Lambda expression

Question

How can i make out

Expression<Func<T, bool>> predicate 

to

 Expression<Func<SomeType, bool>> predicate 

?

So far, no way to find a way. Or at least create a new Expression<Func<SomeType, bool>> using the first string representation of the predicate.

If this helps, T limited to the implementation type of ISomeInterface , and SomeType implements it.

LE: further clarification

The interface looks something like this:

 public interface ICacheable { List<T> GetObjects<T>(Expression<Func<T, bool>> predicate) where T : ICacheable; } 

then you have

 public partial class Video : ICacheable { public List<T> GetObjects<T>(Expression<Func<T, bool>> predicate) where T : ICacheable { // implementation here that returns the actual List<Video> // but when I try to query the dbcontext I can't pass a predicate with type T, I have to cast it somehow List<Video> videos = db.Videos.Where(predicate).ToList(); // not working } } 

then you have:

 public class RedisCache { public List<T> GetList<T>(Expression<Func<T, bool>> predicate) where T : ICacheable { List<T> objList = // get objects from cache store here if(objList == null) { List<T> objList = GetObjects<T>(predicate); // cache the result next } return objList; } } 

I use the above from any class as follows:

 // If the list is not found, the cache store automatically retrieves // and caches the data based on the methods enforced by the interface // The overall structure and logic has more to it. List<Video> videos = redisCache.GetList<Video>(v => v.Title.Contains("some text")); List<Image> images = redisCache.GetList<Image>(v => v.Title.Contains("another text")); 

And I would extend this to any type of object that I need to cache, with methods that allow the Cache repository to automatically get an object or a list of objects if they are not found in the cache. I could do it completely wrong though.

Answer 1

I can't scratch my Entity Framework, but I know that the DatabaseContext inside LINQ has a GetTable<T> , which returns a table based on a common one. If the " GetTable equivalent for ObjectContext " is something like that, is it also available in EF?

To make your statement truly universal, you can try the following:

 public MyBaseObject<T> { public List<T> GetObjects<T>(Expression<Func<T, bool>> predicate) where T : ICacheable { return db.CreateObjectSet<T>().Where(predicate).ToList(); } } public partial class Image : MyBaseObject<Image>, ICacheable { } public partial class Video : MyBaseObject<Video>, ICacheable { } 

Answer 2

Here is something (extremely) basic that I hope can help you with the generic caching process.

 // ICacheable interface is used as a flag for cacheable classes public interface ICacheable { } // Videos and Images are ICacheable public class Video : ICacheable { public String Title { get; set; } } public class Image : ICacheable { public String Title { get; set; } } // CacheStore will keep all objects loaded for a class, // as well as the hashcodes of the predicates used to load these objects public class CacheStore<T> where T : ICacheable { static List<T> loadedObjects = new List<T>(); static List<int> loadedPredicatesHashCodes = new List<int>(); public static List<T> GetObjects(Expression<Func<T, bool>> predicate) { if (loadedPredicatesHashCodes.Contains(predicate.GetHashCode<T>())) // objects corresponding to this predicate are in the cache, filter all cached objects with predicate return loadedObjects.Where(predicate.Compile()).ToList(); else return null; } // Store objects in the cache, as well as the predicates used to load them public static void StoreObjects(List<T> objects, Expression<Func<T, bool>> predicate) { var hashCode = predicate.GetHashCode<T>(); if (!loadedPredicatesHashCodes.Contains(hashCode)) { loadedPredicatesHashCodes.Add(hashCode); loadedObjects = loadedObjects.Union(objects).ToList(); } } } // DbLoader for objets of a given class public class DbStore<T> where T : ICacheable { public static List<T> GetDbObjects(Expression<Func<T, bool>> predicate) { return new List<T>(); // in real life, load objects from Db, with predicate } } // your redis cache public class RedisCache { public static List<T> GetList<T>(Expression<Func<T, bool>> predicate) where T:ICacheable { // try to load from cache var objList = CacheStore<T>.GetObjects(predicate); if(objList == null) { // cache does not contains objects, load from db objList = DbStore<T>.GetDbObjects(predicate); // store in cache CacheStore<T>.StoreObjects(objList,predicate); } return objList; } } // example of using cache public class useRedisCache { List<Video> videos = RedisCache.GetList<Video>(v => v.Title.Contains("some text")); List<Image> images = RedisCache.GetList<Image>(i => i.Title.Contains("another text")); } // utility for serializing a predicate and get a hashcode (might be useless, depending on .Equals result on two equivalent predicates) public static class PredicateSerializer { public static int GetHashCode<T>(this Expression<Func<T, bool>> predicate) where T : ICacheable { var serializer = new XmlSerializer(typeof(Expression<Func<T, bool>>)); var strw = new StringWriter(); var sw = XmlWriter.Create(strw); serializer.Serialize(sw, predicate); sw.Close(); return strw.ToString().GetHashCode(); } }