How to sort properties on a class using custom property attributes

I have a custom attribute that I apply to properties on a class. This attribute is used for exporting the class's properties to a flat file.

One of the attribute's properties is FieldOrder. I need to make sure the order in which I export the properties of the class is correct. Also, not all properties on the class will have the custom attribute.

I found this article: How do I sort a generic list based on a custom attribute? This solution assumes all properties have the custom attribute, which isn't my case. I was also hoping for more elegant solution.

Your help is greatly appreciated!

public interface IFileExport{}

public class ExportAttribute: Attribute
{
    public int FieldOrder { get; set; }
    public int FieldLength { get; set; }
    public ExportAttribute() { }
}

public class ExportClass: IFileExport
{
    [ExportAttribute( FieldOrder = 2, FieldLength = 25 )]
    public string LastName { get; set; }

    [ExportAttribute( FieldOrder=1, FieldLength=25)]
    public string FirstName { get; set; }

    [ExportAttribute( FieldOrder = 3, FieldLength = 3 )]
    public int Age { get; set; }

    public ExportClass() { }
}

public class TestClass
{
    public static List<PropertyInfo> GetPropertiesSortedByFieldOrder
                                                            (IFileExport fileExport)
    {
        //get all properties on the IFileExport object
        PropertyInfo[] allProperties = fileExport
                         .GetType()
                         .GetProperties( BindingFlags.Instance | BindingFlags.Public );
        // now I need to figure out which properties have the ExportAttribute 
        //and sort them by the ExportAttribute.FieldOrder
    }
}

UPDATE: I'm ordering the properties by ExportAttribute.FieldOrder Ascending