Remove matching C# region directives using LINQ

var flag = false;
var modified = false;
var lines = File.ReadLines(fileName)
    .Where(line =>
        {
            if ((!flag && line.Contains("#region " + regionName)) ||
                (flag && line.Contains("#endregion")))
            {
                flag = !flag;
                modified = true;
                return false;
            }
            return true;
        })
    .ToArray();
if (modified)
{
    File.WriteAllLines(fileName, lines);
}