Could not load file or assembly 'AutoMapper' or one of its dependencies

Question

I have this problem when I deploy, I really don't know how to solve it. Any idea?

Server Error in '/' Application.
Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.BadImageFormatException: Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.

Source Error: 

An unhandled exception was generated during the execution of the current web request.    Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Assembly Load Trace: The following information can be helpful to determine why the assembly 'AutoMapper' could not be loaded.


=== Pre-bind state information ===
LOG: User = IIS APPPOOL\DefaultAppPool
LOG: DisplayName = AutoMapper
(Partial)
LOG: Appbase = file:///C:/inetpub/wwwroot/
LOG: Initial PrivatePath = C:\inetpub\wwwroot\bin
Calling assembly : (Unknown).
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: C:\inetpub\wwwroot\web.config
LOG: Using host configuration file:C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Aspnet.config
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework64\v2.0.50727\config\machine.config.
LOG: Policy not being applied to reference at this time (private, custom, partial, or location-based assembly bind).
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/e22c2559/92c7e946/AutoMapper.DLL.
LOG: Attempting download of new URL file:///C:/Windows/Microsoft.NET/Framework64/v2.0.50727/Temporary ASP.NET Files/root/e22c2559/92c7e946/AutoMapper/AutoMapper.DLL.
LOG: Attempting download of new URL file:///C:/inetpub/wwwroot/bin/AutoMapper.DLL.
ERR: Failed to complete setup of assembly (hr = 0x8013101b). Probing terminated.

Stack Trace: 


[BadImageFormatException: Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.]
 System.Reflection.Assembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, Assembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection) +0
 System.Reflection.Assembly.InternalLoad(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +416
  System.Reflection.Assembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +166
  System.Reflection.Assembly.Load(String assemblyString) +35
  System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +190

 [ConfigurationErrorsException: Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.]
 System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +11224200
     System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +388
 System.Web.Configuration.CompilationSection.LoadAssembly(AssemblyInfo ai) +232
 System.Web.Configuration.AssemblyInfo.get_AssemblyInternal() +48
 System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +210
 System.Web.Compilation.BuildProvidersCompiler..ctor(VirtualPath configPath, Boolean supportLocalization, String outputAssemblyName) +76
 System.Web.Compilation.ApplicationBuildProvider.GetGlobalAsaxBuildResult(Boolean isPrecompiledApp) +283

System.Web.Compilation.BuildManager.CompileGlobalAsax() +50 System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +676

 [HttpException (0x80004005): Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.]
  System.Web.Compilation.BuildManager.ReportTopLevelCompilationException() +76
  System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +1012
  System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager,     IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters) +1025

 [HttpException (0x80004005): Could not load file or assembly 'AutoMapper' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.]
  System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +11318198
  System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +88
  System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +4348404

Version Information: Microsoft .NET Framework Version:2.0.50727.5446; ASP.NET Version:2.0.50727.5420

Answer 1

The application pool is configured for .NET 2.0:

Version Information: Microsoft .NET Framework Version:2.0.50727.5446; ASP.NET Version:2.0.50727.5420

The deployed AutoMapper assembly is built for .NET 4.0:

"This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded."

This will not work together. You can probably fix this by switching the app pool to .NET 4.0.