Develop Reference

Expose a Collection But Exclude Parts

In a parent class, I have a collection. In a child class, I want to expose a part of the parent class collection. I want changes from either location to be affect the other.
My real life situation is I am creating a part of an application that will record a database design. I have a ConstraintList collection inside of a Database class. The ConstraintList contains a Constraint class for each constraint in the database. I also have a TablesList collection in the Database class, that contains Table classes. In the Table class I have a ForeignKeyConstraintList where I want to expose the constraints from the parent (Database class) ConstraintList that are foreign key constraints for this Table class.
+-Database Class
|
+--ConstraintList <-----------
| |
+--TableList Same List
| |
+-Table Class |
| |
+-ForeignKeyConstraintList
I have tried using an existing List class from the primary collection and using Linq to filter it to another List collection. However this doesn't work because this makes two List classes. If an entry is removed from the one List it still exists in the other List.
I thought about having the ForeignKeyConstraintList property of the Table class pull directly from the ConstraintList property of the Database class each time it is called but the act of filtering it causes it to create a new List class and thus any entries removed from ForeignKeyConstraintList would not be removed from the ConstraintList.
Another option I came up with so far is creating a new class that implements the same interfaces as List but doesn't subclass from it. Then using a private field to store a reference to the primary List class. Then writing custom Add and Remove methods that sync any changes back to the ConstraintList. I would also need to create a custom implementation of the IEnemerable and IEnumerable to skip items that don't meet the filter criteria.
In a parent class, I have a collection. In a child class, I want to expose a part of the parent class collection. I want changes from either location to be affect the other.

I decided to write a custom List type class to accomplish this. I will post the code below. I haven't tested yet but I figured this would be a good start for anyone else who wants to do the same thing.
hmmmm, seems the class is too large to fit in here. I will just post the key parts and skip the public methods, which just implement the various interfaces.
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
namespace CodeWriter.Collections.Generic
{
/// <summary>
/// This represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort and manipulate the list.
/// This class serves as a wrapper for a <see cref="List{T}"/>. The internal class can be reached by the <see cref="SourceList"/> property.
/// The elements that this class exposes from the <see cref="SourceList"/> can be controlled by changing the <see cref="Filter"/> property.
/// </summary>
/// <typeparam name="T">The type of elements in the list.</typeparam>
/// <remarks>
/// This class was created to support situations where the functionality of two or more <see cref="List{T}"/> collections are needed where one is the Master Collection
/// and the others are Partial Collections. The Master Collection is a <see cref="List{T}"/> and exposes all elements in the collection. The Partial Collections
/// are <see cref="FilteredList{T}"/> classes (this class) and only expose the elements chosen by the <see cref="FilteredList{T}"/> property of this class. When elements are modified,
/// in either type of collection, the changes show up in the other collections because in the backend they are the same list. When elements are added or deleted from the Partial Collections,
/// they will disappear from the Master Collection. When elements are deleted from the Master Collection, they will not be available in the Partial Collection but it
/// may not be apparent because the <see cref="Filter"/> property may not be exposing them.
/// </remarks>
public class FilteredList<T> : IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>
{
#region Public Constructor
public FilteredList(List<T> SourceList)
{
if (SourceList == null)
{
throw new ArgumentNullException("SourceList");
}
_SourceList = SourceList;
}
public FilteredList()
{
_SourceList = new List<T>();
}
public FilteredList(IEnumerable<T> Collection)
{
if (Collection == null)
{
throw new ArgumentNullException("Collection");
}
_SourceList = new List<T>(Collection);
}
#endregion
#region Protected Members
protected List<T> _SourceList;
protected Func<T, bool> _Filter;
#endregion
#region Public Properties
#region Source List Properties
/// <summary>
/// Gets or sets the base class that this class is a wrapper around.
/// </summary>
public List<T> SourceList
{
get
{
return _SourceList;
}
set
{
_SourceList = value;
}
}
/// <summary>
/// Gets or sets the value used to filter the <see cref="SourceList"/>.
/// </summary>
public Func<T, bool> Filter
{
get
{
return _Filter;
}
set
{
_Filter = value;
}
}
#endregion
#region Normal List<T> Implementation
/// <summary>
/// Provides access to the collection the in the same manner as an <see cref="Array"/>.
/// </summary>
/// <param name="Index">The Index of the element you want to retrieve. Valid values are from zero to the value in the <see cref="Count"/> property.</param>
/// <returns>The element at the position provided with the indexer.</returns>
public T this[int Index]
{
get
{
List<T> Selected = _SourceList.Where(_Filter).ToList();
return Selected[Index];
}
set
{
List<T> Selected = _SourceList.Where(_Filter).ToList();
Selected[Index] = value;
}
}
/// <summary>
/// Provides access to the collection the in the same manner as an <see cref="Array"/>.
/// </summary>
/// <param name="Index">The Index of the element you want to retrieve. Valid values are from zero to the value in the <see cref="Count"/> property.</param>
/// <returns>The element at the position provided with the indexer.</returns>
/// <remarks>This is required for IList implementation.</remarks>
object IList.this[int Index]
{
get
{
return this[Index];
}
set
{
if ((value is T) == false)
{
throw new ArgumentException("Value passed is not a valid type.");
}
this[Index] = (T)value;
}
}
/// <summary>
/// Gets or sets the total number of elements the internal data structure can hold without resizing.
/// </summary>
public int Capacity
{
get
{
return _SourceList.Capacity;
}
set
{
// We cannot let them shrink capacity because this class is a wrapper for the List<T> in the _SourceList property.
// They don't get to see all the entries in that list because it is filtered. Therefore it is not safe for them to shrink capacity.
// We check if they are shrinking the capacity.
if (value >= _SourceList.Capacity)
{
_SourceList.Capacity = value;
}
}
}
/// <summary>
/// Gets the number of elements contained in the <see cref="FilteredList{T}"/>.
/// </summary>
public int Count
{
get
{
List<T> Selected = _SourceList.Where(_Filter).ToList();
return Selected.Count();
}
}
/// <summary>
/// Gets a value indicating whether the <see cref="FilteredList{T}"/> has a fixed size.
/// </summary>
public bool IsFixedSize
{
get
{
return false;
}
}
/// <summary>
/// Gets a value indicating whether the <see cref="FilteredList{T}"/> is read-only.
/// </summary>
public bool IsReadOnly
{
get
{
return false;
}
}
/// <summary>
/// Gets a value indicating whether access to the <see cref="FilteredList{T}"/> is synchronized (thread safe).
/// </summary>
public bool IsSynchronized
{
get
{
return false;
}
}
/// <summary>
/// Gets an object that can be used to synchronize access to the <see cref="FilteredList{T}"/>.
/// </summary>
public object SyncRoot
{
get
{
return _SourceList;
}
}
#endregion
#endregion
}
}

