Q:

DTOs. Properties or fields?

I need to create some DTO classes to transport our business objects across WCF.
Since these are just bags of data with no functionality, is there any reason I can't just use fields, or is there some good reason to expose them properly as properties?
//fields
[DataContract]
class CustomerDTO
{
    [DataMember] public int     Id;
    [DataMember] public string  Name;
}

//or properties?
[DataContract]
class CustomerDTO
{
    [DataMember] public int    Id               { get; set; }
    [DataMember] public string Name             { get; set; }
}

A:

I mostly favour immutable DTOs with read-only fields if I can get away with it:
public class CustomerDTO
{
    public CustomerDTO(int id, string name)
    {
        Id = id;
        Name = name;
    }

    public readonly int     Id;
    public readonly string  Name;

    // Override Equals and GetHashCode as well...
}

There's lots of advantages to be had from immutable records, such as structural equality, which makes automated test assertions much simpler to write. It also dispenses with the need to write and maintain separate Test Data Builders.
It depends on the serializer, though. JSON.NET can handle immutable records, but many other serializers can't.
For those that handle public fields, I prefer fields over properties, simply because it's more honest; automatically implemented read/write properties provide no encapsulation.
Some serializers insist on public properties, and don't serialize fields. If that's the scenario, you have to go with that.
Honestly, considering how much thought I've put into this, it's not really something that keeps me awake at night, because ultimately, at the boundaries, applications aren't object-oriented. Thus, the rules of OOD don't really apply to DTOs anyway.

A:

Since these are just bags of data with no functionality, is there any reason I can't just use fields

There are no strong arguments against public fields here. But do realize that it is only because there is no logic (behaviour) inside the DTOs so that the normal argument of encapsulation doesn't hold. 
I would still prefer properties but they're not really necessary here. 

