what 's the use of creating a metadata class and a partial class for the model class c#

The first class you show, the entity classes, are generated from the database every time you save the EDMX (or when you execute the T4 Template).

This causes the file containing public partial class MyClass under the EDMX to be regenerated. So you cannot alter it, because the next time someone refreshes a table or adds one, your changes are gone.

That's why entity classes are generated as a partial: so you can create another partial to the same class to do your modifications in.

However, if you want to annotate your entity's properties with metadata, you cannot redefine the same property in the other partial class: the same name can only be used by one member of a type. So you can't do this:

// Entity class
public partial class FooEntity
{
    public string Name { get; set;} 
}

// User-created partial class
public partial class FooEntity
{
    [Required]
    public string Name { get; set;} 
}

Because that code expresses you want two properties named Name in the FooEntity class, which is not valid.

So you'll have to come up with another way to add metadata to the type. Enter the [MetadataType] attribute. This works by creating a new class with the same properties as the class to be annotated. Here, using reflection, the metadata is resolved based on member name.

So when you create a metadata class for the above annotation:

public class FooEntityMetadata
{
    [Required]
    public string Name { get; set;} 
}

You can apply it to the user-created partial:

// User-created partial class
[MetadataType(typeof(FooEntityMetadata))]
public partial class FooEntity
{
}

And also, in the latter partial, you can add members that add functionality to the entity model. New ([NotMapped]) properties and new methods for example.