Beginner-Friendly C# Setup for AtCoder Using Visual Studio

If you're using C# for AtCoder and prefer Visual Studio over command-line tools, this guide walks you through the simplest and most practical way to set up a local testing environment using input files. This setup allows you to run AtCoder problems with file-based input making it easier to test and iterate quickly.

Why This Setup?

• Run AtCoder problems locally with real input samples
• Use Console.ReadLine() as-is-no need to rewrite logic
• Stay entirely within Visual Studio-no external tools required
• Easily switch between test cases for debugging
• Ideal for beginners, VBA-to-C# learners, or anyone who prefers GUI-based workflows

Environment Overview

Component	Tool / Version	Notes
OS	Windows 11	Any modern Windows version works
IDE	Visual Studio 2022+	Community Edition is fine
Language	C# 12 / .NET 8	Installed via Visual Studio
Input Method	input.txt file	Redirected to standard input
Execution	F5 (Start Debugging)	Output shown in console window

Step-by-Step Setup

1. Create a New Project

1. Launch Visual Studio
2. Click "Create a new project"
3. Choose "Console App(.NET Core or .NET 6/8)"
4. Name the project (e.g.,ABC001A)
5. Click Create

2. Add an Input File

1. In Solution Explorer, right-click the project → Add → New Item
2. Select Text File name it input.txt
3. Paste the sample input from AtCoder (e.g):

2
3

1. Right-click input.txt → Properties
   • Set "Copy to Output Directory" to "Copy if newer"

3. Edit Program.cs

Replace the default code with:

using System;
using System.IO;

class Program {
    static void Main() {
        Console.SetIn(new StreamReader("input.txt"));  // Redirect standard input

        int a = int.Parse(Console.ReadLine());
        int b = int.Parse(Console.ReadLine());
        Console.WriteLine(a + b);
    }
}

4. Run and Verify

• Press F5 or click "Start Debugging"
• The output window should display the result (e.g,5)

Optional Multiple Test Cases

If you want to test multiple input scenarios:

1. Create additional files like input1.txt,input2.txt,etc
2. In Program.cs, change the file name accordingly:

Console.SetIn(new StreamReader("input1.txt"));

Or make it dynamic using command-line arguments:

string path = args.Length > 0 ? args[0] : "input.txt";
Console.SetIn(new StreamReader(path));

This allows you to switch inputs without editing the code each time.

Folder Structure

After setup, your project folder should look like this:

ABC001A/
├── Program.cs
├── input.txt
└── ABC001A.csproj

Summary

With this setup, you can:

• Quickly test AtCoder problem in C# using real input
• Stay inside Visual Studio with no need for external tools
• Easily switch between test cases
• Use Console.ReadLine() without modification
• Extend the template later for stopwatch timing, debug flags, or automation

If you found this guide helpful or have suggestions for improvement, feel free to leave a comment. I'm also learning as I go!