Linq query on IReliableDictionary

I have done a bunch of looking and messing with code and I haven't found a way to do Linq Queries against the IReliableDictionary. I know it's not the same as standard IDictionary, but I was curious if anyone has had any luck. I'm starting to think it's just not possible unfortunately.

Currently, there's no normal way to do a linq query on a reliable dictionary. That being said, there are a few things you can do. As previously stated, the reason the CreateEnumerableAsync method exists is because service fabric pages reliable dictionaries to disk, but if you know that the underlying collection is small and you're okay with a performance hit, then the following class will work.
public static class AsyncEnumerableExtensions
{
/// <summary>
/// Converts the collection to a list
/// </summary>
/// <typeparam name="TValType">value type of the collection</typeparam>
/// <param name="enumerator">enumerator to convert</param>
/// <param name="ct">cancellation token for the async operations</param>
/// <param name="tx">tx to enforce that this is called in a transactional context</param>
/// <returns>a list containing all elements in the origin collection</returns>
public static async Task<IList<TValType>> ToListAsync<TValType>(
this IAsyncEnumerator<TValType> enumerator,CancellationToken ct, ITransaction tx)
{
IList<TValType> ret = new List<TValType>();
while (await enumerator.MoveNextAsync(ct).ConfigureAwait(false))
{
ret.Add(enumerator.Current);
}
return ret;
}
/// <summary>
/// Converts the collection to a list
/// </summary>
/// <typeparam name="TValType">value type of the collection</typeparam>
/// <param name="enumerator">enumerator to convert</param>
/// <param name="tx">tx to enforce that this is called in a transactional context</param>
/// <returns>a list containing all elements in the origin collection</returns>
public static Task<IList<TValType>> ToListAsync<TValType>(
this IAsyncEnumerator<TValType> enumerator, ITransaction tx)
{
return enumerator.ToListAsync(CancellationToken.None,tx);
}
/// <summary>
/// Converts the collection to a list
/// </summary>
/// <typeparam name="TValType">value type of the collection</typeparam>
/// <param name="enumerable">enumerator to convert</param>
/// <param name="ct">cancellation token for the async operations</param>
/// <param name="tx">tx to enforce that this is called in a transactional context</param>
/// <returns>a list containing all elements in the origin collection</returns>
public static Task<IList<TValType>> ToListAsync<TValType>(this IAsyncEnumerable<TValType> enumerable,
CancellationToken ct, ITransaction tx)
{
return enumerable.GetAsyncEnumerator().ToListAsync(ct,tx);
}
/// <summary>
/// Converts the collection to a list
/// </summary>
/// <typeparam name="TValType">value type of the collection</typeparam>
/// <param name="enumerable">enumerator to convert</param>
/// <param name="tx">tx to enforce that this is called in a transactional context</param>
/// <returns>a list containing all elements in the origin collection</returns>
public static Task<IList<TValType>> ToListAsync<TValType>(this IAsyncEnumerable<TValType> enumerable, ITransaction tx)
{
return enumerable.GetAsyncEnumerator().ToListAsync(tx);
}
}
You could also implement your own custom enumerators and extension methods to perform linq-query like operations on your data in an efficient manner. I have written a few that I would like to release but they need some polish first. I have a sneaking feeling that the service fabric team might already be on it, but if or when that happens your guess is as good as mine.

Looks like MS removed the ability to do linq queries in the latest SF release.

If you need to handle operations that are more involved than simple GetByKey(), you would need to explicitly create a local collection of it via the IReliableDictionary.CreateEnumerable() method, which you would then be able to query similar to the following example from the comments in this discussion :
IAsyncEnumerable<KeyValuePair<int, string="">> enumerable = await myDictionary.CreateEnumerableAsync(tx);
using (IAsyncEnumerator<KeyValuePair<int, string="">> e = enumerable.GetAsyncEnumerator())
{
while (await e.MoveNextAsync(cancellationToken).ConfigureAwait(false))
{
doSomething(e.Current);
}
}
This may not fit your scenario at all, but it seems that it's likely the only way to perform any kind of "advanced queries".

