最好的重载方法是匹配字符串.Split(部分参数[])有无效参数

我在mMSH.Split("|")(6).ToUpper;和mMSA_ACK.Split("|")[2].ToUpper ();和CheckNullString(mMSA_ACK.Split("|")[1])表示最好的重载方法是匹配字符串。Split(部分参数[])有无效参数。请帮我一下，下面是我的代码。

private int FindMSH(string strMessage_ACK, string mMsh_Send) {    
    try {
        //MsgBox("FindMSH")
        writeMessage("Inside FindMSH function");
        //-> get current MSH segment 
        string[] mSplittxt = strMessage_ACK
            .Replace(System.Convert.ToChar(11).ToString(),"")
            .Replace(System.Convert.ToChar(28).ToString(), "")
            .Split(System.Convert.ToChar(13));
        string mMSH_ACK = mSplittxt
            .Where(x => x.ToString().StartsWith("MSH|"))
            .Select(x => x).FirstOrDefault().ToString();
        string mMSA_ACK = mSplittxt
            .Where(x => x.ToString().StartsWith("MSA|"))
            .Select(x => x).FirstOrDefault().ToString();
        //1.ackcode(1) = "AA" in MSA segment

        string sendMSH_msgid = mMSH.Split("|")(6).ToUpper;
        string mAck = string.Empty;
        string recACK_msgid = string.Empty;
        if (mMSA_ACK.Length > 0)
        {
            recACK_msgid = mMSA_ACK.Split("|")[2].ToUpper();
            mAck = CheckNullString(mMSA_ACK.Split("|")[1]);
        }

mMSH.Split('|')[6].ToUpper();