In my Base repo I have this code which works fine:
abstract class BaseRepo <T> : IRepo <T>
{
private ISession _session;
public Entity GetById<Entity>(int Id)
{
return _session.Get<Entity>(Id);
}
// other methods
}
I want to add another method to return all rows for an object (entity). I want to do something like:
public IList<Entity> GetAll<Entity>()
{
return _session.CreateCriteria<Entity>().List<Entity>;
}
but I get an error saying:
The type 'Entity' must be a reference type in order to use it as parameter 'T' in the generic type or method 'NHibernate.ISession.CreateCriteria<T>()'
Here's my DAL design for reference: Should I use generics to simplify my DAL?
CreateCriteria method requires you to use reference types - add constraint on your DAL method:
public IList<Entity> GetAll<Entity>()
where Entity : class
{
return _session.CreateCriteria<Entity>().List<Entity>();
}
This naturally implies that any Entity type you pass to this method must be a reference type.
I also suggest naming your generic type parameter TEntity - Entity alone is a bit confusing (as it's perfectly fine name for say, entity base class).
Related
In my project using Entity Framework, I have a bunch of functions that look almost exactly alike, so I want to created a generic method they call:
private IHttpActionResult GetData<TEntity>(DbSet<TEntity> data)
The problem I'm having is that the data parameter is saying TEntity has to be a reference type to work, but the type of the entity comes from auto-generated code that doesn't have any base class that I can constrain via a where clause on the method definition.
I'd basically want to call it by getting a context and passing the table in like so:
using (var context = new DataModel.MyEntities()) {
GetData(context.Lab_SubSpace_Contact);
}
To expand on #Igor's answer, you don't have to pass the DbSet<TEntity>, you can also get that dynamically through the type parameter:
private IHttpActionResult GetData<TEntity>() where TEntity : class
{
using (var context = new YourContext())
{
var dbSet = context.Set<TEntity>();
}
}
You do not need a base class, you only have to specify a constraint that it has to be a class (not a struct). This can be done with where TEntity : class
Constraints on Type Parameters
where T : class : The type argument must be a reference type; this applies also to any class, interface, delegate, or array type.
Modified code
private IHttpActionResult GetData<TEntity>(DbSet<TEntity> data) where TEntity : class
I'm trying to create a generic controller on my C#/MVC/Entity Framework application.
public class GenericRecordController<T> : Controller
{
private DbSet<T> Table;
// ...
public action()
{
// ...
db.Entry(T_Instance).State = System.Data.Entity.EntityState.Modified;
}
}
However the DbSet<T> and T_Instance line has a compiler error.
The type T must be a reference type in order to use it as parameter.
When I constrain it as a class, it was solved.
Controller where T : class
What does the error mean? I'm not asking for a solution, I would like to understand why this error occurs and why constraining it as a class solves it.
If you look at the definition of DbSet<TEntity>:
public class DbSet<TEntity> : DbQuery<TEntity>, IDbSet<TEntity>, IQueryable<TEntity>, IEnumerable<TEntity>, IQueryable, IEnumerable, IInternalSetAdapter
where TEntity : class
Because it has a type constraint that the generic type must be a class then you must initialize it with a type that also matches this condition:
public class GenericRecordController<T> : Controller where T : class
{ ... }
They apparently have a constraint on the generic type.
All you need to change is:
public class GenericRecordController<T> : Controller where T : class
This tells the compiler that only reference types may be supplied as a type for T.
You can do it on just a method as well:
public bool HasKey<T>(T obj) where T : class
{
return _db.Entry<T>(obj).IsKeySet;
}
This question already has answers here:
The type must be a reference type in order to use it as parameter 'T' in the generic type or method
(3 answers)
Closed 6 years ago.
I have an interface IMstTuver which is implemented by the MstTuver class. IMstTuver contains MaxVersion and Agenttype parameters.
public class GetTable<T> where T : IMstTuver
{
public IMstTuver GetEntities(DbContext context, string agenttype)
{
long maxVersion = context.Set<T>().Max(x => x.MaxVersion);
IMstTuver mstTuver = context.Set<T>()
.Where(x => x.MaxVersion == maxVersion &&
x.AgentType == agenttype)
.FirstOrDefault();
return mstTuver;
}
}
In my class:
table.GetEntities(MyDbContext, "MSMA") as MstTuver;
I am getting an error
The type 'T' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method 'System.Data.Entity.DbSet'
Please help.
Set<T> has a generic constraint that T must be a reference type, which means that your class needs to have one too. The fix is to apply the following modification to all generic classes and/or methods up the call chain:
public class GetTable<T> where T : class, IMstTuver
You can declare T as a reference type by adding the class type-constraint to your method:
public class GetTable<T> where T : class, IMstTuver
{
//...
I have done something like this in the past and found it easier to put the generic and constraint on the methods than the type. This will allow the same repository type to serve multiple entity types (as long as they meet the constraints).
Example:
public class GetTable
{
public T GetEntities<T>(DbContext context, string agenttype)where T : class, IMstTuver
{
long maxVersion = context.Set<T>().Max(x => x.MaxVersion);
IMstTuver mstTuver = context.Set<T>().Where(x => x.MaxVersion == maxVersion && x.AgentType == agenttype).FirstOrDefault();
return mstTuver;
}
}
In the future when you are returning a generic as a generic from an existing library you can check the constraints on the method you are calling and implement those same constraints. In this case the Set<T> method is constrained to class. You can then add additional non conflicting constraints as you see fit.