Right LINQ will not work with IAsyncEnumerator
var wgEnumerable = await voteDictionary.CreateEnumerableAsync(tx);
using(IAsyncEnumerator<KeyValuePair<string, int>> enumerator = wgEnumerable.GetAsyncEnumerator())
{
while(await enumerator.MoveNextAsync(CancellationToken.None))
{
//Do something
}
}

It is possible to use Async Linq after converting SerivceFabric's IAsyncEnumerable to a dotnet one.
Please refer to this answer:
Convert IReliableDictionary to IList

ASP.NET MVC Transfer HTML Table Contents to Another View

Is this do-able?
Here's my situation and could you suggest an easier or more efficient way if what I'm trying to do isn't advisable.
We're talking about report generation page here.First, I have a stored procedure that takes a REALLY long time to finish executing if no filters/condition is set. Meaning it is a view all, this stored proc returns a list. This list then populates a table in my view. It could be just 10 up to thousands records but the execution is pretty long because it computes this and that against thousands of record, to make it short, I won't alter my stored procedure.
Now from this first view, I have a printable version button which calls another page with the same contents but print-friendly page. I dont want to execute the painful stored proc to get the same list, I want to re-use what is already generated. How can I do this?

From what I understand you are thinking of implementing some way of caching the data that you calculated through a painfully slow stored procedure?
One option would be to implement a CacheManager and cache the results for a certain period of time:
/// <summary>
/// Cache Manager Singleton
/// </summary>
public class CacheManager
{
/// <summary>
/// The instance
/// </summary>
private static MemoryCache instance = null;
/// <summary>
/// Gets the instance of memoryCache.
/// </summary>
/// <value>The instance of memoryCache.</value>
public static MemoryCache Instance
{
get
{
if (instance == null)
{
instance = new MemoryCache();
}
return instance;
}
}
}
/// <summary>
/// Cache Manager
/// </summary>
public class MemoryCache
{
/// <summary>
/// Gets the expiration date of the object
/// </summary>
/// <value>The no absolute expiration.</value>
public DateTime NoAbsoluteExpiration
{
get { return DateTime.MaxValue; }
}
/// <summary>
/// Retrieve the object in cache
/// If the object doesn't exist in cache or is obsolete, getItemCallback method is called to fill the object
/// </summary>
/// <typeparam name="T">Object Type to put or get in cache</typeparam>
/// <param name="httpContext">Http Context</param>
/// <param name="cacheId">Object identifier in cache - Must be unique</param>
/// <param name="getItemCallback">Callback method to fill the object</param>
/// <param name="slidingExpiration">Expiration date</param>
/// <returns>Object put in cache</returns>
public T Get<T>(string cacheId, Func<T> getItemCallback, TimeSpan? slidingExpiration = null) where T : class
{
T item = HttpRuntime.Cache.Get(cacheId) as T;
if (item == null)
{
item = getItemCallback();
if (slidingExpiration == null)
{
slidingExpiration = TimeSpan.FromSeconds(30);
}
HttpRuntime.Cache.Insert(cacheId, item, null, this.NoAbsoluteExpiration, slidingExpiration.Value);
}
return item;
}
/// <summary>
/// Retrieve the object in cache
/// If the object doesn't exist in cache or is obsolete, null is returned
/// </summary>
/// <typeparam name="T">Object Type to put or get in cache</typeparam>
/// <param name="httpContext">Http Context</param>
/// <param name="cacheId">Object identifier in cache - Must be unique</param>
/// <returns>Object put in cache</returns>
public T Get<T>(string cacheId) where T : class
{
T item = HttpRuntime.Cache.Get(cacheId) as T;
if (item == null)
{
return null;
}
return item;
}
/// <summary>
/// Delete an object using his unique id
/// </summary>
/// <param name="httpContext">Http Context</param>
/// <param name="cacheId">Object identifier in cache</param>
/// <returns><c>true</c> if XXXX, <c>false</c> otherwise</returns>
public bool Clear(string cacheId)
{
var item = HttpRuntime.Cache.Get(cacheId);
if (item != null)
{
HttpRuntime.Cache.Remove(cacheId);
return true;
}
return false;
}
/// <summary>
/// Delete all object in cache
/// </summary>
/// <param name="httpContext">Http Context</param>
public void ClearAll(string filter = null)
{
var item = HttpRuntime.Cache.GetEnumerator();
while (item.MoveNext())
{
DictionaryEntry entry = (DictionaryEntry)item.Current;
var key = entry.Key.ToString();
if (filter != null && (key.ToLower().Contains(filter.ToLower()) || filter == "*" )) //if filter, only delete if the key contains the filter value
{
HttpRuntime.Cache.Remove(entry.Key.ToString());
}
else if (filter == null) //no filter, delete everything
{
HttpRuntime.Cache.Remove(entry.Key.ToString());
}
}
}
}
Note: I didn't write this myself but can't find the original source.
This is how you use it:
// Retrieve object from cache if it exist, callback is performed to set and return object otherwise
UserObject userObject = CacheManager.Instance.Get(httpContext, "singleId", ()=> {
return new UserObject() {
Id = Guid.NewGuid()
};
});
// Retrieve object from cache if it exist, return null otherwise
UserObject userObjectRetrieved = CacheManager.Instance.Retrieve<UserObject>(httpContext, "singleId");
// Remove object from cache
CacheManager.Instance.Clear(httpContext, "singleId");
// Remove all object from cache
CacheManager.Instance.ClearAll(httpContext);

Not sure how your View design is meant to be but you could populate the printable version at the same time and hide it until the button is clicked.

