Missing using directive with imported c# .ddl

I have created a c# .dll to do some bitmap stuff with “System.Drawing” inside a unity game.

So I set up a c# classlibrary-project in visual studio, changed targetframework to “.NET Framework 3.5”, added UnityEngine.dll as a reference and built the tiler.dll.
It has the namespace “Tiler” and a public class with a public method in it.

I tested the class in visualStudio and everything works.
Then I placed this .dll in my Assetsfolder and wrote using Tiler;
I instanciated the class and called the method. Everything is fine and monodevelop recognices everything.
But in unityeditor there is this error:

The type or namespace name `CreateTiledGraphic' could not be found. Are you missing a using directive or an assembly reference?

My .dll:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Drawing;
using UnityEngine;

namespace Tiler
{
    class CreateTiledGraphic
    {
        public string TileGraphic(String path, int[,] colors, int xBlocks)
        {
            ...
        }
    }
}

And my c#-script in unity:

using UnityEngine;
using System.Collections;
using Tiler;

public class PNG_FileFinder : MonoBehaviour {

	protected string m_graphicPath;
	protected string m_graphicTiledPath;
	protected FileBrowser m_fileBrowser;

	private CreateTiledGraphic createTG;
	public int xBlocks;
	public int[,] colors;
	
	[SerializeField]
	protected Texture2D	m_directoryImage,
	m_fileImage;

	void Start()
	{
		createTG = new CreateTiledGraphic ();
	}

	protected void OnGUI () {
		if (m_fileBrowser != null) {
			m_fileBrowser.OnGUI();
		} else {
			OnGUIMain();
		}
	}
	
	protected void OnGUIMain() {
		
		GUILayout.BeginHorizontal();
		GUILayout.Label("PNG File", GUILayout.Width(100));
		GUILayout.FlexibleSpace();
		GUILayout.Label(m_graphicPath ?? "none selected");
		if (GUILayout.Button("...", GUILayout.ExpandWidth(false))) {
			m_fileBrowser = new FileBrowser(
				new Rect(100, 100, 600, 500),
				"Choose PNG File",
				FileSelectedCallback
				);
			m_fileBrowser.SelectionPattern = "*.png";
			m_fileBrowser.DirectoryImage = m_directoryImage;
			m_fileBrowser.FileImage = m_fileImage;
		}
		GUILayout.EndHorizontal();
	}
	
	protected void FileSelectedCallback(string path) {
		m_fileBrowser = null;
		m_graphicPath = path;
		m_graphicTiledPath = createTG.TileGraphic(m_graphicPath, colors, xBlocks);
	}
}

(I use a script from the unify wiki and just added the few lines for calling the dll-method.)

I think this is just a stupid small mistake but I cannot find it. Thanks in advance for your help!

Only almost 3 years to late, but you can add this class as an asset to your project and afterwards you can Include it the way you did. No need to compile a dll at first.