c# – 从SOAP消息中提取SOAP主体

public string Load_XML(string SoapMessage)
{
    //check soap message
    if (SoapMessage == null || SoapMessage.Length <= 0)
        throw new Exception("Soap message not valid");

    //declare some local variable
    int iSoapBodyStartIndex = 0;
    int iSoapBodyEndindex = 0;

    //load the Soap Message
    //Učitaj string XML-a i pretvori ga u XML
    XmlDocument doc = new XmlDocument();

    try
    {
        doc.Load(SoapMessage);
    }

    catch (XmlException ex)
    {
        WriteErrors.WritetoLogFile("WS.LOAD_DOK_LoadXML",ex.ToString());

        throw ex;
    }

    //search for the "http://schemas.xmlsoap.org/soap/envelope/" URI prefix
    string prefix = string.Empty;
    for (int i = 0; i < doc.ChildNodes.Count; i++)
    {
        System.Xml.XmlNode soapNode = doc.ChildNodes[i];
        prefix = soapNode.GetPrefixOfNamespace("http://schemas.xmlsoap.org  /soap/envelope/");

        if (prefix != null && prefix.Length > 0)
            break;
    }

    //prefix not founded. 
    if (prefix == null || prefix.Length <= 0)
        throw new Exception("Can't found the soap envelope prefix");

    //find soap body start index
    int iSoapBodyElementStartFrom = SoapMessage.IndexOf("<" + prefix + ":Body");
    int iSoapBodyElementStartEnd = SoapMessage.IndexOf(">",iSoapBodyElementStartFrom);    -> HERE I HAVE AN ERROR!!!!   
    iSoapBodyStartIndex = iSoapBodyElementStartEnd + 1;

    //find soap body end index
    iSoapBodyEndindex = SoapMessage.IndexOf("</" + prefix + ":Body>") - 1;

    //get soap body (xml data)
    return SoapMessage.Substring(iSoapBodyStartIndex,iSoapBodyEndindex - iSoapBodyStartIndex + 1);
}

我在这里有一个错误：

int iSoapBodyElementStartEnd = SoapMessage.IndexOf(">",iSoapBodyElementStartFrom);

错误：

Index was out of range. Must be non-negative and less than the size of
the collection.

如果有人知道如何解决这个问题？