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Unable to load System.Threading.Tasks.Extensions

I have a web project build on .net framework 4.5.1. We are trying to added PostgreSQL support for the project. Using Nuget, I have installed 4.0.4 npgsql to the project. Under references, I see the following being added to the project.
Npgsql - 4.0.4.0 - Runtime version v4.0.30319
System.Threading.Tasks.Extensions - 4.2.0.0 - Runtime version v4.0.30319
When I tried run the project and connect and get the data from the database, I am getting the following error saying FileNotFoundException:
System.TypeInitializationException
HResult=0x80131534
Message=The type initializer for 'com.rsol.RConfig' threw an exception.
Source=RConfig
StackTrace:
at com.rsol.RConfig.getInstance() in C:\Workspaces\PS\RConfig\RConfig.cs:line 1113
at RAdmin.Global.Application_Start(Object sender, EventArgs e) in C:\Workspaces\PS\RAdmin\Global.asax.cs:line 528
Inner Exception 1:
TypeInitializationException: The type initializer for 'com.rsol.Db.DbMgr' threw an exception.
Inner Exception 2:
FileNotFoundException: Could not load file or assembly 'System.Threading.Tasks.Extensions, Version=4.2.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.
Inner Exception 3:
FileNotFoundException: Could not load file or assembly 'System.Threading.Tasks.Extensions, Version=4.2.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The system cannot find the file specified.
System.Threading.Tasks.Extensions which is installed using Nuget is not getting loaded to the project. When I checked the properties of System.Threading.Tasks.Extensions reference, the dll file exists in the location. I have also tried installing System.Threading.Tasks.Extensions.dll file to assembly using gacutil. I am still getting the same error.
Please let me know if you need any additional information.
Any help is really appreciated.

In my case, I got the issue after upgrading to version 4.5.4 and tried #user2713341 answer. It didn't work but put me in the right direction.
My project had no bindings for this library, so I added the binding and it worked
<dependentAssembly>
<assemblyIdentity name="System.Threading.Tasks.Extensions" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-4.2.0.1" newVersion="4.2.0.1" />
</dependentAssembly>
and it worked.
Note that it should be 4.2.0.1 even though the version is 4.5.4.

Update Nuget Package
https://www.nuget.org/packages/System.Threading.Tasks.Extensions/
will solve your problem

Update Nuget Package is not working for me.
What works?
In app.config need to change
<bindingRedirect oldVersion="0.0.0.0-4.2.0.1" newVersion="4.2.0.1" />
To
<bindingRedirect oldVersion="0.0.0.0-4.5.4" newVersion="4.5.4" />
for current version 4.7.2 should work.
Microsoft like ;)

The response from #Keyjote was at the root of the solution for me, but rather than cherry-picking the assemblies, I was able to just reinstall. This seemed to automatically repair the app.config file.
Tools -> Nuget Packet Manager -> Packet Manager Console
Update-Package -reinstall -Project <your project name>
This way you don't need to mess with the syntax or have to figure out the PublicKeyToken values.
If you want to do it for the whole solution, you can omit the -Project <> parameter.

I got the error in a different context when trying to migrate using Entity Framework Core (EFC) Version 3.1.8 using the Package Manager Console. The project built successfully.
Trying the binding redirects as suggested in this thread and (re)installing different versions of System.Threading.Tasks.Extensions as well as EFC (as suggested also elsewhere) did not work for me.
I managed to solve the problem by
deleting the packages/ directory in my project's root directory
using manual NuGet Package Restore (link to Microsoft Docs) via Visual Studio

problem lies in *.csproj file. having wrong reference for System.Threading.Tasks.Extensions.4.5.4 because this dll not loading.
after referring correct framework version folder from lib its start working
old Reference:
<Reference Include="System.Threading.Tasks.Extensions, Version=4.2.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
<HintPath>..\packages\System.Threading.Tasks.Extensions.4.5.2\lib\netstandard2.0\System.Threading.Tasks.Extensions.dll</HintPath>
</Reference>
corrected one:
<Reference Include="System.Threading.Tasks.Extensions, Version=4.2.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51, processorArchitecture=MSIL">
<HintPath>..\packages\System.Threading.Tasks.Extensions.4.5.4\lib\net472\System.Threading.Tasks.Extensions.dll</HintPath>
</Reference>
please clean the nuget cache first, then run test case it will work

I have been struggling a lot with bindingRedirect.
I finally found a real gotcha that solved my problems.
I have a WCF server project in .net 4.8, depending on projects in .net standard 2.0.
I was updating Nuget packages in the referred projects.
I got al sorts of problems, I won't try to mention them here.
I had bindingRedirects in web.config. I had to rediscover that those were unnecessary, and maybe even conflicting, because of automatic generation of those into the dll.config. There are various aspects to make that work, see elsewhere.
But the real gotcha for me was that for IIS, I had to LINK web.config to the dll.config. The latter being the complete config-file, with all the bindingRedirects, which turned out to be working after all.

