c# - Initialize generic type with non-empty constructor -

referencing question: cannot create instance of variable type 'item' because not have new() constraint

i create instance generic type non empty constructor.

public class a{};  public class b {    public b(int){/*...*/} }  public class c {    public static t validate<t>(int index)        t : a,        new(int) //error    {       if(/*validation*/)          return null;       return new t(index);    } } 

if try call b b = validate<b>(42); got error:

'b' must non-abstract type public parameterless constructor in order use parameter 't' in generic type or method 'c.validate(int)'

question 1: why can use parameterless constructors only?

question 2: how can solve problem without delegates or interfaces? (i looking fancy solutions.)

you can use reflection invoke constructor:

var constructorinfo = typeof(t).getconstructor(new[] { typeof(int) }); if (constructorinfo != null) {     object[] parameters = new object[] { index };     return (t)constructorinfo.invoke(parameters); } else {   // handle } 

(adapted https://stackoverflow.com/a/13523659/5962841)

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or, can use activator create instance (see @datrax's answer)

(t)activator.createinstance(typeof(t), index) 

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the feature asked has been requested.

the request feature being tracked here (github csharp design repo), don't expect year, it's not prototyped or accepted.