Caching strategy with an abstract Type Mapper implementation on top of Dapper

Overview
We use Dapper to execute stored procedures on our internal applications. I need to build out a set of APIs that we could use that sit on top of Dapper, so the enterprise can avoid being tightly coupled with dapper. I wrote the set of APIs and have them working and performing great.
A simple example of the usage is:
private async Task Delete()
{
// Get an instance of the graph builder from our factory.
IGraph graphBuilder = EntityGraphFactory.CreateEntityGraph();
// Associate the builder to a stored procedure, and map an entity instance to it.
// We provide the graph the entities primary key and value.
graphBuilder.MapToProcedure("DeleteAddress").MapEntity(this.CustomerAddress)
.DefineProperty(address => address.AddressId).IsKey();
// Get an instance of our repository and delete the entity defined in the graph.
IGraphRepository repository = GraphRepositoryFactory.CreateRepository();
await repository.DeleteAsync(graphBuilder);
this.CustomerAddress = new Address();
}
The problem
The challenge I now have is caching. I want the repository to handle the caching for us automatically. When we query for lookup data like this:
private async Task RestoreAddress()
{
IGraph graphBuilder = EntityGraphFactory.CreateEntityGraph();
IGraphRepository repository = GraphRepositoryFactory.CreateRepository();
// Map ourself to a stored procedure. Tell the graph we are going to
// take the entered Id, and pass it in to the stored procedure as a
// "AddressId" parameter.
// We then define each of the properties that the returned rows
// must map back to, renaming the columns to their associated properties.
graphBuilder.MapToProcedure("GetAddressbyId")
.MapFromEntity(this.CustomerAddress)
.DefineProperty(address => address.AddressId.ToString())
.MapToEntity<Address>()
.DefineProperty(address => address.Street == "AddressLine1")
.DefineProperty(address => address.City)
.DefineProperty(address => address.RowGuid.ToString() == "rowguid")
.DefineProperty(address => address.LastModified.ToString() == "ModifiedDate")
.DefineProperty(address => address.PostalCode)
.DefineProperty(address => address.AddressId)
.MapToEntity<StateProvince>()
.DefineProperty(province => province.StateProvinceId.ToString() == "StateProvinceId");
IEnumerable<Address> addresses = await repository.GetAsync<Address>(graphBuilder);
this.CustomerAddress = addresses.FirstOrDefault() ?? new Address();
this.SelectedProvince = this.Provinces.FirstOrDefault(
province => province.StateProvinceId == this.CustomerAddress.StateProvinceId);
}
Addresses in this example is a set of lookup data that won't change during the runtime of the app. Not until a sync is performed, at which point the cache could be cleared. The issue though is that I'm not sure how to go about caching. In this example, I am executing GetAddressById, but I could have executed GetAddressByStateId or GetAllAddresses. Then I don't know what data was already fetched and still needs to be fetched.
Potential solutions
I have a few ideas on how to go about doing this, but I'm not sure if they're going to cause conflicts or issues if I were to implement them. So before I outline them, I want to show you the implementation of the IGraph interface.
/// <summary>
/// Exposes methods for retrieving mapping information and entity definitions.
/// </summary>
internal class Graph : IGraph
{
/// <summary>
/// Initializes a new instance of the <see cref="Graph"/> class.
/// </summary>
internal Graph()
{
this.ProcedureMapping = new ProcedureBuilder(this);
this.GraphMap = new Dictionary<Type, List<PropertyDefinition>>();
}
/// <summary>
/// Gets the graph definitions created for each Type registered with it.
/// </summary>
internal Dictionary<Type, List<PropertyDefinition>> GraphMap { get; private set; }
/// <summary>
/// Gets or sets the key used by the graph as it's Primary Key.
/// </summary>
internal PropertyDefinition RootKey { get; set; }
/// <summary>
/// Gets the procedure mapping.
/// </summary>
internal ProcedureBuilder ProcedureMapping { get; private set; }
/// <summary>
/// Gets the graph generated for the given entity
/// </summary>
/// <typeparam name="TEntity">The entity type to retrieve definition information from.</typeparam>
/// <returns>
/// Returns a collection of PropertyDefinition objects
/// </returns>
public IEnumerable<PropertyDefinition> GetEntityGraph<TEntity>() where TEntity : class, new()
{
return this.GetEntityGraph(typeof(TEntity));
}
/// <summary>
/// Gets a collection of PropertyDefinition objects that make up the data graph for the Entity speified.
/// </summary>
/// <param name="entityType">The entity type to retrieve definition information from.</param>
/// <returns>
/// Returns a collection of PropertyDefinition objects
/// </returns>
public IEnumerable<PropertyDefinition> GetEntityGraph(Type entityType)
{
if (GraphMap.ContainsKey(entityType))
{
return GraphMap[entityType];
}
return Enumerable.Empty<PropertyDefinition>();
}
/// <summary>
/// Gets the graph generated by the graph for all entities graphed on it.
/// </summary>
/// <returns>
/// Returns a dictionary where the key is a mapped type and the value is its definition data.
