I would like to show in a messagebox the records with line break.
but the result is this:
I'dlikeyoutostaylinebyline;
Myimplementationlookslikethis: 
When I'm debugging, in builder.append I realize that the line break is not inserted.
At the end in return json it returns to the initial call in JavaScript and displays the message:
modal.window({ alert: data.Mensagem });
This will be shown in an HTML, so, instead of adding Environment.NewLine , add <br/> .
var builder = new StringBuilder();
foreach(var item in tributoPagar)
{
builder.Append("***");
builder.Append(item.CodigoDocumentoPagar);
builder.Append(" ");
builder.Append("<br/>");
}
return Json(new RetornoFuncao
{
Sucesso = false,
Mensagem = "Os documentos abaixo já foram adicionados: " + builder
});