Попробуйте:
var csv = string.Join(Environment.NewLine, XDocument.Load(@"D:\New Text Document.xml").Descendants()
.Where(element => element.Name.LocalName.Equals("element", StringComparison.CurrentCultureIgnoreCase))
.Select(x => new CustomElements()
{
Type = x.Attribute("type")?.Value,
Label = x.Attribute("label")?.Value,
CompTypes = x.Attribute("CompTypes")?.Value,
Readonly = x.Attribute("readonly")?.Value,
Hidden = x.Attribute("hidden")?.Value,
Require = x.Attribute("required")?.Value
}).Select(x => $"{x.Type}, {x.Label}, {x.CompTypes?.Replace(";", ",")}, readonly=\"{x.Readonly}\", hidden=\"{x.Hidden}\", required=\"{x.Require}\""));