c# - TFS build custom parameter always null -

i created custom process parameter , use dialog enter values in guid of queue new build work fine when run build value of parameter null or if define default value default.

i read because

process parameters aren’t restricted simple types (such boolean, string, , integer); can complex types (such enumerations, arrays, , custom classes) can further comprised of simple types , other complex types. process parameters stored against build definition or build serializing them xaml, requirement complex type supports

how can "serializing them xaml"???

my parameter this:

public class imversion  {     public imversion()     {         version = new version(svbuildactivity.findlatest(svbuildactivity.im_releases_path));     }     public imversion(version v)     {         version = v;     }     version version { get; set; }      public override string tostring()     {         return version.tostring();     }       }