution Explorer, right-click the project node and click Add Reference.
b. In the Add Reference dialog box, select the .NET tab.
c. Select System.Numerics, and then click OK.
Add a using directive importing the System.Numerics namespace:
using System.Numerics;
Use the BigInteger structure:
var i = new BigInteger(934157136952);

Did you add a reference to System.Numerics?
Right click on References -> Add Reference -> .NET tab -> System.Numerics -> OK

Add a Reference to System.Numerics assembly.
Add using System.Numerics; statement

Have you added a project reference (Project... Add Reference...) to System.Numerics?