Is there an attribute to skip empty arrays in the xml-serialization of c#?

Well, you could perhaps add a ShouldSerializeFoo() method:

using System;
using System.ComponentModel;
using System.Xml.Serialization;
[Serializable]
public class MyEntity
{
    public string Key { get; set; }

    public string[] Items { get; set; }

    [EditorBrowsable(EditorBrowsableState.Never), Browsable(false)]
    public bool ShouldSerializeItems()
    {
        return Items != null && Items.Length > 0;
    }
}

static class Program
{
    static void Main()
    {
        MyEntity obj = new MyEntity { Key = "abc", Items = new string[0] };
        XmlSerializer ser = new XmlSerializer(typeof(MyEntity));
        ser.Serialize(Console.Out, obj);
    }
}

The ShouldSerialize{name} patten is recognised, and the method is called to see whether to include the property in the serialization. There is also an alternative {name}Specified pattern that allows you to also detect things when deserializing (via the setter):

[EditorBrowsable(EditorBrowsableState.Never), Browsable(false)]
[XmlIgnore]
public bool ItemsSpecified
{
    get { return Items != null && Items.Length > 0; }
    set { } // could set the default array here if we want
}