public class Program
{
  static void Main()
  {
    foreach(var s in FindWindows("Visual"))
      Console.WriteLine(s);
  }

  static IEnumerable<string> FindWindows(string captionPart)
  {
    var found = new List<string>();
    // hwnd и lParam в делегат передаёт внутренняя реализация EnumWindows
    EnumWindows((hwnd, lParam) => 
    {
      var caption = GetWindowText(hwnd);
      if (caption.IndexOf(captionPart, StringComparison.Ordinal) != -1)
        found.Add(caption);
      return true;
    }, IntPtr.Zero);
    return found;
  }

  static string GetWindowText(IntPtr hWnd)
  {
    var size = GetWindowTextLength(hWnd);
    if (size <= 0) return string.Empty;
    var builder = new StringBuilder(size + 1);
    GetWindowText(hWnd, builder, builder.Capacity);
    return builder.ToString();
  }

  delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);
  [DllImport("user32.dll", CharSet = CharSet.Unicode)]
  static extern int GetWindowText(IntPtr hWnd, StringBuilder strText, int maxCount);
  [DllImport("user32.dll", CharSet = CharSet.Unicode)]
  static extern int GetWindowTextLength(IntPtr hWnd);
  [DllImport("user32.dll")]
  static extern bool EnumWindows(EnumWindowsProc enumProc, IntPtr lParam);
}

Microsoft Visual Studio
test (Running) - Microsoft Visual Studio
Visual Studio Application Management Window

class Foo
{
  public void Bar()
  {
    SomeAction func = n => Console.WriteLine("Passed: {0}", n);
    var ptr = Marshal.GetFunctionPointerForDelegate(func);
    Task.Run(() => Zot(ptr));
  }

  void Zot(IntPtr ptr)
  {
    /*some long operation */
    CallFromNative(ptr);
  }

  delegate void SomeAction(int n);
  [DllImport("mylib.dll")]
  static extern void CallFromNative(IntPtr ptr);
}