If you have installed Visual Studio 2011, after that changed back to VS 2010, and you are receiving error like:

Error   20: The "EntityHydrate" task failed unexpectedly.
System.TypeInitializationException: The type initializer for 'Microsoft.Data.Entity.Design.Model.SchemaManager' threw an exception. ---> System.MissingFieldException: Field not found: 'System.Data.Entity.Design.EntityFrameworkVersions.Version3'.
   at Microsoft.Data.Entity.Design.Model.SchemaManager..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.Data.Entity.Design.Model.SchemaManager.GetSchemaVersion(XNamespace xNamespace)
   at Microsoft.Data.Tools.Integration.Build.EntityHydrate.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask, Boolean& taskResult)   
BuildingModule

then do as it is described here, delete files in folder:

C:\Program Files (x86)\MSbuild\4.0\Microsoft.Common.Targets\ImportAfter\Microsoft.Data.Tools.Integration.targets

 

Note: Use “Program Files” without “(x86)” if you have the 32 bits version of Windows.

and then restart your computer.