Given a Generic how to create a new E and return it?

Question

This code returns a variable set of fields and I want to return a strongly typed <E>:

public IList<E> Get(Expression<Func<E, object>> selectLambda == null)
{
    if (selectLambda == null)
        selectLambda = p => p;
    var partialSet = DC.CreateQuery<E>("[" + typeof(E).Name + "]");
    foreach ( var record in partialSet)
    {
        var tempEntity = new E();  // This is the error
        MapIt( record, tempContract);
        result.Add(tempContract);
    }
    return result;
}

Duplicate of http://stackoverflow.com/questions/840261/c-generic-new-constructor-problem ... ? — JeffFerguson, Oct 28 '10 at 15:24

score 3 · Accepted Answer · answered Oct 28 '10 at 15:23

3

The simplest way is to add a constraint:

public IList<E> Get(Expression<Func<E, object>> selectLambda == null)
    where E : new()

Then the rest of your code will compile :)

If you can't use constraints there (e.g. because they would propagate all over the place) but you happen to know that it will work at execution time, you could use:

var tempEntity = Activator.CreateInstance<E>();

answered Oct 28 '10 at 15:23

Jon Skeet

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What if you have public class MyClass : IClass : where E : EntityObject, then later have your code. The compiler error is "Constraints are not allowed on non-generic declarations" – Zachary Scott Oct 28 '10 at 15:32
That's a very nice answer. TYVM – Zachary Scott Oct 28 '10 at 15:38
@Dr. Zim: I assume you've sorted out your first error now then? (Basically you've got an extra colon). – Jon Skeet Oct 28 '10 at 16:16

score 2 · Answer 2 · answered Oct 28 '10 at 15:24

2

You need a constraint for E:

public IList<E> Get() where E : new()

This way you make sure E does have a parameterless constructor.

Cheers Matthias

answered Oct 28 '10 at 15:24

Matthias Meid

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score 1 · Answer 3 · answered Oct 28 '10 at 15:23

1

E must support the new () definition as per generic constraint (i.e. E must be ": new ()")

answered Oct 28 '10 at 15:23

TomTom

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score 0 · Answer 4 · answered Oct 28 '10 at 15:25

If E does not have empty constructor you can pass delegate to your method which you can use to create E. In this case caller of the method would be responsible for passing proper delegate.

public IList<E> Get(Expression<Func<E, object>> selectLambda == null, Func<E> creator)
{
    // ...
    var tempEntity = creator();  
    // ...
}

Given a Generic how to create a new E and return it?

4 Answers4