Develop Reference

How to use System.Xml.Linq in Unity C# Script?

working with unity and c# and when I try these line
using System.Xml.Linq;
I get this error
The type or namespace name Linq' does not exist in the namespace
System.Xml'. Are you missing an assembly reference?
So I found online, through a lot of searching, that Linq can cause problems, and you need to manually add a reference in MonoDevelop. So I added a reference there (by right clicking References in the solutions pane, clicking Edit References and browsing for System.Xml.Linq)
Still no luck,
Any ideas?

I think this SO post will answer your question...
https://stackoverflow.com/a/875704/1246574
It's pretty much the same issue you're having.

You can't modify the monodevelop (or visual studio) solution or project manually. Because they are generated by Unity. Even if you modify it, Unity can overwrite your changes when it regenerates the solution files.
The reason you can't use some namespaces is the .net/mono version selected in your Unity project settings.
Search for "API Compatibility Level" in this documantation http://docs.unity3d.com/Documentation/Manual/class-PlayerSettings.html
C'mon people, Stop modifying the generated code files. This is 2013!

from this post, I found that Unity doesn't support .NET 3.5 by default (which is required to use Linq). In the settings Xtro talked about, you can only choose .NET 2.0. Instead, you need to drag the desired .dll file into Unity like a texture. Kinda hackish, but it works. Once again, like Xtro said, there is NO REASON to edit the MonoDevelop settings. They have no affect on the final compilation.
EDIT: See Xtro's answer for an alternate solution. He was able to make Linq work by changing the API Compatibility Level to .NET 2.0 (not Subset).
EDIT 2: I just updated from Unity 4.1 to 4.2 today and got an error that Linq was defined twice. by deleting the .dll, I solved the error. Note I'm still using .NET 2.0 Subset. Looks like they added Linq support by default.

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.

Referencing OpenWebKitSharp to use WebKit in C#

I am trying to reference OpenWebKitSharp, the C# library for WebKit. However, I keep getting an error on compiling: "The type or namespace WebKitBrowser does not exist in the namespace WebKit".
I am trying to follow the directions at http://code.google.com/p/open-webkit-sharp/ but I do not know what this means:
Copy the contents of the cairo build to your debug/release folder.
What is a cairo build? Why do I keep getting this error even though I am referencing both WebKit.Interop and OpenWebKitSharp?
On a similar note, is there a good site for documentation about the OpenWebKitSharp library?

If you are using .Net Framework Client Profile, change it to .Net Framework 4

The type or namespace name could not be found [duplicate]

This question already has answers here:
Getting "type or namespace name could not be found" but everything seems ok?
(44 answers)
Closed 8 years ago.
I have a C# solution with several projects in Visual Studio 2010.
One is a test project (I'll call it "PrjTest"), the other is a Windows Forms Application project (I'll call it "PrjForm"). There is also a third project referenced by PrjForm, which it is able to reference and use successfully.
PrjForm references PrjTest, and PrjForm has a class with a using statement:
using PrjTest;
Reference has been correctly added
using statement is correctly in place
Spelling is correct
PrjTest builds successfully
PrjForm almost builds, but breaks on the using PrjTest; line with the error:
The type or namespace name 'PrjTest' could not be found (are you missing a using directive or an assembly reference?)
I've tried the following to resolve this:
Removed Resharper (since Resharper had no trouble recognizing the referenced project, I thought it might be worth a shot)
Removed and re-added the reference and using statement
Recreated PrjForm from scratch
PrjForm currently resides inside the PrjTest folder, I tried moving it to an outside folder
Loaded the solution on a different computer with a fresh copy of VS 2010
I have done my homework and spent far too long looking for an answer online, none of the solutions has helped yet.
What else could I try?

See this question.
Turns out this was a client profiling issue.
PrjForm was set to ".Net Framework 4 Client Profile"
I changed it to ".Net Framework 4", and now I have a successful build.
Thanks everyone!
I guess it figures that after all that time spent searching online, I find the solution minutes after posting, I guess the trick is knowing the right question to ask..

In my case I had:
Referenced DLL : .NET 4.5
Project : .NET 4.0
Because of the above mismatch, the 4.0 project couldn't see inside the namespace of the 4.5 .DLL. I recompiled the .DLL to target .NET 4.0 and I was fine.

