无法使用泛型隐式转换类型'System.Collections.ObjectModel.ObservableCollection ...'

Question

我的抽象泛型类遇到了一些问题：

public abstract class ViewModel<TPrimaryModel> : ViewModel
    where TPrimaryModel : TwoNames, new()
{
    private ObservableCollection<ViewModel<TPrimaryModel>> _searchResults = new ObservableCollection<ViewModel<TPrimaryModel>>();

    public ObservableCollection<ViewModel<TPrimaryModel>> SearchResults
    {
        get { return _searchResults; }
        set
        {
            if (_searchResults != value)
            {
                _searchResults = value;
                RaisePropertyChanged("SearchResults");
            }
        }
    }
}

我的具体课程是：

public class RecipientViewModel : ViewModel<Recipient>
{
    protected override void Find()
    {
        try
        {
            SelectedSearchResult = null;
            //Following row leads to an error
            SearchResults = new ObservableCollection<RecipientViewModel>();
            ...
        }
    }
}

此行会导致错误：

SearchResults = new ObservableCollection<RecipientViewModel>(); 

编译器消息是：

Cannot implicitly convert type 
'System.Collections.ObjectModel.ObservableCollection<...RecipientViewModel>' in 
'System.Collections.ObjectModel.ObservableCollection<...ViewModel<...Recipient>>'

收件人是一个模型，继承自TwoNames。