Create instance from generic class? [duplicate]

Question

I have the following classes :

public abstract class Repository<TEntity, TIdentifier> : IRepository<TEntity, TIdentifier> where TEntity : class
{ --- }

public abstract class BaseBusinessObject<TEntity, TIdentifier> : Repository<TEntity, TIdentifier> where TEntity : class
    { --- }

public class AttachmentBusinessObject : BaseBusinessObject<Attachment, long>
    { --- }

Somewhere, The only things that I have are "AttachmentBusinessObject" in string and "Attachment" as TEntity in string.

How can I create an instance of BaseBusinessObject or AttachmentBusinessObject ?

BaseBusinessObject is an abstract class, you cannot instantiate it. — Jeroen Vannevel
– Jeroen Vannevel, Commented Nov 29, 2013 at 21:28
@JeroenVannevel: So,How we can create instance from AttachmentBusinessObject ? — Mohammad Dayyan
– Mohammad Dayyan, Commented Nov 29, 2013 at 21:36
@Mohammad: using reflection? Trough the Activator. Look at the duplicate question for a lot more information. — Jeroen Vannevel
– Jeroen Vannevel, Commented Nov 29, 2013 at 21:37

Sherlock · Accepted Answer · 2013-11-29 21:45:58Z

2

You can do this using Activator.CreateInstance:

public class Base
{ }

public class Derived : Base
{ }

[TestMethod]
public void BaseDerivedTest()
{
    string type = "Derived"; // type is found somewhere upstream
    if (type == "Derived")
    {
        var b = Activator.CreateInstance(typeof(Derived).BaseType);
        Assert.IsInstanceOfType(b, typeof(Base));
    }
}

edited Nov 29, 2013 at 21:45

answered Nov 29, 2013 at 21:37

Sherlock

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3 Comments

Mohammad Dayyan Over a year ago

Thanks for your response, but your solution doesn't work in my sample, because I have to cast b to BaseBusinessObject for using its methods

Sherlock Over a year ago

I don't see how it couldn't work for you. I made an edit for clarity. Please let me know if you need anything else.

Mohammad Dayyan Over a year ago

I have several BusinessObjects class in my project instead of "Derived", I have to write if statement for each of them ? e.g: ContentBusinessObject, BrandBusinessObject, ... Is there a better way?

Grant Winney · Accepted Answer · 2013-11-29 22:47:44Z

1

I'm not sure if I'm understanding your correctly, but you may want to check out Activator.CreateInstance.

Something like this:

var yourObject
    = Activator.CreateInstance(null, "AttachmentBusinessObject").Unwrap();

This isn't very useful though, if you want to pass the object around, because CreateInstance has no idea what the type might be, so it just returns an object.

One way around it - any classes you'll be creating an instance of can all implement a common interface. So you might end up with something like:

IBusinessObject yourObject
    = (IBusinessObject)Activator.CreateInstance(null, "AttachmentBusinessObject").Unwrap();

Another option is to use the first line of code above, and then test for the correct class type in an if/else statement:

Type objType = yourObject.GetType();

if (objType == typeof(AttachmentBusinessObject))
{
    var myAttachBusObject = (AttachmentBusinessObject)yourObject;
    ...
}
else
    ...

edited Nov 29, 2013 at 22:47

answered Nov 29, 2013 at 21:37

Grant Winney

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2 Comments

Mohammad Dayyan Over a year ago

Can we cast yourObject to BaseBusinessObject, If so how ? I have to convert yourObject to BaseBusinessObject for using its methods

Mohammad Dayyan Over a year ago

I have several BusinessObjects class in my project, I have to write if statement for each of them ? e.g: ContentBusinessObject, BrandBusinessObject, ... Is there a better way?

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