Converts C# source files to use file-scoped namespaces

Program.cs

// There may be corner cases where the naive string matching incorrectly
// changes your source file. Check your diff before committing any changes.

using System.Text;

// Change to whatever your favourite indentation is
const string Indent = "    ";

// Get all the C# files in the specified directory
var fileNames = Directory.GetFiles(args[0], "*.cs", SearchOption.AllDirectories);

foreach (var fileName in fileNames)
{
    // Ignore any files that are auto-generated
    if (fileName.EndsWith(".designer.cs", StringComparison.OrdinalIgnoreCase) ||
        fileName.EndsWith(".g.cs", StringComparison.OrdinalIgnoreCase) ||
        fileName.EndsWith(".generated.cs", StringComparison.OrdinalIgnoreCase))
    {
        continue;
    }

    var hasBom = HasBom(fileName);
    var text = await File.ReadAllLinesAsync(fileName, Encoding.UTF8);

    bool foundNamespace = false;
    bool hasFileScopedNamespaceAlready = false;
    bool isDirty = false;

    for (int i = 0; i < text.Length; i++)
    {
        var line = text[i];

        if ((!hasFileScopedNamespaceAlready && line.StartsWith('{')) ||
            (line.StartsWith('}') && line.Trim().Length == 1 ))
        {
            // Remove the starting and trailing namespace braces
            line = string.Empty;
            isDirty = true;
        }
        else if (line.StartsWith("namespace "))
        {
            foundNamespace = true;

            if (line.Contains(';'))
            {
                hasFileScopedNamespaceAlready = true;
                break;
            }

            // Convert to a file-scoped namespace
            line += ';';
            isDirty = true;
        }
        else if (line.StartsWith(Indent))
        {
            // Remove one level of indentation
            line = line[Indent.Length..];
            isDirty = true;
        }

        text[i] = line;
    }

    if (foundNamespace && !hasFileScopedNamespaceAlready && isDirty)
    {
        // Remove the extra blank line at the end of the file left
        // over from removing the closing brace from the namespace.
        if (string.IsNullOrEmpty(text[^1]))
        {
            text = text.Take(text.Length - 1).ToArray();
        }

        await File.WriteAllLinesAsync(fileName, text, new UTF8Encoding(encoderShouldEmitUTF8Identifier: hasBom));
    }
}

static bool HasBom(string fileName)
{
    // From https://stackoverflow.com/a/19283954/1064169
    var bom = new byte[4];
    using (var file = new FileStream(fileName, FileMode.Open, FileAccess.Read))
    {
        file.Read(bom, 0, bom.Length);
    }

    if (bom[0] == 0xef && bom[1] == 0xbb && bom[2] == 0xbf) return true;
    if (bom[0] == 0xff && bom[1] == 0xfe && bom[2] == 0 && bom[3] == 0) return true;
    if (bom[0] == 0xff && bom[1] == 0xfe) return true;
    if (bom[0] == 0xfe && bom[1] == 0xff) return true;
    if (bom[0] == 0 && bom[1] == 0 && bom[2] == 0xfe && bom[3] == 0xff) return true;

    return false;
}