DbContext.Set Method
public virtual DbSet<TEntity> Set<TEntity>() where TEntity : class
I have some entities, that may or may not inherit from other objects, but they will implement an interface, lets call it IMyInterface.
public interface IMyInterface {
long MyPropertyName { get; set; }
}
An object will always implement this interface, but it may have been implemented on a class that the object inherits from. How can i get the name of the class that has this interface implemented?
Examples should give these results
public class MyClass : IMyInterface {
}
public class MyHighClass : MyClass {
}
public class MyPlainClass {
}
public class PlainInheritedClass : MyPlainClass, IMyInterface {
}
If i pass in MyClass, it should return MyClass, because MyClass implements the interface.
If i pass in MyHighClass, it should return MyClass, because MyClass was inherited, and it implements the interface.
If i pass in PlainInheritedClass, it should return PlainInheriedClass, because it inherited from MyPlainClass, but that did not implement the interface, PlainInheritedClass did
EDIT/ EXPLAINATION
I am working with entity framework 6. I have created a sort of recycle bin feature, that allows users to delete data on the database, but really it just hides it. In order to use this feature, an entity must implement an interface, which has a particular property against it.
Most of my entities do not inherit from anything, but just implement the interface. But i have a couple of entities that do inherit from another object. Sometimes the object they are inheriting from implement the interface and sometimes the object itself will implement the interface.
When i set the value, i use the entities and entity framework works out which table to update. But when i "unset" the property, i am using my own SQL statements. In order to create my own SQL statements, i need to find out which table has the column i need to update.
I cannot use entity framework to load the entities based on the type only, because .Where doesnt exist on a generic DbSet class.
So i want to create an SQL statement similar to this
UPDATE tableX SET interfaceProperty = NULL WHERE interfaceProperty = X
I was just over thinking the whole thing, the function was very easy. Just encase someone needs something siliar, here it is, i have made it generic. You could always make it an extension instead.
Code just interates all the way down, to the base class, and then checks each class on the way back up through the tree.
public Type GetImplementingClass(Type type, Type interfaceType)
{
Type baseType = null;
// if type has a BaseType, then check base first
if (type.BaseType != null)
baseType = GetImplementingClass(type.BaseType, interfaceType);
// if type
if (baseType == null)
{
if (interfaceType.IsAssignableFrom(type))
return type;
}
return baseType;
}
So i had to call this like so, with my examples
// result = MyClass
var result = GetClassInterface(typeof(MyClass), typeof(IMyInterface));
// result = MyClass
var result = GetClassInterface(typeof(MyHighClass), typeof(IMyInterface));
// result = PlainInheritedClass
var result = GetClassInterface(typeof(PlainInheritedClass), typeof(IMyInterface));
I'm working on a small class library at work, and it naturally involves using generics for this task. But there is this thing that I don't really understand with generics:
Why would I need to use generic type parameters, and then constrain the the type parameter to a specific base class or interface.
Here's an example to what I mean:
public class MyGenericClass<T> where T : SomeBaseClass
{
private T data;
}
And here's the implementation without generics
public class MyClass
{
private SomeBaseClass data;
}
Are these two definitions the same (if yes, then i don't see the advatage of using generics here)?
If not, what do we benefit from using generics here?
As with almost all uses of generics, the benefit comes to the consumer. Constraining the type gives you the same advantages that you get by strongly typing your parameter (or you can do other things like ensure that there's a public parameterless constructor or ensure that it's either a value or reference type) while still retaining the niceties of generics for the consumer of your class or function.
Using generics also, for example, allows you to obtain the actual type that was specified, if that's of any particular value.
This example is a little contrived, but look at this:
public class BaseClass
{
public void FunctionYouNeed();
}
public class Derived : BaseClass
{
public void OtherFunction();
}
public class MyGenericClass<T> where T: BaseClass
{
public MyGenericClass(T wrappedValue)
{
WrappedValue = wrappedValue;
}
public T WrappedValue { get; set; }
public void Foo()
{
WrappedValue.FunctionYouNeed();
}
}
...
var MyGenericClass bar = new MyGenericClass<Derived>(new Derived());
bar.Foo();
bar.WrappedValue.OtherFunction();
The difference is that the former defines the new class as a specific type; the latter simply defines a plain class with a field of that type.
It's all about type safety. Using generics you can return a concrete type (T) instead of some base type which defines the API you need in your generic class. Therefore, the caller of your method won't have to cast the result to the concrete type (which is an error-prone operation).
The main difference is in usage. In the first case, the usage can have:
MyGenericClass<SomeDerivedClass> Variable
Variable.data.SomeDerivedProperty = X
And so that when you use that class, you can still access anything from SomeDerivedClass without casting back to it.
The second example will not allow this.
MyClass.data = SomeDerivedClassInstance
MyClass.data.SomeDerivedProperty = X //Compile Error
((SomeDerivedClass)MyClass.data).SomeDerivedProperty = X //Ewwwww
You will have to cast back up to the SomeDerivedClass (which is unsafe) to use something specific to the derived class.
I don't think that there is a huge amount of difference except that the generic version is constraining your Class, whereas the second is just a constraint on a member of the class. If you added more members and methods to your first Class, you would have the same constraint in place.