/// </returns>
public Dictionary<Type, IEnumerable<PropertyDefinition>> GetBuilderGraph()
{
// Return a new dictionary containing the same values. This prevents someone from adding content to the
// dictionary we hold internally.
return this.GraphMap.ToDictionary(keySelector => keySelector.Key, valueSelector => valueSelector.Value as IEnumerable<PropertyDefinition>);
}
/// <summary>
/// Resets the graph so that it may be used in a fresh state.
/// </summary>
public void ClearGraph()
{
this.GraphMap.Clear();
this.RootKey = null;
this.ProcedureMapping = new ProcedureBuilder(this);
}
/// <summary>
/// Gets the primary key defined for this data graph.
/// </summary>
/// <returns>Returns the PropertyDefinition associated as the Builder Key.</returns>
public PropertyDefinition GetKey()
{
return this.RootKey;
}
/// <summary>
/// Gets the stored procedure for the operation type provided.
/// </summary>
/// <param name="operationType">Type of operation the procedure will perform when executed.</param>
/// <returns>
/// Returns the ProcedureDefinition mapped to this graph for the given operation type.
/// </returns>
public ProcedureDefinition GetProcedureForOperation(ProcedureOperationType operationType)
{
string procedureName = this.ProcedureMapping.ProcedureMap[operationType];
return new ProcedureDefinition(operationType, procedureName);
}
/// <summary>
/// Gets all of the associated stored procedure mappings.
/// </summary>
/// <returns>
/// Returns a collection of ProcedureDefinition objects mapped to this data graph.
/// </returns>
public IEnumerable<ProcedureDefinition> GetProcedureMappings()
{
// Convert the builders dictionary mapping of stored procedures to OperationType into a collection of ProcedureDefinition objects.
return this.ProcedureMapping.ProcedureMap
.Where(kvPair => !string.IsNullOrEmpty(kvPair.Value))
.Select(kvPair => new ProcedureDefinition(kvPair.Key, kvPair.Value));
}
/// <summary>
/// Maps the data defined in this graph to a stored procedure.
/// </summary>
/// <param name="procedureName">Name of the procedure responsible for receiving the data in this graph.</param>
/// <returns>
/// Returns the data graph.
/// </returns>
public IGraph MapToProcedure(string procedureName)
{
this.ProcedureMapping.DefineForAllOperations(procedureName);
return this;
}
/// <summary>
/// Allows for mapping the data in this graph to different stored procedures.
/// </summary>
/// <returns>
/// Returns an instance of IProcedureBuilder used to perform the mapping operation
/// </returns>
public IProcedureBuilder MapToProcedure()
{
return this.ProcedureMapping;
}
/// <summary>
/// Defines what Entity will be used to building out property definitions
/// </summary>
/// <typeparam name="TEntity">The type of the entity to use during the building process.</typeparam>
/// <returns>
/// Returns an instance of IEntityDefinition that can be used for building out the entity definition
/// </returns>
public IPropertyBuilderForInput<TEntity> MapFromEntity<TEntity>() where TEntity : class, new()
{
this.CreateDefinition<TEntity>();
return new PropertyBuilder<TEntity>(this, DefinitionDirection.In);
}
/// <summary>
/// Defines what Entity will be used to building out property definitions
/// </summary>
/// <typeparam name="TEntity">The type of the entity to use during the building process.</typeparam>
/// <param name="entity">An existing instance of the entity used during the building process.</param>
/// <returns>
/// Returns an instance of IEntityDefinition that can be used for building out the entity definition
/// </returns>
public IPropertyBuilderForInput<TEntity> MapFromEntity<TEntity>(TEntity entity) where TEntity : class, new()
{
this.CreateDefinition<TEntity>();
return new PropertyBuilder<TEntity>(this, DefinitionDirection.In, entity);
}
public IPropertyBuilderForOutput<TEntity> MapToEntity<TEntity>() where TEntity : class, new()
{
this.CreateDefinition<TEntity>();
return new PropertyBuilder<TEntity>(this, DefinitionDirection.Out);
}
public IPropertyBuilderForInput<TEntity> MapEntity<TEntity>() where TEntity : class, new()
{
this.CreateDefinition<TEntity>();
return new PropertyBuilder<TEntity>(this, DefinitionDirection.Both);
}
public IPropertyBuilderForInput<TEntity> MapEntity<TEntity>(TEntity entity) where TEntity : class, new()
{
this.CreateDefinition<TEntity>();
return new PropertyBuilder<TEntity>(this, DefinitionDirection.Both, entity);
}
private void CreateDefinition<TEntity>() where TEntity : class, new()
{
// A definition has already been created, so return.
if (this.GraphMap.ContainsKey(typeof(TEntity)))
{
return;
}
this.GraphMap.Add(typeof(TEntity), new List<PropertyDefinition>());
}
}
The point of this class is to let you map a Type to it, and then use interfaces that are returned on the MapEntity methods, to define properties and their characteristics. The repository then is given the builder, and pulls the Mappings from it, generating a Dapper DynamicParameters collection from it. The map is also used in a custom Dapper TypeMapper.
Since I am only caching things that are queried, I'll save some page-space and just share the query method on my repository, and its TypeMapper.
