Trying to call function where T: TheClass, new() within where T: class

Question

This is a simplified version of what I need to do. In its current state, what I am trying to do doesn't make sense. However it shows the problem in a simple way.

I need to call a function2<T>() where T: TheClass, new() within a function1<T>() where T: class

Something<T> function1<T>() where T : class {
  //cannot call function2 directly due to compile error
  //return function2<T>();

  //What I need as a pseudo code
  if (T is TheClass and T is new())
    return Then function2<T>()
  else
    throw Exception
} 

Something<T> function2<T>() where T : TheClass, new() {
  //...
}

Since you throw exception anyway if input is not valid - why not apply the same restrictions (TheClass, new()) to function1? — Evk, May 17 '16 at 14:33
what compiler error are you getting trying to call function2 directly and for what type T? — Marco Fatica, May 17 '16 at 14:43
@MarcoFatica It doesn't matter what `T` is Since the constraint on `function1` doesn't force it to have a default constructor it can't be used at the parameter for `function2`. — D Stanley, May 17 '16 at 14:45
Since you want different logic based on what `T` is you're going to have to resort to reflection, or use one method for new-able types and another method for other types. — D Stanley, May 17 '16 at 14:46

score 2 · Answer 1 · edited May 23 '17 at 12:16

2

new() restriction can't be verified at run-time (unlike inheritance where you can use as/is) - so there is no C# construct you can write to invoke function2 from function1 and keep it strongly typed.

Your only option is to construct method via reflection and than call it. See How do I use reflection to call a generic method?

edited May 23 '17 at 12:16

Community

1
1

answered May 17 '16 at 15:04

Alexei Levenkov

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score 0 · Answer 2 · answered May 17 '16 at 15:04

You really cannot do that. I'm guessing that function2 is at some point doing new T(), and this might just not be possible for T : class constrained in function1.

You might try for example removing the new() constraint and doing it manually using checks against ConstructorInfo, and get an instance from it as well:

   Type type = typeof(T);
   ConstructorInfo ctor = type.GetConstructor(new Type[0]);
   if(ctor == null) throw new InvalidOperationException();
   object instance = ctor.Invoke(new object[0]);

Trying to call function where T: TheClass, new() within where T: class

2 Answers2