Func<T> with out parameter

Question

Can I pass a method with an out parameter as a Func?

public IList<Foo> FindForBar(string bar, out int count) { }

// somewhere else
public IList<T> Find(Func<string, int, List<T>> listFunction) { }

Func needs a type so out won't compile there, and calling listFunction requires an int and won't allow an out in.

Is there a way to do this?

Answer 1

ref and out are not part of the type parameter definition so you can't use the built-in Func delegate to pass ref and out arguments. Of course, you can declare your own delegate if you want:

delegate V MyDelegate<T,U,V>(T input, out U output);

Answer 2

Why not create a class to encapsulate the results?

public class Result
{
     public IList<Foo> List { get; set; }
     public Int32 Count { get; set; }
}

Answer 3

The Func family of delegates (or Action for that matter) are nothing but simple delegate types declared like

//.NET 4 and above
public delegate TResult Func<out TResult>()
public delegate TResult Func<in T, out TResult>(T obj)

//.NET 3.5
public delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
public delegate TResult Func<T1, T2, T3, TResult>(T1 obj1, T2 obj2, T3 obj3)

etc. Delegates as such can have out/ref parameters, so in your case its only a matter of custom implementation by yourself as other answers have pointed out. As to why Microsoft did not pack this by default, think of the sheer number of combinations it would require.

delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(T1 obj1, out T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, out T2 obj2)

for just two parameters. We have not even touched ref . It would actually be cumbersome and confusing for developers.

Answer 4

您可以将它包装在一个 lambda/delegate/function/method 中，该方法暴露了正确的接口并称为 FindForBar，但我怀疑 FindForBar 已将其视为 out 参数作为一个原因，因此您需要确保将这些信息扔掉ok/safe/desirable/有正确的结果（即使您可以直接传入 FindForBar，您也需要确定这一点）。