public async Task<IEnumerable<TEntity>> GetAsync<TEntity>(IGraph builder, IDataContext context = null)
{
IEnumerable<TEntity> items = null;
DynamicParameters parameters = this.GetParametersFromDefinition(builder, DefinitionDirection.In);
// Setup our mapping of the return results.
this.SetupSqlMapper<TEntity>(builder);
ProcedureDefinition mapping = builder.GetProcedureForOperation(ProcedureOperationType.Select);
// Query the database
await this.SetupConnection(
context,
async (connection, transaction) => items = await connection.QueryAsync<TEntity>(
mapping.StoredProcedure,
parameters,
commandType: CommandType.StoredProcedure,
transaction: transaction));
return items;
}
private async Task SetupConnection(IDataContext context, Func<IDbConnection, IDbTransaction, Task> communicateWithDatabase)
{
SqlDataContext connectionContext = await this.CreateConnectionContext(context);
IDbConnection databaseConnection = await connectionContext.GetConnection();
// Fetch the transaction, if any, associated with the context. If none exists, null is returned and passed
// in to the ExecuteAsync method.
IDbTransaction transaction = connectionContext.GetTransaction();
try
{
await communicateWithDatabase(databaseConnection, transaction);
}
catch (Exception)
{
this.RollbackChanges(connectionContext);
throw;
}
// If we are given a shared connection, we are not responsible for closing it.
if (context == null)
{
this.CloseConnection(connectionContext);
}
}
private DynamicParameters GetParametersFromDefinition(IGraph builder, DefinitionDirection direction)
{
// Fetch the model definition, then loop through each property we are saving and add it
// do a Dapper DynamicParameer dictionary.
Dictionary<Type, IEnumerable<PropertyDefinition>> definition = builder.GetBuilderGraph();
var parameters = new DynamicParameters();
foreach (var pair in definition)
{
IEnumerable<PropertyDefinition> properties =
pair.Value.Where(property => property.Direction == direction || property.Direction == DefinitionDirection.Both);
foreach (PropertyDefinition data in properties)
{
parameters.Add(data.ResolvedName, data.PropertyValue);
}
}
return parameters;
}
/// <summary>
/// Sets up the Dapper SQL Type mapper.
/// </summary>
/// <param name="type">The type we want to map the build definition to.</param>
/// <param name="graph">The graph.</param>
private void SetupSqlMapper(Type type, IGraph builder)
{
SqlMapper.SetTypeMap(
type,
new CustomPropertyTypeMap(type, (typeToMap, columnName) =>
{
// Grab all of the property definitions on the entity defined with the IGraph
IEnumerable<PropertyDefinition> entityDefinition = builder.GetEntityGraph(typeToMap);
PropertyInfo propertyForColumn;
// Lookup a PropertyDefinition definition from the IGraph that can map to the columnName provided by the database.
PropertyDefinition propertyData = null;
if (this.dataStoreConfig.EnableSensitiveCasing)
{
propertyData = entityDefinition.FirstOrDefault(
definition => definition.ResolvedName.Equals(columnName) || definition.Property.Name.Equals(columnName));
}
else
{
propertyData = entityDefinition.FirstOrDefault(
definition =>
definition.ResolvedName.ToLower().Equals(columnName.ToLower()) ||
definition.Property.Name.ToLower().Equals(columnName.ToLower()));
}
// If a mapping definition was not found, use the TypePool to fetch the property info from the type cache.
// Otherwise we assign the property from the definition mapping.
if (propertyData == null)
{
propertyForColumn = this.dataStoreConfig.EnableSensitiveCasing
? TypePool.GetProperty(typeToMap, (info) => info.Name.Equals(columnName))
: TypePool.GetProperty(typeToMap, (info) => info.Name.ToLower().Equals(columnName.ToLower()));
}
else
{
propertyForColumn = propertyData.Property;
}
if (propertyForColumn == null)
{
Debug.WriteLine(string.Format("The column {0} could not be mapped to the Type {1}. It does not have a definition associated with the data graph, nor a property with a matching name.", columnName, typeToMap.Name));
}
return propertyForColumn;
}));
}
There are a couple different paths I'm considering here.
Override GetHashCode() on my IGraph implementation and have it return a hashed value of the Dictionary & RootKey properties. Then in the repository, I ask for the graphs hashcode, query the database and cache the return results in a Dictionary of <HashCode, ResultSet>. The next time I create the builder, or re-use an existing one, the repository could do a key lookup and return back the cache.
In this approach, is that safe? Can I call GraphMap.GethashCode() and rest assured that the hash will be based on the contents of the dictionary, and therefore (mostly)unique? Would I have to iterate over each item in the dictionary, asking for HashCodes on their members to prevent hash code collisions?
Cache the expression used to generate the map. Within my repository, I can generate a hash, based on the hashcode of each expression used on the builder. This way, if you ever use the same series of expressions to build the mapping, the repository would know and could return the previously fetched data?
Are hashcodes a safe way to go, or should I be exploring different routes? Is there an industry standard way of going about this?

