How do you build and handle file paths so that a .NET application works on both Windows and Linux? Explain absolute vs relative paths, separators, Path.Combine vs Path.Join, and case sensitivity.

1. Anatomy of a path

Windows:   C:\Users\sam\app\data\report.csv
           │   └──── directories ────┘└ file name ┘
           └ drive root           extension ──┘

UNC:       \\fileserver\share\data\report.csv
Linux:     /home/sam/app/data/report.csv
           └ root

2. Normalisation

Path.GetFullPath resolves . and .., fixes separators on Windows and makes the path absolute (against the current directory or a base you pass).

"/app/data/./reports/../a.csv"  ──GetFullPath──►  "/app/data/a.csv"

It does not touch the disk, so it does not resolve symbolic links and does not check existence.

3. Useful Path members

4. Case sensitivity traps

Dev laptop (Windows/macOS):   Config.json == config.json   ✔
Linux container:              Config.json != config.json   ✘ FileNotFoundException

The same applies to <Content Include="..."> items, static files served by ASP.NET Core and import paths in front-end bundles. Git on Windows/macOS may also not notice a rename that only changes case; use git mv Config.json config.json.

5. Long paths

Windows historically limited paths to 260 characters (MAX_PATH). Modern .NET handles longer paths on Windows, but other tools in your pipeline (old installers, some zip tools, Explorer in some cases) may not. Deep node_modules or nested build output are the usual triggers. Linux allows 4,096 bytes for a path and 255 bytes per name.

6. Try it yourself

1. In a console app, print Environment.CurrentDirectory and AppContext.BaseDirectory, then run it with dotnet run and with dotnet bin/Debug/net10.0/App.dll from another folder.
2. Print Path.Combine("/app", "/etc/hosts") and Path.Join("/app", "/etc/hosts").
3. Run docker run --rm -v $PWD:/src -w /src mcr.microsoft.com/dotnet/sdk:10.0 dotnet test to catch case-sensitivity bugs locally.

// Files shipped with the app: resolve from the app's folder, not the working directory
string templatePath = Path.Join(AppContext.BaseDirectory, "Templates", "welcome.html");
string html = await File.ReadAllTextAsync(templatePath, ct);

// Data folders: from configuration, normalised once at startup
var dataRoot = Path.GetFullPath(builder.Configuration["Storage:Root"] ?? "/var/lib/myapp");
Directory.CreateDirectory(dataRoot);

// Building a safe file name from user-provided text
static string ToSafeFileName(string input)
{
    var invalid = Path.GetInvalidFileNameChars();
    var chars = input.Select(c => invalid.Contains(c) ? '_' : c).ToArray();
    return new string(chars).Trim();
}

// Path.Combine vs Path.Join
Console.WriteLine(Path.Combine("/app/data", "reports", "2026.csv")); // /app/data/reports/2026.csv
Console.WriteLine(Path.Combine("/app/data", "/etc/passwd"));         // /etc/passwd
Console.WriteLine(Path.Join("/app/data", "/etc/passwd"));            // /app/data//etc/passwd
Console.WriteLine(Path.GetRelativePath("/app", "/app/data/a.csv"));  // data/a.csv