mef Import always null

Question

when I'm trying to import some service that gets some data from db the dataprovider import is always null ( Object reference not set to an instance of an object). I had a look into the aggregatecatalog and the Assembly which contains Service seems to be loaded correctly. A reference to dataprovider is used in Loaded event of userControl.

interface

public interface IGetSampleData
{
   IEnumerable<SampleDataValueObject> GetAllSampleData();
}

service

[Export(typeof(IGetSampleData))]
    public class SampleDataService : IGetSampleData
    {
        private ISampleData dax = new SampleDataRepository();

        public IEnumerable<SampleDataItem> GetAllSampleData()
        {
            throw new NotImplementedException();
        }
    }

ViewModel

[Export]
public class SampleDataViewModel : BaseViewModel
{
        [Import(typeof(IGetSampleData))]
        public IGetSampleData DataProvider { get; set; }

        //in Loaded event dataprovider is null
        var src = this.DataProvider.GetAllSampleData();
}

Thanks for help. Regards