Is it there any LRU implementation of IDictionary?

I would like to implement a simple in-memory LRU cache system and I was thinking about a solution based on an IDictionary implementation which could handle an hashed LRU mechanism.
Coming from java, I have experiences with LinkedHashMap, which works fine for what I need: I can't find anywhere a similar solution for .NET.
Has anyone developed it or has anyone had experiences like this?

This a very simple and fast implementation we developed for a web site we own.
We tried to improve the code as much as possible, while keeping it thread safe.
I think the code is very simple and clear, but if you need some explanation or a guide related to how to use it, don't hesitate to ask.
namespace LRUCache
{
public class LRUCache<K,V>
{
private int capacity;
private Dictionary<K, LinkedListNode<LRUCacheItem<K, V>>> cacheMap = new Dictionary<K, LinkedListNode<LRUCacheItem<K, V>>>();
private LinkedList<LRUCacheItem<K, V>> lruList = new LinkedList<LRUCacheItem<K, V>>();
public LRUCache(int capacity)
{
this.capacity = capacity;
}
[MethodImpl(MethodImplOptions.Synchronized)]
public V get(K key)
{
LinkedListNode<LRUCacheItem<K, V>> node;
if (cacheMap.TryGetValue(key, out node))
{
V value = node.Value.value;
lruList.Remove(node);
lruList.AddLast(node);
return value;
}
return default(V);
}
[MethodImpl(MethodImplOptions.Synchronized)]
public void add(K key, V val)
{
if (cacheMap.TryGetValue(key, out var existingNode))
{
lruList.Remove(existingNode);
}
else if (cacheMap.Count >= capacity)
{
RemoveFirst();
}
LRUCacheItem<K, V> cacheItem = new LRUCacheItem<K, V>(key, val);
LinkedListNode<LRUCacheItem<K, V>> node = new LinkedListNode<LRUCacheItem<K, V>>(cacheItem);
lruList.AddLast(node);
// cacheMap.Add(key, node); - here's bug if try to add already existing value
cacheMap[key] = node;
}
private void RemoveFirst()
{
// Remove from LRUPriority
LinkedListNode<LRUCacheItem<K,V>> node = lruList.First;
lruList.RemoveFirst();
// Remove from cache
cacheMap.Remove(node.Value.key);
}
}
class LRUCacheItem<K,V>
{
public LRUCacheItem(K k, V v)
{
key = k;
value = v;
}
public K key;
public V value;
}
}

There is nothing in the base class libraries that does this.
On the free side, maybe something like C5's HashedLinkedList would work.
If you're willing to pay, maybe check out this C# toolkit. It contains an implementation.

I've recently released a class called LurchTable to address the need for a C# variant of the LinkedHashMap. A brief discussion of the LurchTable can be found here.
Basic features:
Linked Concurrent Dictionary by Insertion, Modification, or Access
Dictionary/ConcurrentDictionary interface support
Peek/TryDequeue/Dequeue access to 'oldest' entry
Allows hard-limit on items enforced at insertion
Exposes events for add, update, and remove
Source Code: http://csharptest.net/browse/src/Library/Collections/LurchTable.cs
GitHub: https://github.com/csharptest/CSharpTest.Net.Collections
HTML Help: http://help.csharptest.net/
PM> Install-Package CSharpTest.Net.Collections

The LRUCache answer with sample code above uses MethodImplOptions.Synchronized, which is equivalent to putting lock(this) around each method call. Whilst correct, this global lock will significantly reduce throughput under concurrent load.
To solve this I implemented a thread safe pseudo LRU designed for concurrent workloads. Performance is very close to ConcurrentDictionary, ~10x faster than MemoryCache and hit rate is better than a conventional LRU. Full analysis provided in the github link below.
Usage looks like this:
int capacity = 666;
var lru = new ConcurrentLru<int, SomeItem>(capacity);
var value = lru.GetOrAdd(1, (k) => new SomeItem(k));
GitHub: https://github.com/bitfaster/BitFaster.Caching
Install-Package BitFaster.Caching

Found you answer while googling, also found this:
http://code.google.com/p/csharp-lru-cache/
csharp-lru-cache: LRU cache collection class library
This is a collection class that
functions as a least-recently-used
cache. It implements ICollection<T>,
but also exposes three other members:
Capacity, the maximum number of items
the cache can contain. Once the
collection is at capacity, adding a
new item to the cache will cause the
least recently used item to be
discarded. If the Capacity is set to 0
at construction, the cache will not
automatically discard items.
Oldest,
the oldest (i.e. least recently used)
item in the collection.
DiscardingOldestItem, an event raised
when the cache is about to discard its
oldest item. This is an extremely
simple implementation. While its Add
and Remove methods are thread-safe, it
shouldn't be used in heavy
multithreading environments because
the entire collection is locked during
those methods.

This takes Martin's code with Mr T's suggestions and makes it Stylecop friendly. Oh, it also allows for disposal of values as they cycle out of the cache.
namespace LruCache
{
using System;
using System.Collections.Generic;
/// <summary>
/// A least-recently-used cache stored like a dictionary.
/// </summary>
/// <typeparam name="TKey">
/// The type of the key to the cached item
/// </typeparam>
/// <typeparam name="TValue">
/// The type of the cached item.
/// </typeparam>
/// <remarks>
/// Derived from https://stackoverflow.com/a/3719378/240845
/// </remarks>
public class LruCache<TKey, TValue>
{
private readonly Dictionary<TKey, LinkedListNode<LruCacheItem>> cacheMap =
new Dictionary<TKey, LinkedListNode<LruCacheItem>>();
private readonly LinkedList<LruCacheItem> lruList =
new LinkedList<LruCacheItem>();
private readonly Action<TValue> dispose;
/// <summary>
/// Initializes a new instance of the <see cref="LruCache{TKey, TValue}"/>
/// class.
/// </summary>
/// <param name="capacity">
/// Maximum number of elements to cache.
/// </param>
/// <param name="dispose">
/// When elements cycle out of the cache, disposes them. May be null.
/// </param>
public LruCache(int capacity, Action<TValue> dispose = null)
{
this.Capacity = capacity;
this.dispose = dispose;
}
/// <summary>
/// Gets the capacity of the cache.
/// </summary>
public int Capacity { get; }
/// <summary>Gets the value associated with the specified key.</summary>
/// <param name="key">
/// The key of the value to get.
/// </param>
/// <param name="value">
/// When this method returns, contains the value associated with the specified
/// key, if the key is found; otherwise, the default value for the type of the
/// <paramref name="value" /> parameter. This parameter is passed
/// uninitialized.
/// </param>
/// <returns>
/// true if the <see cref="T:System.Collections.Generic.Dictionary`2" />
/// contains an element with the specified key; otherwise, false.
/// </returns>
public bool TryGetValue(TKey key, out TValue value)
{
lock (this.cacheMap)
{
LinkedListNode<LruCacheItem> node;
if (this.cacheMap.TryGetValue(key, out node))
{
value = node.Value.Value;
this.lruList.Remove(node);
this.lruList.AddLast(node);
return true;
}
value = default(TValue);
return false;
}
}
/// <summary>
/// Looks for a value for the matching <paramref name="key"/>. If not found,
/// calls <paramref name="valueGenerator"/> to retrieve the value and add it to
/// the cache.
/// </summary>
/// <param name="key">
/// The key of the value to look up.
/// </param>
/// <param name="valueGenerator">
/// Generates a value if one isn't found.
/// </param>
/// <returns>
/// The requested value.
/// </returns>
public TValue Get(TKey key, Func<TValue> valueGenerator)
{
lock (this.cacheMap)
{
LinkedListNode<LruCacheItem> node;
TValue value;
if (this.cacheMap.TryGetValue(key, out node))
{
value = node.Value.Value;
this.lruList.Remove(node);
this.lruList.AddLast(node);
}
else
{
value = valueGenerator();
if (this.cacheMap.Count >= this.Capacity)
{
this.RemoveFirst();
}
LruCacheItem cacheItem = new LruCacheItem(key, value);
node = new LinkedListNode<LruCacheItem>(cacheItem);
this.lruList.AddLast(node);
this.cacheMap.Add(key, node);
}
return value;
}
}
/// <summary>
/// Adds the specified key and value to the dictionary.
/// </summary>
/// <param name="key">
/// The key of the element to add.
/// </param>
/// <param name="value">
/// The value of the element to add. The value can be null for reference types.
/// </param>
public void Add(TKey key, TValue value)
{
lock (this.cacheMap)
{
if (this.cacheMap.Count >= this.Capacity)
{
this.RemoveFirst();
}
LruCacheItem cacheItem = new LruCacheItem(key, value);
LinkedListNode<LruCacheItem> node =
new LinkedListNode<LruCacheItem>(cacheItem);
this.lruList.AddLast(node);
this.cacheMap.Add(key, node);
}
}
private void RemoveFirst()
{
// Remove from LRUPriority
LinkedListNode<LruCacheItem> node = this.lruList.First;
this.lruList.RemoveFirst();
// Remove from cache
this.cacheMap.Remove(node.Value.Key);
// dispose
this.dispose?.Invoke(node.Value.Value);
}
private class LruCacheItem
{
public LruCacheItem(TKey k, TValue v)
{
this.Key = k;
this.Value = v;
}
public TKey Key { get; }
public TValue Value { get; }
}
}
}

The Caching Application Block of EntLib has an LRU scavenging option out of the box and can be in memory. It might be a bit heavyweight for what you want tho.

I don't believe so. I've certainly seen hand-rolled ones implemented several times in various unrelated projects (which more or less confirms this. If there was one, surely at least one of the projects would have used it).
It's pretty simple to implement, and usually gets done by creating a class which contains both a Dictionary and a List.
The keys go in the list (in-order) and the items go in the dictionary.
When you Add a new item to the collection, the function checks the length of the list, pulls out the last Key (if it's too long) and then evicts the key and value from the dictionary to match. Not much more to it really

I like Lawrence's implementation. Hashtable + LinkedList is a good solution.
Regarding threading, I would not lock this with[MethodImpl(MethodImplOptions.Synchronized)], but rather use ReaderWriterLockSlim or spin lock (since contention usually fast) instead.
In the Get function I would check if it's already the 1st item first, rather than always removing and adding. This gives you the possibility to keep that within a reader lock that is not blocking other readers.

I just accidently found now LruCache.cs in aws-sdk-net: https://github.com/aws/aws-sdk-net/blob/master/sdk/src/Core/Amazon.Runtime/Internal/Util/LruCache.cs

If it's an asp.net app you can use the cache class[1] but you'll be competing for space with other cached stuff, which may be what you want or may not be.
[1] http://msdn.microsoft.com/en-us/library/system.web.caching.cache.aspx