here is my code

Telerik.WinForms.Controls.Spreadsheet.Worksheets.Selection selection = this.radSpreadsheet1.SpreadsheetElement.ActiveWorksheetEditor.Selection;

MessageBox.Show(selection.ActiveCellIndex.ToString());

here is my build error

The type 'System.Windows.IWeakEventListener' is defined in an assembly that is not referenced. You must add a reference to assembly 'WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

Hello, Nathan,  

Following the provided information, I was unable to reproduce the issue you are facing. Please refer to the following screenshot:

I have the following assemblies added to the project and it runs as expected:

• Telerik.WinControls
• Telerik.WinControls.RadSpreadsheet
• Telerik.WinControls.UI
• Telerik.Windows.Documents.Spreadsheet
• TelerikCommon

You can find attached my sample project for your reference. Am I missing something? Do I need to perform any changes in order to replicate the problem you are facing?

I am looking forward to your reply.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik

Interesting your project works fine for me.  I had to upgrade your project to 2018.2.621.  I run Telerik in VS 2013.  I don't have the Excel Ribbon in my other App.  I removed it from your app and it still works.  I also have plenty of other libraries.  Here is all the libraries I have included in my form.  Perhaps there is some conflict.  I look forward to your reply.

All my code & attached screenshots of the reference folder.  FYI Most is commented out.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using Telerik.WinControls;
using System.Net;
using System.Text.RegularExpressions;
using Telerik.WinControls.UI;
using Telerik.WinControls.Spreadsheet;
using Telerik.Windows.Documents.Spreadsheet;
using Telerik.Windows.Documents.Core;
using Telerik.Documents.SpreadsheetStreaming;
using Telerik.Windows.Data;
using Telerik.Windows.Zip;
using Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx;
using System.IO;
using System.Windows;

    public partial class ToolsURLChecker : Telerik.WinControls.UI.RadForm
    {
        public ToolsURLChecker()
        {
            InitializeComponent();
        }

        private void radButton1_Click(object sender, EventArgs e)
        {
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                var formatProvider = new XlsxFormatProvider();
                using (Stream input = new FileStream(openFileDialog1.FileName, FileMode.Open))
                {
                    radSpreadsheet1.Workbook = formatProvider.Import(input);
                }
            }
        }

        private void ToolsURLChecker_Load(object sender, EventArgs e)
        {
            radSpreadsheet1.ElementTree.EnableApplicationThemeName = false;
        }

        private void btnPageTitles_Click(object sender, EventArgs e)
        {
            Telerik.WinForms.Controls.Spreadsheet.Worksheets.Selection selection = this.radSpreadsheet1.SpreadsheetElement.ActiveWorksheetEditor.Selection;
            MessageBox.Show(selection.ActiveCellIndex.ToString());
            //MessageBox.Show(radSpreadsheet1.ActiveSheet.Name);
           
            //{

            /*
            try
            {
                WebClient wc = new WebClient();
                string html = wc.DownloadString("https://Nathan.sertaretailers.com");

                Regex x = new Regex("<title>(.*)</title>");
                MatchCollection m = x.Matches(html);

                if (m.Count > 0)
                {
                     MessageBox.Show(m[0].Value.Replace("<title>", "").Replace("</title>", ""));
                }
                else
                     MessageBox.Show("");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Could not connect. Error:" + ex.Message);
            }
             */
            //}
        }

        private void btnStatusCodes_Click(object sender, EventArgs e)
        {
            /* 
            Uri urlCheck = new Uri("https://Nathan.sertaretailers.com");
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(urlCheck);
            request.Timeout = 15000;

            HttpWebResponse response;
            try
            {
                response = (HttpWebResponse)request.GetResponse();
                MessageBox.Show("Code:  " + response.StatusCode + "  Desc:  " + response.StatusDescription);
                
            }
            catch (Exception)
            {
                MessageBox.Show("UH OH");
            }
            */
        }
    }

I added your form to my Project and it no longer works.

I get the same build error

Error1The type 'System.Windows.IWeakEventListener' is defined in an assembly that is not referenced. You must add a reference to assembly 'WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.C:\Users\nathan.herritt\Google Drive\GWA_Projects\GWA_ERP\GWA_ERP\ToolsURLCheckerV2.cs2313GWA_ERP

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik