I have an abstract class called EntityTypeTransform with a single abstract method designed to hold a Func delegate that converts an IDataRecord into an instance of T.
public abstract class EntityTypeTransform<TEntityType> where TEntityType : class
{
public abstract Func<IDataRecord, TEntityType> GetDataTransform();
}
An implementation of that class might look like (does look like) this:
public class TaskParameterEntityTypeTransform : EntityTypeTransform<TaskParameter>
{
public override Func<IDataRecord, TaskParameter> GetDataTransform()
{
return dataRecord => new TaskParameter()
{
TaskId = (int)dataRecord["task_id"],
Name = (string)dataRecord["p_name"],
Value = (string)dataRecord["p_value"]
};
}
}
Now I want to keep an instance of each of these classes in a generic Dictionary, something like:
Dictionary<Type, EntityTypeTransform<T>>
But this doesn't work because (for example) an instance of EntityTypeTransform Of Task is not the same as an instance of EntityTypeTransform Of TaskParameter.
Can anyone help me out?
Edit: I should add that the Type key = typeof(T)
Actually, you don't need to use a dictionary at all! You can use the fact that GenericClass<T>
is actually a different type for each T, so it can have its own static fields (ie GenericClass<Foo>.SomeField
is not shared with GenericClass<Bar>.SomeField
)
For instance you can implement your cache like this:
static class TransformCache<TEntityType>
{
public static EntityTypeTransform<TEntityType> Transform { get; set; }
}
And use it like this:
TransformCache<TaskParameter>.Transform = new TaskParameterEntityTypeTransform();
You can't specify a strong-typed collection that would hold different generic types. Here's the approach I've used in a similar problem, modified to match your requirement:
class TransformCollection
{
private Hashtable cache = new Hashtable();
public void Add<T>(EntityTypeTransform<T> transform) where T : class
{
this.cache[typeof(T)] = itemToCache;
}
public bool Exists<T>() where T : class
{
return this.cache.ContainsKey(typeof(T));
}
public EntityTypeTransform<T> Get<T>() where T : class
{
if (!this.Exists<T>())
throw new ArgumentException("No cached transform of type: " + typeof(T).Name);
return this.cache[typeof(T)] as EntityTypeTransform<T>;
}
}
This gives you type-safe cache for your generic type (though type-safety is enforced by the class's logic, not C#). You can use it as follows:
var collection = new TransformCollection();
collection.Add(SomeMethodToGetTransform<Task>());
//...
if (collection.Exists<Task>())
{
var transform = collection.Get<Task>();
//...
}
You could use an interface that is non-generic and then implement that interface explicitly inside that abstract class, It's pretty common in the .Net library itself:
public interface IEntityTypeTransform
{
Func<IDataRecord, object> GetDataTransform();
}
public abstract class EntityTypeTransform<TEntityType> : IEntityTypeTransform
where TEntityType : class
{
public virtual Func<IDataRecord, TEntityType> GetDataTransform()
{
return this.GetDataTransformImpl();
}
public abstract Func<IDataRecord, TEntityType> GetDataTransformImpl();
Func<IDataRecord, object> IEntityTypeTransform.GetDataTransform()
{
return this.GetDataTransform();
}
}
You would have to create a non-generic base class, eg
public abstract class EntityTypeTransformBase
{
public abstract Func<IDataRecord, object> GetDataTransform();
}
public abstract class EntityTypeTransform<TEntityType> : EntityTypeTransformBase where TEntityType : class
{
public abstract Func<IDataRecord, TEntityType> GetDataTransformImpl();
public override Func<IDataRecord, object> GetDataTransform()
{
return GetDataTransformImpl();
}
}
public class TaskParameterEntityTypeTransform : EntityTypeTransform<TaskParameter>
{
public override Func<IDataRecord, TaskParameter> GetDataTransformImpl()
{
return dataRecord => new TaskParameter()
{
