The type or namespace name 'System' could not be found

Question

I have the following errors (and more) in all my views (*.cshtml) when opening my project in Visual Studio 2015 Professional.

Error CS0246 The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?) Severity Code Description Project File Line

Error CS0518 Predefined type 'System.Object' is not defined or imported Severity Code Description Project File Line

Error CS0518 Predefined type 'System.String' is not defined or imported

I got these references:

<ItemGroup>
<Reference Include="Microsoft.CSharp" />
<Reference Include="Sitecore.Kernel, Version=7.0.0.0, Culture=neutral, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\Sitecore.Kernel.dll</HintPath>
</Reference>
<Reference Include="Sitecore.Mvc">
  <HintPath>N:\XXX\Sitecore.Mvc.dll</HintPath>
</Reference>
<Reference Include="System.Web.DynamicData" />
<Reference Include="System.Web.Entity" />
<Reference Include="System.Web.ApplicationServices" />
<Reference Include="System.ComponentModel.DataAnnotations" />
<Reference Include="System" />
<Reference Include="System.Data" />
<Reference Include="System.Core" />
<Reference Include="System.Data.DataSetExtensions" />
<Reference Include="System.Web.Extensions" />
<Reference Include="System.Web.Helpers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.Helpers.dll</HintPath>
</Reference>
<Reference Include="System.Web.Mvc, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.Mvc.dll</HintPath>
</Reference>
<Reference Include="System.Web.Providers">
  <HintPath>N:\XXX\System.Web.Providers.dll</HintPath>
</Reference>
<Reference Include="System.Web.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.Razor.dll</HintPath>
</Reference>
<Reference Include="System.Web.WebPages, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.WebPages.dll</HintPath>
</Reference>
<Reference Include="System.Web.WebPages.Deployment, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.WebPages.Deployment.dll</HintPath>
</Reference>
<Reference Include="System.Web.WebPages.Razor, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL">
  <SpecificVersion>False</SpecificVersion>
  <HintPath>N:\XXX\System.Web.WebPages.Razor.dll</HintPath>
</Reference>
<Reference Include="System.Xml.Linq" />
<Reference Include="System.Drawing" />
<Reference Include="System.Web" />
<Reference Include="System.Xml" />
<Reference Include="System.Configuration" />
<Reference Include="System.Web.Services" />
<Reference Include="System.EnterpriseServices" />

I found some solutions saying that a mscorlib wasn't referenced.. but when I tried to add it it sayd that it's already there (even though I don't see it on the csproj file).

System.dll reference:

SOLUTION:

Managed the issue.. I had the build path set to a "c:...\bin" instead of "\bin". I believe this happened because I don't have any web.config in the root of the project, so VS didn't identify my project as a web project.

Answer 1

I attempted to re-create your issue and came up with a similar error when the solution was created in visual studio 2013 and then tried building it in in vs 2015.

I was able to get a successful build once I reinstalled NuGet Package Manager (and closed, then reopened VS 2015).

References / Credit

There are several SO questions relating to build issues via with earlier version of NPM for VS 2015 (i.e. I'm just passing along what I've tried and worked out). Recurring resolution is usually update / reinstall NPM or change execution policy in power shell. I tend to like the update + restart approach to avoid tinkering with the black boxes in windows. one source: https://stackoverflow.com/a/32251092/1158842 There may also be an issue of MSBuild Integrated solutions, in which case migrating away from the NuGet resources in the solution could do the trick.

Source: https://stackoverflow.com/a/31811461/1158842

Answer 2

I had the same problem earlier when I tried to edit an open source project from the internet.

Solved it by just Cleaning the solution and rebuilding it.

Answer 3

Cleaning solution worked for me.

I would also advise to close and relaunch Visual Studio once doing so.

Answer 4

Follow these steps :

1. right click on Solution > Restore NuGet packages
2. right click on Solution > Clean Solution
3. right click on Solution > Build Solution
4. Close Visual Studio and re-open.
5. Rebuild solution. If these steps don't initially resolve your issue try repeating the steps a second time.

Thats All.

Answer 5

If cleaning the solution didn't work and for anybody who see's this question and tried moving a project or renaming.

Open Package manager console and type "dotnet restore".

Answer 6

Unload project -> Reload project normally fixes this weird, Nuget related, error to me.

Answer 7

right click you project name then open the properties windows. downgrade your Target framework version, build Solution then upgrade your Target framework version to the latest, build Solution .

Answer 8

Open Project References, I had lots of yellow warning triangles on the various packages. Click Add Reference, just doing that seems to get VS to refresh it references, and the yellow triangles disappeared and the errors from the code also.

You might have to do this in a few projects if your solution reference to other projects

Answer 9

For the folks that like me got here because they're trying to host aspnet.core mvc in a console application: The ONLY way I was able to solve this was by converting the .csproj to the new format and add the Sdk property to the Project tag on the very first line.

<Project Sdk="Microsoft.NET.Sdk.Razor">

Answer 10

i had to upgrade from visual studio 2017 to 2019. no other proposed solution here helped me, i installed and configured, rebooted every one here plus many other options from ms site.

my root cause was probably due to the fact the git repository developer used 2019 and i tried to clone the original git repo into ma 2017.

Answer 11

You can check the framework on which system was created and developed, and then changes the framework of your current solution to the old framework. You can do this by right click on the solution and then choose properties. In Application section you can change the framework.

Answer 12

I've tried all answers above. For me works only removal and adding the reference again described in the following steps:

1. Open 'References' under the project.
2. Right click on 'System' reference.
3. Click on 'Remove'.
4. Right click on 'References'.
5. Click 'Add Reference...'.
6. From right menu choose an 'Assemblies',
7. In a search field type 'System'.
8. Choose 'System' from the list.
9. Click 'Add' button.
10. IMPORTANT: Restart the Visual Studio.

Answer 13

Try to

1. Clean Solution
2. Rebuild Solution

Answer 14

For me it only happened in one project and I tried everything I found online and nothing seemed to help. I was ready to delete and recreate the project when after messing through the projects properties changing the .NET framework target from 4.7.2 to 4.8 fixed the issue. I changed it back to 4.7.2 later and the error is gone.

Hopefully this helps other users as well.

Answer 15

Check the Target framework. changed target framework from .NET framework 4.6.1 to .NET framework 4.6 has worked for me.

Answer 16

I had to delete the bin and obj folders from each folder in order to resolve this.

Answer 17

I encountered this message while trying to extract part of a solution to set it up as a stand-alone solution.

I found that all the <HintPath> references in my .csproj files were now pointing to the wrong place. In my case they were originally:

<Reference Include="System.IO.FileSystem.Primitives, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\..\packages\System.IO.FileSystem.Primitives.4.0.1\lib\net46\System.IO.FileSystem.Primitives.dll</HintPath> 
</Reference>

And to make the compile problems go away, they just needed to be fixed up as:

<Reference Include="System.IO.FileSystem.Primitives, Version=4.0.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, processorArchitecture=MSIL">
      <HintPath>..\packages\System.IO.FileSystem.Primitives.4.0.1\lib\net46\System.IO.FileSystem.Primitives.dll</HintPath>
</Reference>

Answer 18

Using VS Professional 2019, I was trying to run a downloaded solution from a Udemy class on selenium automation testing, and most of the projects had errors in the project references sections. I tried cleaning, rebuilding, closing VS. Then, in VS, when I right clicked on the solution in Solution Explorer and chose Manage Nuget Packages for Solution, there were 2 available updates: for MSTest.TestAdapter and MSTest.TestFramework, and when I installed those, the error messages on the references for all the projects went away, for the references to those, as well as for the references to System and System.Core.