我可以在分部类中定义属性，然后用另一个分部类中的属性标记它们吗？

Not as such; the compiler will complain that the member is defined in multiple parts. However, as the use of custom attributes is reflective in nature, you could define a "metadata" class and use it to contain decorators.

public class A
{
public string MyString;
}


public class AMeta
{
[TheAttribute("etc")]
public object MyString;
}


...


var myA = new A();
var metaType = Type.GetType(myA.GetType().Name + "Meta");
var attributesOfMyString = metaType.GetMember("MyString").GetCustomAttributes();

I've seen something like this done in an article by Scott Guthrie (near the end of it) - didn't try it myself, though.
http://weblogs.asp.net/scottgu/archive/2010/01/15/asp-net-mvc-2-model-validation.aspx

[MetadataType(typeof(Person_Validation))]
public partial class Person
{
// Partial class compiled with code produced by VS designer
}


[Bind(Exclude="ID")]
public class Person_Validation
{
[Required(ErrorMessage = "First Name Required")]
[StringLength(50, ErrorMessage = "Must be under 50 characters")]
public string FirstName { get; set; }


[Required(ErrorMessage = "Last Name Required")]
[StringLength(50, ErrorMessage = "Must be under 50 characters")]
public string LastName { get; set; }


[Required(ErrorMessage = "Age Required")]
[Range(0, 120, ErrorMessage = "Age must be between 0 and 120")]
public int Age { get; set; }


[Required(ErrorMessage = "Email Required")]
[Email(ErrorMessage = "Not a valid email")]
public string Email { get; set; }
}

Here is the solution I have been using for such cases. It is useful when you have auto-generated classes that you want to decorate with attributes. Let's say this is the auto-generated class:

public partial class UserProfile
{
public int UserId { get; set; }
public string UserName { get; set; }
public string Firstname { get; set; }
public string Lastname { get; set; }
}

And let's say, I would like to add an attribute to specify that UserId is the key. I would then create a partial class in another file like this:

[Table("UserProfile")]
[MetadataType(typeof(UserProfileMetadata))]
public partial class UserProfile
{
internal sealed class UserProfileMetadata
{
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int UserId { get; set; }
}
}

This is my answer
different class files or you can combine the metadatas in a same file but keep the namespace the same..so they can see each other obviously.

keep in mind when you update your model like add more columns you have to update the project class too.

--your model class
public partial class A {
public string a {get; set;}
}


--your project class
public class Ametadata {
[Attribute("etc")]
public string a {get; set;}
}




[MetadataType(typeof(Ametadata))]
public partial class A
{
}

You need to define a partial class for your A class just like below example

using System.ComponentModel.DataAnnotations;


// your auto-generated partial class
public partial class A
{
public string MyProp { get; set; }
}


[MetadataType(typeof(AMetaData))]
public partial class A
{


}


public class AMetaData
{
[System.ComponentModel.DefaultValue(0)]
public string MyProp { get; set; }
}