Fun with reflection: Invoking a private method

Posted by Michael Doyon / on 11/19/2009 01:58:00 PM / /

I was recently working on a project where the use of reflection became one of the proposed solutions.  Before I go any further, allow me to preface this by saying that reflection can be costly and have negative performance impacts.  It is not always type safe and may lead to run-time exceptions.  Other alternatives should be considered before committing to using reflection as a solution. 

The scenario we had was we needed to access a private method inside a legacy .NET assembly.  I'm not going to get into the reasons why.  The alternative was to duplicate all of the logic that was needed from legacy code.  I mentioned to a co-worker that we could use reflection to invoke a private method.  He was skeptical so I provided him with the following spike solution code showing how to do just that. It's a simple, but complete working example that I thought I would share with all of you.

1. using System;
2. using System.Collections.Generic;
3. using System.Reflection;
4.  
5. namespace ConsoleApplication1 {
6.     class Program {
7.         static void Main(string[] args) {
8.             HelloWorld hw = new HelloWorld();
9.  
10.             hw.SayWhatsUp();
11.             hw.name = "World";
12.  
13.             BindingFlags eFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
14.             MethodInfo hi = null;
15.             
16.             hi = hw.GetType().GetMethod("SayHi", eFlags);
17.  
18.             object[] a = new List<object>().ToArray();
19.             if (hi != null) {
20.                 hi.Invoke(hw, a);
21.             } else {
22.                 Console.WriteLine("Method Not Found.");
23.             }
24.  
25.             Console.ReadLine();
26.         }
27.     }
28.  
29.  
30.     public class HelloWorld {
31.         public string name { get; set; }
32.  
33.         public void SayWhatsUp() {
34.             Console.WriteLine("Whats Up");
35.         }
36.  
37.         private void SayHi() {
38.             Console.WriteLine("Hi " + name);
39.         }
40.     }
41. }