Question

In Java, one can declare a variable parameterised by an ‘unknown’ generic type, which looks like this:

Foo<?> x; 

Is there an equivalent construct to this question-mark, in C#?

Answer 1

The short answer is no. There isn’t an equivalent feature in C#.

A workaround, from C# from a Java developer’s perspective by Dare Obasanjo:

In certain cases, one may need create a method that can operate on data structures containing any type as opposed to those that contain a specific type (e.g. a method to print all the objects in a data structure) while still taking advantage of the benefits of strong typing in generics. The mechanism for specifying this in C# is via a feature called generic type inferencing while in Java this is done using wildcard types. The following code samples show how both approaches lead to the same result.

C# Code

using System; using System.Collections; using System.Collections.Generic;   class Test{      //Prints the contents of any generic Stack by      //using generic type inference      public static void PrintStackContents<T>(Stack<T> s){         while(s.Count != 0){             Console.WriteLine(s.Pop());          }      }      public static void Main(String[] args){      Stack<int> s2 = new Stack<int>();      s2.Push(4);      s2.Push(5);      s2.Push(6);       PrintStackContents(s2);           Stack<string> s1 = new Stack<string>();      s1.Push('One');      s1.Push('Two');      s1.Push('Three');       PrintStackContents(s1);      } } 

Java Code

import java.util.*;   class Test{      //Prints the contents of any generic Stack by      //specifying wildcard type      public static void PrintStackContents(Stack<?> s){         while(!s.empty()){             System.out.println(s.pop());          }     }      public static void main(String[] args){      Stack <Integer> s2 = new Stack <Integer>();      s2.push(4);      s2.push(5);      s2.push(6);       PrintStackContents(s2);           Stack<String> s1 = new Stack<String>();      s1.push('One');      s1.push('Two');      s1.push('Three');         PrintStackContents(s1);      } }