I have an xlsx file consisting of 10 sheets.How can i read and import these sheets to a textbox in a c# winform project?

I tried something similar some time back and this came in handy for me. Give it a look if u have not seen it yet. And if i am seeing ur problem right, you are getting the data from the xlsx to be displayed on textboxes in the form. For this i would read the data and put it into a data structure and then put that into the UI (I understood the use of this the hard way). It gives you greater control over the data if you are doing manipulations on it.

I usually rather use a DataGridView instead of a TextBox.
I use this class (can be extended if you like) to handle stuff in with Excel in C#.

// ****************************************************
//
// Class to manipulate and work with Excel files in C#
// DM 2.2.2009
//
// ****************************************************

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Excel = Microsoft.Office.Interop.Excel;

namespace Peaker
{
    class ExcelC

    {
        const int FirstColWidth = 50;

        private Excel.Application  app = null;
        private Excel.Workbook workbook = null;
        private Excel.Worksheet worksheet = null;
        //private Excel.Range range = null;

        public ExcelC()
        {            
        }

        private int WSCount;

        public int WorkSheetCount
        {
            get { return WSCount = workbook.Worksheets.Count; }//if null?           
        }

        public void CreateADoc()
        {
            try
            {
                app = new Excel.Application();
                app.Visible = false;
                workbook = app.Workbooks.Add(1); 
                worksheet = (Excel.Worksheet)workbook.Sheets[1];
            }
            catch (Exception e)
            {
                MessageBox.Show("Error:" + e.Message);
            }
        }

        public void OpenADoc(string fileName)
        {
            try
            {
                app = new Excel.Application();
                workbook = app.Workbooks.Open
                    ( Filename : fileName, 
                    UpdateLinks : Type.Missing, 
                    ReadOnly : false, 
                    Format : 5, 
                    Password : Type.Missing,
                    WriteResPassword : Type.Missing,
                    IgnoreReadOnlyRecommended : true, 
                    Origin : Excel.XlPlatform.xlWindows,
                    Delimiter : "\t", 
                    Editable : true, 
                    Notify : false, 
                    Converter : Type.Missing,
                    AddToMru : true,
                    Local : false,
                    CorruptLoad : Type.Missing);
                worksheet = (Excel.Worksheet)workbook.Worksheets.get_Item(1);

            }
            catch (Exception e)
            {
                MessageBox.Show("Error:" + e.Message);
            }
        }

        public void Show()
        {
            app.Visible = true;
        }

        public void Hide()
        {
            app.Visible = false;
        }

        public void CloseADoc()
        {
            workbook.Close(true, Type.Missing, Type.Missing);           
        }

        public void Quit()
        {
            app.Quit();
            releaseObject(worksheet);
            releaseObject(workbook);
            releaseObject(app);
        }

        // Put data in an excel cell
        public void PutData(int row, int col, string data)
        {
            worksheet.Cells[row, col] = data;
        }

        // Get data out of an excel cell
        public string GetData(int row, int col)
        {
            string Val = string.Empty;
            if (worksheet.Cells[row, col].Value2 != null)
            {
                Val = worksheet.Cells[row, col].Value2.ToString();
            }
            return Val;
        }

        public string GetData(int worksheetNr, int row, int col)
        {
            worksheet = (Excel.Worksheet)workbook.Worksheets.get_Item(worksheetNr);
            return GetData(row, col);
        }

        public double GetDouble(int row, int col)
        {
            double d = -999.0;
            if (worksheet.Cells[row, col].Value2 != null)
            {
                d = worksheet.Cells[row, col].Value2;
            }
            return d;
        }

        public void PutHeader(int startrow, int startcol, List<string> H)
        {
            foreach (string item in H)
            {
                PutData(startrow, startcol, item);
                startcol++;
            }
        }

        public void WidenCols(int Ncols) //start with col nr 1
        {
            const int B_charASCIIcode = 66;

            worksheet.Range["A:A", Type.Missing].ColumnWidth = FirstColWidth;
            char ch = (char)(B_charASCIIcode + Ncols - 1);
            worksheet.Range["B:" + ch.ToString(), Type.Missing].ColumnWidth = 17;
        }

        public int GetLastFreeRowNr(int worksheetNr)
        {
            worksheet = (Excel.Worksheet)workbook.Worksheets.get_Item(worksheetNr);
            return worksheet.UsedRange.Rows.Count + 1; 
        }

        private void releaseObject(object obj)
        {
            try
            {
                System.Runtime.InteropServices.Marshal.ReleaseComObject(obj);
                obj = null;
            }
            catch (Exception ex)
            {
                obj = null;
                MessageBox.Show("Unable to release the Object " + ex.ToString());
            }
            finally
            {
                GC.Collect();
            }
        } 
    }
}
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