How to create generic method which return instance of generic?

Question

I want to create simple factory class which implements interface like this:

IFactory 
{
   TEntity CreateEmpty<TEntity>(); 
}

In this method I want to return an instance of type TEntity (generic type). Example:

TestClass test = new Factory().CreateEmpty<TestClass>();

Is it possible? Does interface is correct?

I've tried something like this:

private TEntity CreateEmpty<TEntity>() {
   var type = typeof(TEntity);
   if(type.Name =="TestClass") {
      return new TestClass();
   }
   else {
     ...
   }
}

But it doesn't compile.

Olivier Jacot-Descombes · Accepted Answer · 2012-05-05 12:53:41Z

6

You need to specify the new() constraint on the generic type parameter

public TEntity CreateEmpty<TEntity>() 
    where TEntity : new()
{
    return new TEntity();
}

The new constraint specifies that the concrete type used must have a public default constructor, i.e. a constructor without parameters.

public TestClass
{
    public TestClass ()
    {
    }

    ...
}

If you don't specify any constructors at all, then the class will have a public default constructor by default.

You cannot declare parameters in the new() constraint. If you need to pass parameters, you will have to declare a dedicated method for that purpose, e.g. by defining an appropriate interface

public interface IInitializeWithInt
{
     void Initialize(int i);
}

public TestClass : IInitializeWithInt
{
     private int _i;

     public void Initialize(int i)
     {
         _i = i;
     }

     ...
}

In your factory

public TEntity CreateEmpty<TEntity>() 
    where TEntity : IInitializeWithInt, new()
{
    TEntity obj = new TEntity();
    obj.Initialize(1);
    return obj;
}

edited May 5, 2012 at 12:53

answered May 5, 2012 at 12:40

Olivier Jacot-Descombes

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1 Comment

fl4izdn4g Over a year ago

Thanks for comprehensive answer.

FiftiN · Accepted Answer · 2012-05-05 12:41:10Z

2

interface IFactory<TEntity> where T : new()
{
   TEntity CreateEmpty<TEntity>(); 
}

answered May 5, 2012 at 12:41

FiftiN

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Comments

igofed · Accepted Answer · 2012-05-05 12:43:49Z

2

This method will help you, pass parameters in that order, in which they in constructor:

private T CreateInstance<T>(params object[] parameters)
{
    var type = typeof(T);

    return (T)Activator.CreateInstance(type, parameters);
}

answered May 5, 2012 at 12:43

igofed

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