Try to download the package and add reference to your project explicitly . should work , I just resolved it .

I got a similar error message - but for a different reason. In packages.config set by NUGET manager There was a ref for the new version - but in project reference there was a ref for an old version. The solution - delete the ref in project reference

How to reference a strongly-named DLL at compile time, or for ASP.NET view compilation?

I have an ASP.NET 4.7.2 Framework project. At runtime, upon loading a view, it says System.Runtime version 4.0.0.0 cannot be found. We have written about this extensively here: ASP.NET: .NET Framework/Standard/Core DLL collisions, caused by Nuget. "You must add a reference to assembly System.Runtime..."
It's been over a week and we still don't have a solution. All development has been halted.
Yesterday we tried side-by-side loading by simply calling on application start up:
Assembly systemRuntime = Assembly.Load("System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
Upon start up. We don't do anything with systemRuntime. It's only there for us to confirm that indeed 4.0.0.0 was loaded. Unfortunately, that didn't fix anything.
This makes me think that maybe since views are compiled at run-time, maybe somehow they are not referencing the right System.Runtime, or that we need to do the side-by-side loading there.
This problem is so severe we've started porting the site to .NET Core, because no one has any ideas how to solve it. According to analyzer, we'll have to rewrite approximately 15% of the site. We cannot go without deploying to production for potentially weeks, so a temporary solution must be found as soon as physically possible. We already have fixes that need to go to up.

Dai, in the first comment, directed me in the right direction: the view engine doesn't necessarily use the same references as the main project.
After some Googling, I found a way to reference assemblies for the view engine. In web.config:
<configuration>
...
<system.web>
<compilation debug="true" targetFramework="4.7.2" batch="false">
<assemblies>
<add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</assemblies>
</compilation>
</system.web>
...
</configuration>
I also followed Dai's recommendation to try precompiling views. This is done by editing the MVC project's .csproj file with:
<MvcBuildViews>true</MvcBuildViews>
Precompiling the views is not necessary, but did prove it was indeed views compiling differently than the main project, and made it a tiny bit faster to test because I didn't have to run the site to see it blow up.

loading assembly GAC_MSIL

I have the problem in my project ,I must load the assembly .
my problem is "impossible de charger l'assembly nom_dll" I am working with c# and framework 4 in my project I have v4.0
I use gacutil / if nom_dll to installe my dll in GAC ,it is install in path C:\Windows\Microsoft.NET\assembly\GAC_MSIL

Since it is for an ASP.NET application there are 2 options:
The new dll is in your bin directory but in your web.config you should not include the version, culture and PublicKeyToken details in your reference.
or
Your new dll is in the GAC (C:\Windows\Assembly) and your web.config should contain at least a valid PublicKeyToken.
<compilation targetFramework="4.0">
<assemblies>
<add assembly="SomeNS.SomeDllName, Version=1.0.0.0, Culture=neutral, PublicKeyToken=568C4712810GD043"/>

I have been taught that there are essentially two GACS:
One at "C:\Windows\assembly", which is for .Net Framework 3.x and lower, and
One at "C:\Windows\Microsoft.NET\assembly\", which is for .Net Framework 4.x and above.
We are going through this right now, with some discussion as to which GAC to use. Our folder structure under "C:\Windows\assembly" only has a "download" folder, which is empty.
The folder structure under "C:\Windows\Microsoft.NET\assembly\" is:
"C:\Windows\Microsoft.NET\assembly\GAC_32"
"C:\Windows\Microsoft.NET\assembly\GAC_64"
"C:\Windows\Microsoft.NET\assembly\GAC_MSIL"
Our latest .DLLs are in the "C:\Windows\Microsoft.NET\assembly\GAC_MSIL".
I hope this helps.
Randy

Intellisense in Asp.Net Mvc (Razor) View not working

In Asp.Net MVC, I am not getting options Goto View and Add View if I right on Controller. And in View, intelligence is not working for c# code.
Most importantly I am getting a window and message as below
The web project requires missing web components to run with Visual Studio. Would you like to download and install them using the Web Platform Installer now?
(Note: Some components might require restart to take effect.)
Asp.Net web oages with Razor syntax 3.0.0.0
Please help me to solve this. Thanks in advance

Install Microsoft Asp.net and web tools
https://visualstudiogallery.msdn.microsoft.com/c94a02e9-f2e9-4bad-a952-a63a967e3935

You may need to check the web.config in you views folder and Compare System.Web.WebPages.Razor version with Runtime dependentAssembly in the project web.config.
View web.config
type="System.Web.WebPages.Razor.Configuration.HostSection, System.Web.WebPages.Razor, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false"
Project Web.Config
assemblyIdentity name="System.Web.WebPages" bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0"
Once they are consistent, the errors should be gone. I hope this helps.

HttpRequestMessage could not be found, yet System.Net.Http is referenced

I have a new .Net 4.5.2 (checked that this is the target framework) Web API Application (in VS 2015). I have referenced System.Net.Http, and as you can see the version is 4.0.0.0:
In my Filter/Attribute I am trying to reference HttpRequestMessage which is in the System.Net.Http namespace (I have the correct using at the top of my file), yet am getting the message that
The type 'HttpRequestMessage' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
And although I can't reference that class, I can browse to it in the Object Exploerer (I assume this uses the .xml file of the same name next to the .dll).
I have tried (edit: as well as the usual restarting VS and rebuilds etc):
Removing and re-adding the reference (from both the Framework and
Extensions - although I'm certain it's meant to be the Framework version, I was getting desparate)
Installing from Nuget and referencing the version in my
local packages folder, which seemed to just default back to the system-wide version.
Reinstalling all my dependencies in Nuget PM with: update-package -reinstall -ignoreDependencies
Adding the assembly manually to the web.config Adding the assembly manually to the .csproj file
[Edit: Solution:
OK, I created a few new VS Web API projects (and solutions) to see if I could reproduce the problem. In the end I created a new "Filters" folder, created a new Filter class in there, copied the contents of my pared-down original over, and lo-and-behold HttpRequestMessage was recognised. Did the same thing going back the other way (my filter was originally created in an App_Code folder) and it was still working. I think perhaps there was an unseen reference to an old System.Net.Http that was throwing a spanner in the works and even though I may have commented out the code that called it, somehow it was being remembered. That's just a guess. But anyway, paring it down to barest bones and going from there was the final approach that led to a solution.]

In my case, willing to actually use dynamically compiled files inside the App_Code, I had to add explicitly the assembly in the compilation node of the Web.Config:
<system.web>
<compilation targetFramework="4.8">
<assemblies>
<add assembly="System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
</assemblies>
</compilation>
...
This solved my case. Of course, adjust the target framework according to your project needs.

I also managed to resolve this by moving my custom RouteConstraints out of the App_Code folder. Just added a new folder to my project and copied them there.

I've tried this highly suggested solution (on Web.Config)
<system.web>
<compilation debug="true" targetFramework="4.5" />
<compilation debug="true" targetFramework="4.0">
<assemblies>
<add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
</assemblies>
</compilation>
</system.web>
But unfortunately, this didn't work me. At last, I've found a solution (this worked for me)
Right-click the References folder > Add Reference...
Expand Assemblies on the left side of the window and select Framework.
Scroll to and select System.Net.Http in the list of assemblies.
Make sure the box next to System.Net.Http is checked, then click OK.
Rebuild the project.

If you're coming from creating web sites, but are now creating a web application, don't add the App_Code folder or any code into it as each class file will either be set to "Content" (click on the class, look in the Properties pane under "Build Action") and so won't be picked up by Intellisense in the rest of the application, or you can set to "Compile" but then that has its own problems at release time: See Vishel Joshi's blog all about it.

I am having the same problem, (which is how I stumbled on this question) and there is one thing that I noticed that nobody seems to have latched onto yet.
I am building a WebApi2 application, and I added in a class library to turn into an nunit test library. I started getting conflicts with System.Web.Http. It appears that the webapi2 uses 5.2.6.0, where other things will try to default to 4.0.0.0.
Make sure that you are using the correct version of System.Web.Http, and that you also have the System.Net.Http assembly referenced. Once both assemblies were referenced and running the same version numbers across all solutions, all problems were resolved. VS doesn't always load the correct versions by itself.