How to force Lazy<T> object to create/re-create value? - asp.net-mvc

Question

I try to Lazy< SelectList > for lazy caching any lookup data in my ASP.NET MVC project. But I cannot force Lazy object to reload lookup data when it is changed.
I create derived class like the following code. I found that Lazy< T > use IsValueCreated property to keep current state. However, in MappingFunc method I cannot change value of IsValueCreated because it is static method.
public class LazySelectList : Lazy<SelectList>
{
public LazySelectList(Func<LimeEntities, IEnumerable> initFn, string dataValueField, string dataTextField)
: base(MapingFunc(initFn, dataValueField, dataTextField))
{
}
public new bool IsValueCreated { get; set; }
public static Func<SelectList> MapingFunc(Func<DbContext, IEnumerable> valueFactory, string dataValueField, string dataTextField)
{
return () =>
{
var context = ObjectFactory.GetInstance<DbContext>();
return new SelectList(valueFactory(context), dataValueField, dataTextField);
};
}
}
I use the below code the call this function. But it always creates new value because IsValueCreated value is always false.
LookupCache.DocTypeList = new LazySelectList(db => db.DocTypes.OrderBy(x => x.Name), "ID", "Name");

Answer 1

After a several hours for searching & testing, I think it is impossible to reset state of lazy object. But I can create wrapper for handling this problem. The wrapper class contains lazy object and necessary object for creating new lazy object. The code should be like this.
public class LazyCache<TSource, TModel> : IValue<TModel>
{
private Lazy<TModel> _lazyObj;
private readonly Func<TSource, TModel> _valueFactory;
protected LazyCache()
{
Reset();
}
public LazyCache(Func<TSource, TModel> valueFactory) : this()
{
_valueFactory = valueFactory;
}
public void Reset()
{
_lazyObj = new Lazy<TModel>(MapingFunc());
}
public TModel Value
{
get { return _lazyObj.Value; }
}
protected virtual Func<TModel> MapingFunc()
{
return () =>
{
var context = ObjectFactory.GetInstance<TSource>();
return _valueFactory(context);
};
}
}
The above code allows us to reset state of object to force it to retrieve new data for defined function.
After that, I try to use the above method to cache SelectList object in ASP.NET MVC. But it always retrieves new from database because SelectList will contain IEnumerable object instead of real object data. So, I solve problem by enumerating data into temp object list like the following class.
public class LazyList<TSource> : LazyCache<TSource, SelectList>
{
private readonly Func<TSource, IEnumerable> _valueFactory;
private readonly string _dataValueField;
private readonly string _dataTextField;
public LazyList(Func<TSource, IEnumerable> valueFactory, string dataValueField, string dataTextField)
{
_valueFactory = valueFactory;
_dataValueField = dataValueField;
_dataTextField = dataTextField;
}
protected override Func<SelectList> MapingFunc()
{
return () =>
{
var context = ObjectFactory.GetInstance<TSource>();
// Force to retrieve data from current IEnumerable to prevent lazy loading data that
// cause system always connect to database every time they generate data from selectlist.
var loop = _valueFactory(context).GetEnumerator();
var tempList = new List<object>();
while (loop.MoveNext())
{
tempList.Add(loop.Current);
}
return new SelectList(tempList, _dataValueField, _dataTextField);
};
}
}
PS. All source code are a part of my Higgs RIA framework that available on Codeplex website.
LazyCache.cs | LazyList.cs