C# Error 01: An object reference is required for the non-static field, method, or property in C#

C# Error 01: An object reference is required for the non-static field, method, or property in C# ? 

The CS0120 error occurs because you're trying to call a non-static method from a static context without an instance of the class.

📌 Solution

Since Main() is static, all methods you call from it must also be static, or you need to create an instance of the class.

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✅ Corrected Code (Make Method Static)

If your method does not depend on instance variables, declare it as static:

using System;
using System.IO;

class Program
{
    static void Main()
    {
        // Get application running path
        string appPath = AppDomain.CurrentDomain.BaseDirectory;
        string logDirectory = Path.Combine(appPath, "Output", "Log");

        // Ensure log directory exists
        Directory.CreateDirectory(logDirectory);

        // Define log file path
        string logFilePath = Path.Combine(logDirectory, "app.log");

        // ✅ Call static method without instance
        WriteLog(logFilePath, "Application started.");
        WriteLog(logFilePath, "Processing data...");
        WriteLog(logFilePath, "Application finished successfully.");

        Console.WriteLine($"Logs written to: {logFilePath}");
    }

    /// <summary>
    /// ✅ Make this method static to call from Main()
    /// </summary>
    static void WriteLog(string filePath, string message)
    {
        string logEntry = $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} | {message}";
        File.AppendAllText(filePath, logEntry + Environment.NewLine);
    }
}

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🛑 If You Cannot Make WriteLog() Static

Then, you need to create an instance of Program in Main():

class Program
{
    static void Main()
    {
        Program p = new Program();  // ✅ Create instance
        p.WriteLog("log.txt", "Hello, log!");
    }

    void WriteLog(string filePath, string message)
    {
        File.AppendAllText(filePath, $"{DateTime.Now} | {message}\n");
    }
}

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🚀 Best Practice: Use a Dedicated Logger Class

For better organization, move logging to a separate Logger class:

class Logger
{
    public static void WriteLog(string filePath, string message)
    {
        string logEntry = $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} | {message}";
        File.AppendAllText(filePath, logEntry + Environment.NewLine);
    }
}

Then call:

Logger.WriteLog("log.txt", "Application started.");

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