TaskId = (int)dataRecord["task_id"],
Name = (string)dataRecord["p_name"],
Value = (string)dataRecord["p_value"]
};
}
}
Now you can create your dictionary:
var d = new Dictionary<Type, EntityTypeTransformBase>();
d.Add(typeof(TaskParameter), new TaskParameterEntityTypeTransform());
You can use KeyedByTypeCollection
to get type-safety and you can define an interface with a covariant type parameter to make sure that only objects of type EntityTypeTransform<T>
can be added to the dictionary:
public interface IEntityTypeTransform<out TEntityType> where TEntityType : class
{
TEntityType Transform(IDataRecord dataRecord);
}
public abstract class EntityTypeTransform<TEntityType> : IEntityTypeTransform<TEntityType> where TEntityType : class
{
public abstract TEntityType Transform(IDataRecord dataRecord);
}
public class TaskParameter
{
public int TaskId;
public string Name;
public string Value;
}
public class TaskParameterEntityTypeTransform : EntityTypeTransform<TaskParameter>
{
public override TaskParameter Transform(IDataRecord dataRecord)
{
return new TaskParameter()
{
TaskId = (int)dataRecord["task_id"],
Name = (string)dataRecord["p_name"],
Value = (string)dataRecord["p_value"]
};
}
}
public class SomeClass
{
public KeyedByTypeCollection<IEntityTypeTransform<object>> TransformDictionary = new KeyedByTypeCollection<IEntityTypeTransform<object>>()
{
new TaskParameterEntityTypeTransform(),
// More transforms here
};
}
Now you can use it like this:
public void SomeMethod(IDataRecord dataRecord)
{
TaskParameter taskParameter = TransformDictionary.Find<TaskParameterEntityTypeTransform>().Transform(dataRecord);
}
Add a non generic interface to your transformers:
public interface IEntityTypeTransform
{
Func<IDataRecord, object> GetDataTransform();
}
public abstract class EntityTypeTransform<T> : IEntityTypeTransform
{
public abstract Func<IDataRecord, object> GetDataTransform();
}
public class TaskParameterEntityTypeTransform : EntityTypeTransform<TaskParameter>
{
public override Func<IDataRecord, object> GetDataTransform()
{
return dataRecord => new TaskParameter()
{
TaskId = (int)dataRecord["task id"],
};
}
}
Then you can encapsulate your dictionary for ensure that datatypes will always match. Never allow to add a IEntityTypeTransform of a bad type :
public class TransformDistributor
{
private readonly Dictionary<Type, IEntityTypeTransform> _transforms = new Dictionary<Type, IEntityTypeTransform>();
public void Add<T>(EntityTypeTransform<T> type)
{
this._transforms.Add(typeof(T), type);
}
public T Transform<T>(IDataRecord record)
{
var transform = this._transforms[typeof(T)].GetDataTransform()(record);
if (transform is T)
{
return (T)transform;
}
else
{
// theorically can't happen
throw new InvalidOperationException("transformer doesn't return instance of type " + transform.GetType().Name);
}
}
}
The advantage are that at compile time, your are sure that nobody can insert a bad transformer, even if your are not using generics.
Usage :
var transforms = new TransformDistributor();
transforms.Add<TaskParameter>(new TaskParameterEntityTypeTransform());
var taskParameter = transforms.Transform<TaskParameter>(new DataRecord());
I have tried to understand what you exactly want I hope this is exactly what you are looking for!
You shall set in TaskParameter class the correct parameters: TaskId, Name, Value
public abstract class EntityTypeTransform<TEntityType> where TEntityType : class
{
public abstract Func<IDataRecord, TEntityType> GetDataTransform();
}
public class TaskParameterEntityTypeTransform : EntityTypeTransform<TaskParameter>
{
public override Func<IDataRecord, TaskParameter> GetDataTransform()
{
return x => new TaskParameter { X = x.FieldCount };
}
}
public class TaskParameter
{
public int X { get; set; }
}
Dictionary<Type, EntityTypeTransform<TaskParameter>> imADict;
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