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I am getting this error.

Compilation Error Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS1704: An assembly with the same simple name 'MyMVCAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null has already been imported. Try removing one of the references or sign them to enable side-by-side.

My solution has 2 ASP.Net MVC projects - splitting the main content and the areas. The main project doesn't have a reference to the 2nd one. Instead, it gets all MVC DLL's in the folder and stores it into the IOC container, which takes care of registering all the controllers.

I get the error when I set the output of my 2nd ASP.Net MVC project into the bin folder of the main one. (I've set all my views in the Areas solution to Copy Always.)

I also tried post-build action of copying the DLL into the main bin instead of setting the output, but the results are the same.

What should I do?

In addition, I have the following setup:

  1. It is set to use the Local IIS Server instead of the VS Dev Server.
  2. In the Output window, I see that the local web server is loading all the DLL's from the GAC and Temporary ASP.Net files folder. The problem I see is that it is also trying to load my DLL file from my Solutions folder.

Any ideas why it is trying to get that sole DLL from the Solution too? It doesn't have that behaviour for the other projects.

UPDATE:

Here's a strange behaviour:

[1] I clear all copies of the offending assembly.

[2] I removed the post-build action that copies that assembly to my main MVC project bin folder.

[3] I run the main project. It works without any issues, but of course the missing assembly does not get loaded.

[4] I manually copy that assembly to the mian MVC project bin folder.

[5] I run the solution. It is getting the error again!

Can someone explain why it is trying to load this very same assembly from two different places?

Ok this may be a clue to the problem: Because I am copying the assembly to the main MVC project's bin folder, it is also getting copied into the Temporary ASP.NEt folder. At the same time, because the project is not being referenced by any other project in the solution, it is also copying the same assembly into IIS. How can I prevent this from happening? If I exclude the step of copying the assembly into the main MVC bin, it does not get loaded into IIS at all. Why is it behaving that way?

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    I have updated my post with more details. Plz check "UPDATE" section. Commented Jan 31, 2013 at 9:46

2 Answers 2

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This error points out that two references have the same assembly identity because the assemblies in question lack strong names, they were not signed, and thus the compiler has no way of distinguishing between them in metadata. Thus, the run time ignores the version and culture assembly name properties. The user should remove the redundant reference, rename one of the references, or provide a strong name for them.

Can explain that like below.

Below sample creates an assembly and saves it to the root directory.

// CS1704_a.cs
// compile with: /target:library /out:c:\\cs1704.dll
public class A {}

Below sample creates an assembly with the same name as the previous sample, but saves it to a different location.

// CS1704_b.cs
// compile with: /target:library /out:cs1704.dll
public class A {}

Below sample attempts to reference both assemblies.The following sample generates CS1704.

// CS1704_c.cs
// compile with: /target:library /r:A2=cs1704.dll /r:A1=c:\\cs1704.dll
// CS1704 expected
extern alias A1;
extern alias A2;

UPDATE

If both assemblies are strong-named (signed), the CLR will always load from the GAC.

You can follow below mentioned steps :

  1. Determines the correct assembly version by examining applicable configuration files, including the application configuration file, publisher policy file, and machine configuration file. If the configuration file is located on a remote machine, the runtime must locate and download the application configuration file first.

  2. Checks whether the assembly name has been bound to before and, if so, uses the previously loaded assembly. If a previous request to load the assembly failed, the request fails immediately without attempting to load the assembly.

  3. Checks the global assembly cache. If the assembly is found there, the runtime uses this assembly.

  4. Probes for the assembly (for more info check below mentioned article )

for more information check How the Runtime Locates Assemblies

I hope this will help to you.

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10 Comments

I came across this explanation from a Microsoft site. What I'm trying to figure out is why is it trying to insert the same assembly twice. I don't have any class or solution that creates the same DLL in different locations then copied over.
@JonasArcangel why don't you give a strong name for one assembly (for Assembly which cause a error) ?
Yup, just tried it. As expected, it gave an error about duplicate references. An assembly with the same identity 'MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=2031ed7ef42c08b6' has already been imported. Try removing one of the duplicate references.
In the debug output window in Visual Studio, I see that it is loading the assembly from "'iisexpress.exe' (Managed (v4.0.30319)): Loaded 'C:\WINDOWS\Microsoft.Net\assembly\GAC_MSIL\...", and then I see it again, this time loading it from my solutions file. Why is it loading the same assembly twice, and from different locations? I'm not even referring to the project at all from other projects. It's on its own.
@JonasArcangel yes.you did very good investigation.Then try to remove your assembly from GAC.try that also.
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I had a similar problem playing around with a test container project having added the wrong reference. I removed it and found that after a solution clean and a rebuild the issue was two DLL's in the bin directory clashing. Having removed them and rebuilt everything was fine.

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