ClosedXML for C# (.NET): Complete Guide with Examples

Introduction to ClosedXML: What It Is and Why You Need It

ClosedXML is a powerful open-source library for C# (.NET) designed to create, read, and edit Microsoft Excel files (formats .xlsx, .xlsm, .xltx). It provides a clean, object-oriented interface for working with spreadsheets without requiring Microsoft Office installation or COM objects.

This library is especially useful for:

• Report automation — generating complex Excel reports from databases, APIs, or other sources
• Data import/export — bulk processing of tabular data between systems
• Template filling — populating pre-designed Excel templates with data
• Financial and analytical calculations — working with formulas, pivot tables, and charts
• VBA migration — moving macros to modern C#

ClosedXML supports all major Excel features: cell formatting, merging, conditional formatting, charts, pivot tables, data validation, sheet protection, and more. The library works entirely in memory and does not require Excel to be installed on the server.

Installing ClosedXML

Install ClosedXML via NuGet Package Manager. Below are all installation methods:

Via .NET CLI

dotnet add package ClosedXML

Via Package Manager Console (Visual Studio)

Install-Package ClosedXML

Via NuGet Package Manager UI

1. Open your project in Visual Studio
2. Right-click the project → "Manage NuGet Packages"
3. Search for "ClosedXML" and click "Install"

Verify Installation

After installation, add the following using directive to your code:

using ClosedXML.Excel;

Quick Start: Create Your First Excel Table

Let's create a simple employee data table and save it to a file:

using ClosedXML.Excel;

class Program
{
    static void Main(string[] args)
    {
        // Create a new workbook
        using (var workbook = new XLWorkbook())
        {
            // Add a worksheet
            var worksheet = workbook.Worksheets.Add("Employees");

            // Fill in headers
            worksheet.Cell("A1").Value = "ID";
            worksheet.Cell("B1").Value = "Name";
            worksheet.Cell("C1").Value = "Position";
            worksheet.Cell("D1").Value = "Salary";

            // Fill in data
            worksheet.Cell("A2").Value = 1;
            worksheet.Cell("B2").Value = "John Smith";
            worksheet.Cell("C2").Value = "Developer";
            worksheet.Cell("D2").Value = 150000;

            worksheet.Cell("A3").Value = 2;
            worksheet.Cell("B3").Value = "Jane Doe";
            worksheet.Cell("C3").Value = "Analyst";
            worksheet.Cell("D3").Value = 120000;

            // Format headers
            var headerRange = worksheet.Range("A1:D1");
            headerRange.Style.Font.Bold = true;
            headerRange.Style.Fill.BackgroundColor = XLColor.LightBlue;

            // Auto-fit column widths
            worksheet.Columns().AdjustToContents();

            // Save the file
            workbook.SaveAs("employees.xlsx");
        }

        Console.WriteLine("File created successfully!");
    }
}

This example creates an employees.xlsx file with three rows of data and formatted headers.

Key ClosedXML Methods and Functions

1. Creating Workbooks and Worksheets

Method: new XLWorkbook() and workbook.Worksheets.Add()

Description: Creates a new workbook and adds worksheets to it.

Parameters: