Set value of private field

Question

Why is the following code not working:

class Program
{
    static void Main ( string[ ] args )
    {
        SomeClass s = new SomeClass( );

        s.GetType( ).GetField( "id" , System.Reflection.BindingFlags.NonPublic ) // sorry reasently updated to GetField from GetProperty...
            .SetValue( s , "new value" );
    }
}


class SomeClass
{
    object id;

    public object Id 
    {
        get
        {
            return id;
        }
    }   
}

I am trying to set the value of a private field.

---

Here is the exeption I get:

 System.NullReferenceException was unhandled   Message=Object reference not set to an instance of an object.   Source=ConsoleApplication7
 StackTrace:
        at Program.Main(String[] args) in C:\Users\Antonio\Desktop\ConsoleApplication7\ConsoleApplication7\Program.cs:line 18
        at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
        at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
        at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
        at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
        at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
        at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
        at System.Threading.ThreadHelper.ThreadStart()   InnerException:

Answer 1

Evidently, adding BindingFlags.Instance seems to have solved it:

> class SomeClass
  {
      object id;

      public object Id
      {
          get
          {
              return id;
          }
      }
  }
> var t = typeof(SomeClass)
      ;
> t
[Submission#1+SomeClass]
> t.GetField("id")
null
> t.GetField("id", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
> t.GetField("id", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)
[System.Object id]
> 

Answer 2

Try this (inspired by Find a private field with Reflection?):

var prop = s.GetType().GetField("id", System.Reflection.BindingFlags.NonPublic
    | System.Reflection.BindingFlags.Instance);
prop.SetValue(s, "new value");

My changes were to use the GetField method - you are accessing a field and not a property, and to or NonPublic with Instance.