无法加载指定的元数据资源Unable to load the specified metadata resource

在使用ADO.NET Entity Framework构建Application时出现如下异常。Solution 中有2个项目，一个为Windows Form项目，另一个为Class Library 项目，包含ADO.NET Entity Data Model。Windows Form 项目增加对Class Library 项目的引用。

异常信息：

System.Data.MetadataException was unhandled

Message="Unable to load the specified metadata resource."

Source="System.Data.Entity"

StackTrace:

       at System.Data.Metadata.Edm.MetadataArtifactLoaderCompositeResource.LoadResources(String assemblyName, String resourceName, ICollection`1 uriRegistry, MetadataArtifactAssemblyResolver resolver)

       at System.Data.Metadata.Edm.MetadataArtifactLoaderCompositeResource.CreateResourceLoader(String path, ExtensionCheck extensionCheck, String validExtension, ICollection`1 uriRegistry, MetadataArtifactAssemblyResolver resolver)

       at System.Data.Metadata.Edm.MetadataArtifactLoader.Create(String path, ExtensionCheck extensionCheck, String validExtension, ICollection`1 uriRegistry, MetadataArtifactAssemblyResolver resolver)

       at System.Data.EntityClient.EntityConnection.SplitPaths(String paths)

       at System.Data.EntityClient.EntityConnection.GetMetadataWorkspace(Boolean initializeAllCollections)

       at System.Data.EntityClient.EntityConnection.InitializeMetadata(DbConnection newConnection, DbConnection originalConnection, Boolean closeOriginalConnectionOnFailure)

       at System.Data.EntityClient.EntityConnection.Open()

在Windows Form 项目中出现异常的代码：

            string customerID = txtCustomerID.Text.Trim();

            // Contains a reference to an Entity Data Model (EDM) and a data source connection.

            using (EntityConnection cn = new EntityConnection("Name=NorthwindEntities"))

            {

                cn.Open();

                EntityCommand cmd = cn.CreateCommand();

                cmd.CommandText =

                     "SELECT VALUE c FROM NorthwindEntities.Customers "+

                     "AS c WHERE c.CustomerID = @customerID";

                cmd.Parameters.AddWithValue("customerID", customerID);

                DbDataReader rdr = cmd.ExecuteReader(CommandBehavior.SequentialAccess);

                while (rdr.Read())

                    Console.WriteLine(rdr["CompanyName"].ToString());

                rdr.Close();

            }

上述代码行 cn.Open(); 抛出异常Exception - Unable to load the specified metadata resource。经检查，问题出现在App.config 配置文件（该配置文件在使用ADO.NET Entity Data Model向导时自动添加），

ADO.NET Entity Data Model向导自动添加的Connection String：

<configuration>

<connectionStrings>

    <add name="NorthwindEntities" connectionString="metadata=res://*/NorthwindDB.csdl|res://*/NorthwindDB.ssdl|res://*/NorthwindDB.msl; provider=System.Data.SqlClient;provider connection string=&quot;Data Source=localhost;Initial Catalog=Northwind;Integrated Security=True;MultipleActiveResultSets=True&quot;"providerName="System.Data.EntityClient" />

</connectionStrings>

</configuration>

需要将上面connectionStrings 配置节中的* 修改为Class Library 项目的Assembly Name。修改后的示例App.config 如下：

<configuration>

<connectionStrings>

    <add name="NorthwindEntities" connectionString=" metadata= res://NorthwindEDM/NorthwindModel.csdl|res://NorthwindEDM/NorthwindModel.ssdl|res://NorthwindEDM/NorthwindModel.msl; provider=System.Data.SqlClient;provider connection string=&quot;Data Source=localhost;Initial Catalog=Northwind;Integrated Security=True;MultipleActiveResultSets=True&quot;"providerName="System.Data.EntityClient" />

</connectionStrings>

</configuration

    The problem sometimes occurs with the Entity Framework where it gives an array of errors when doing simple operations. For example, I received an error about not being able to “unable to load the specified metadata resource”. Uh, the connection string information is right in the config file! The problem often can be resolved simply by:

1. open the EDMX file in the designer mode 
2. go to the properties of the EDMX 
3. set the Metadata Artifact Processing property to “Copy to Output” 
4. build the project 
5. 然后从输出目录将*.msl.*.ssdl,*.csdl文件拷贝到应用程序目录底下即可。呵呵（估计很多人这里出错！）