Validation Metadata and Entity Framework

When using the Entity Framework it is challenging to associate attributes with the database properties defined in the context. The .Net Framework provides a solution to this problem with its MetadataType attribute. The steps to implementing this functionality is:

• Create your Entity Framework model. (In this particular example I’m working with a JobScheduler database that manages computer programs to be run on a particular schedule. Not really important for this post but it does provide a context.)
• I’m going to add validation to the “Title” property of the “Job” class. In theory we would want to add the following code:

<br />
Imports System.ComponentModel.DataAnnotations</p>
<p>Partial Public Class Job</p>
<p><Required(ErrorMessage:="Title is required")> _<br />
Public Property Title As String</p>
<p>End Class<br />

• Unfortunately the above approach generates errors no matter what you try to do. (Really!)
• Instead, what we are going to do is the following:

<br />
Imports System.ComponentModel.DataAnnotations</p>
<p><MetadataType(GetType(Job_Extensions))> _<br />
Partial Public Class Job</p>
<p>End Class<br />

• Referencing the “Job_Extensions” type will generate a “Type not found” error. So, let’s create it.
• Add the following class definition to your project:

<br />
Imports System.ComponentModel.DataAnnotations</p>
<p>Public Class Job_Extensions</p>
<p>    <Required(ErrorMessage:="Title is required")><br />
    Public Title As Object</p>
<p>End Class<br />

• Note that “Title” does not need to be defined as a property nor does its type need to match the database definition.
• In this manner metadata is added to Entity Framework controlled properties.