当R与RdotNet连接时，它说设置环境变量'PATH'

当我运行警告

RDotNet.NativeLibrary.UnmanagedDll.SetDllDirectory(string) is obsolete Set environment variable 'PATH' instead. 

我不知道怎么做。如何设置变量路径请快速帮助。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using RDotNet;
namespace WindowsFormsApplication7
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            string dlldir = @"C:'Program Files'R'R-3.1.1'bin'x64";
            REngine.SetDllDirectory(dlldir);
            REngine.CreateInstance("RDotNet");
        }

            private void button1_Click(object sender, EventArgs e)
        {
            REngine engine = REngine.GetInstanceFromID("RDotNet");
            try
            {
                // import csv file
                engine.Evaluate("dataset<-read.table(file.choose(), header=TRUE, sep = ',')");
                // retrieve the data frame
                DataFrame dataset = engine.Evaluate("dataset").AsDataFrame();
                for (int i = 0; i < dataset.ColumnCount; ++i)
                {
                    dataGridView1.ColumnCount++;
                    dataGridView1.Columns[i].Name = dataset.ColumnNames[i];
                }
                for (int i = 0; i < dataset.RowCount; ++i)
                {
                    dataGridView1.RowCount++;
                    dataGridView1.Rows[i].HeaderCell.Value = dataset.RowNames[i];
                    for (int k = 0; k < dataset.ColumnCount; ++k)
                    {
                        dataGridView1[k, i].Value = dataset[i,k];
                    }
                }
            }
            catch
            {
                MessageBox.Show(@"Equation error.");
            }
        }
        }
    }