'MyObject[]' must be a non-abstract type with a public parameterless constructor to use it as parameter 'T'

Question

In MyRequest class I have:

public static T RetrieveData<T>(Uri uri) where T : class, new()
{
        return requestData<T>(query);
}

The Uri returns an array of strings such as:

{

 "prop1": "JOHN",
 "prop2": "JULIE",
 "value": 9

}, 
{

 "prop1": "KATE",
 "prop2": "Ryan",
 "value": 8

}

This is how I call my method:

var obj = MyRequest.RetrieveData<MyObject[]>("http://.....");

This is how MyObject is defined:

public class MyObject
    {
        public string prop1;
        public string prop2; 
        public double value; 
    }

And this is the error I get:

MyObject[]' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'MyRequest.RetrieveData(uri)'

I got the method to work just fine when I tweak the uri to return just an array of 1 element and call it:

var obj = MyRequest.RetrieveData<MyObject>("http://.....");

instead of

MyObject[]

but I really need to use an array. Any thoughts?

Try List<MyObject> instead? Unfortunately there's not enough information here to answer fully. — DavidG
Like a charm! Thank you very much. May add it as an answer if you like. — Sami

eocron eocron · Accepted Answer · 2017-06-29T08:23:43

It happens becase type T[] is fixed size array and doesn't have parameterless constructor. It is obvious that fixed size array require this size. For example:

var foo = new bar[0];
var foo = new bar[123];
var foo = new bar[count];

You can't initialize something like this:

var foo = new bar[]();
var foo = new bar[];

So it is clear that new() quantifier can't be used with fixed size arrays.

'MyObject[]' must be a non-abstract type with a public parameterless constructor to use it as parameter 'T'

2 Answers