using System.Text.RegularExpressions;
public static void Main()
var contentType = @"Multipart/Related; boundary=""----=_Part_17500_382902305.1682622011945""; action=""urn:ihe:iti:2007:CrossGatewayRetrieve""; type=""application/xop+xml""; start-info=""application/soap+xml""";
var manuallyFixed = @"multipart/related; boundary=""----=_Part_17500_382902305.1682622011945""; action=""urn:ihe:iti:2007:CrossGatewayRetrieve""; type=""application/xop+xml""; start-info=""application/soap+xml""";
var replaced = Regex.Replace(contentType, "multipart/related", "multipart/related", RegexOptions.IgnoreCase);
Console.WriteLine(replaced);
Console.WriteLine(replaced == manuallyFixed);