Question

I’m not sure if this is possible or not.

I have a number of different classes that implement interface IBar, and have constructors that take a couple of values. Rather than create a bunch of almost identical method, is it possible to have a generic method that will create the appropriate constructor?

private function GetFoo(Of T)(byval p1, byval p2) as List(Of IBar)
  dim list as new List(Of IBar)

  dim foo as T

  ' a loop here for different values of x
     foo = new T(x,p1) 
     list.Add(foo)
  ' end of loop
  return list
end function

I get:

'New' cannot be used on a type parameter that does not have a 'New' constraint. 

Answer 1

Unfortunately not – .NET generics only allow you to constrain a generic type to have a parameterless constructor, which you can then call with New T()… you can’t specify a particular set of parameters.

If you don’t mind making your types mutable, you could create an interface which containing a method with the relevant parameters, make all your types implement the interface, and then constrain the type to implement that method and have a parameterless constructor, but it’s not ideal.

Another option is to pass in an appropriate Func which takes x and p1 and returns a new T each time. That would certainly be easy to use from C# – not quite so easy in VB IIRC, but worth considering nevertheless.