PrjForm was set to ".Net Framework 4 Client Profile" I changed it to ".Net Framework 4", and now I have a successful build.
This worked for me too. Thanks a lot. I was trying an RDF example for dotNet where in I downloaded kit from dotnetrdf.
NET4 Client Profile:
Always target NET4 Client Profile for all your client desktop applications (including Windows Forms and WPF apps).
NET4 Full framework:
Target NET4 Full only if the features or assemblies that your app need are not included in the Client Profile. This includes:
If you are building Server apps, Such as:
ASP.Net apps
Server-side ASMX based web services
If you use legacy client scenarios, Such as:
o Use System.Data.OracleClient.dll which is deprecated in NET4 and not included in the Client Profile.
Use legacy Windows Workflow
Foundation 3.0 or 3.5 (WF3.0 , WF3.5)
If you targeting developer scenarios and need tool such as MSBuild or need access to design assemblies such as System.Design.dll

Another thing that can cause this error is having NuGet packages that have been built with a newer version of .NET.
The original error:
frmTestPlanSelector.cs(11,7): error CS0246: The type or namespace name 'DatabaseManager'
could not be found (are you missing a using directive or an assembly reference?)
Further up in the log I found this:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3275: The primary reference "[redacted]\DatabaseManager\bin\Release\DatabaseManager.dll" could not be resolved because it has an indirect dependency on the assembly "System.Data.SQLite, Version=1.0.94.0, Culture=neutral, PublicKeyToken=db937bc2d44ff139" which was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".
The solution was to re-install the NuGet packages:
http://docs.nuget.org/docs/workflows/reinstalling-packages

I solved mine because the other project was coded with .NET 4.5 and the other one was coded 4.0

The using statement refers to a namespace, not a project.
Make sure that you have the appropriately named namespace in your referenced project:
namespace PrjTest
{
public class Foo
{
// etc...
}
}
Read more about namespaces on MSDN:
Using Namespaces

I encountered this issue it turned out to be.
Project B references Project A.
Project A compiled as A.dll (assembly name = A).
Project B compiled as A.dll (assembly name A).
Visual Studio 2010 wasn't catching this. Resharper was okay, but wouldn't compile. WinForms designer gave misleading error message saying likely resulting from incompatbile platform targets.
The solution, after a painful day, was to make sure assemblies don't have same name.

It is also possible, that the referenced projects targets .NET 4.0, while the Console App Project targets .NET 4.0 Client Library.
While it might not have been related to this particular case, I think someone else can find this information useful.

The compiled dll should have public Class.

I had the same issue. The target frameworks were fine for me. Still it was not working.
I installed VS2010 sp1, and did a "Rebuild" on the PrjTest. Then it started working for me.

Other problem that might be causing such behavior are build configurations.
I had two projects with configurations set to be built to specific folders.
Like Debug and Any CPU and in second it was Debug and x86.
What I did I went to Solution->Context menu->Properties->Configuration properties->Configuration and I set all my projects to use same configurations Debug and x86 and also checked Build tick mark.
Then projects started to build correctly and were able to see namespaces.

Changing the framework to
.NET Framework 4 Client Profile
did the job for me.

For COM/ActiveX references, VS 2012 will show this error right on using statement. Which is quite funny, since it's saying that may be you are missing a using statement.
To solve this: register the actual COM/ActiveX dll even if it's in the neighbor project, and add a reference through COM channel, not project channel. It will add Interop.ProjectName instead of ProjectName as a reference and this solves this strange bug.

If your project (PrjTest) does not expose any public types within the PrjTest namespace, it will cause that error.
Does the project (PrjTest) include any classes or types in the "PrjTest" namespace which are public?

just changed Application's target framework to ".Net Framework 4".
And error got Disappeared.
good luck;
:D

check your Project Properties, your Reference Paths should be empty like this:
Regards

Namespace in Referenced Project Present in Autocomplete Before Building, but Causes Compile Error After Building

I have a class library project which uses a namespace (e.g., "Cosmos.Creator.Util"). I then create a solution and windows forms application to test the library. From the windows form application, I add a reference to the library. So now I have two projects open in visual studio, a class library and a windows forms project. The forms project references the library.
When I edit my form's code, code autocompletion works correctly for the namespace that I use in the library. E.g., if I type "using Cosmos." I get autocomplete options like "Creator". But now if I build my solution, all of the "Cosmos" are red-underlined with the compile error: "The type or namespace name "Cosmos" could not be found (are you missing a using directive or an assembly reference?)".
For the purposes of the form application test, I placed my library code into a folder CosmosFormExample\Cosmos. When I check the reference from the form application, the reference is to CosmosFormExample\Cosmos\bin\Debug\Cosmos.dll, so that looks okay. I looked at the GUID referenced in the solution file and it matches the GUID of the project file Cosmos.csproj.
What has happened? How has the build caused my forms application to forget about the Cosmos namespace, despite the fact that it is still referencing the library project? Thanks much in advance.

Are you using VS2010 & .NET 4? If so you're probably using .NET 4 Client Profile instead of full fledged .NET 4. Go to project properties and check your Target Framework.
for more info: http://msdn.microsoft.com/en-us/library/cc656912.aspx

you need to check the framework you are using and the framework yout library was compiled for...