C# 在InfoPath 2010中使用C运行SQL通配符查询

我构建了一个InfoPath 2010表单，用于查询和编辑已输入Access 2010数据库的数据。我遇到的问题是，没有编写代码就无法直接通过InfoPath运行通配符。经过广泛搜索，我找到了以下示例，我已尝试使用InfoPath表单中的字段重新创建该示例：

我遇到的问题是，在运行代码后，我得到的NullReferenceException未被行上的用户代码错误处理：

字符串strPayDate=xnPaymentLookupQuery。SelectSingleNode@Pay_Date，this.NamespaceManager.Value

下面是我的全部脚本。有人对问题的起因有什么建议吗

using Microsoft.Office.InfoPath;
using System;
using System.Windows.Forms;
using System.Xml;
using System.Xml.XPath;
using mshtml;

namespace PaymentLookup
{
    public partial class FormCode
    {
        // Member variables are not supported in browser-enabled forms.
        // Instead, write and read these values from the FormState
        // dictionary using code such as the following:
        //
        // private object _memberVariable
        // {
        //     get
        //     {
        //         return FormState["_memberVariable"];
        //     }
        //     set
        //     {
        //         FormState["_memberVariable"] = value;
        //     }
        // }

        // NOTE: The following procedure is required by Microsoft InfoPath.
        // It can be modified using Microsoft InfoPath.
        public void InternalStartup()
        {
            ((ButtonEvent)EventManager.ControlEvents["btnQuery"]).Clicked += new ClickedEventHandler(btnQuery_Clicked);
        }

        public void btnQuery_Clicked(object sender, ClickedEventArgs e)
        {
            //Create an XPathNavigator object for the main data source
            XPathNavigator xnMain = this.MainDataSource.CreateNavigator();

            //Create an AdoQueryConnection from the main data source by "casting" the default
            //data connection to an "AdoQueryConnection"
            AdoQueryConnection cn = (AdoQueryConnection)this.DataConnections["Main connection"];

            //Obtain the default SQL command for the form.
            string strOrigSQLCommand = cn.Command.ToString();



            // Obtain the query node that you want to change.
            XPathNavigator xnPaymentLookupQuery = xnMain.SelectSingleNode("/dfs:myFields/dfs:queryFields/q:PaymentBatchesToDate/Pay_Date", this.NamespaceManager);

            //Obtain the text that was entered for the wildcard character search, and then clear the current query parameter so that InfoPath will leave the current query parameter blank.
            string strPayDate = xnPaymentLookupQuery.SelectSingleNode("@Pay_Date", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@Pay_Date", this.NamespaceManager).SetValue(string.Empty);

            //Have InfoPath construct an SQL command that is based on all other field values.
            string strMySQLCommand = cn.BuildSqlFromXmlNodes(xnPaymentLookupQuery);

            //Save the other query items, and then clear the other query items before the next query.
            string strPortfolioID = xnPaymentLookupQuery.SelectSingleNode("@Portfolio_ID", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@Portfolio_ID", this.NamespaceManager).SetValue(string.Empty);
            string strTaxEntity = xnPaymentLookupQuery.SelectSingleNode("@Tax_Entity", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@Tax_Entity", this.NamespaceManager).SetValue(string.Empty);
            string strFileType = xnPaymentLookupQuery.SelectSingleNode("@File_Type", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@File_Type", this.NamespaceManager).SetValue(string.Empty);
            string strPayType = xnPaymentLookupQuery.SelectSingleNode("@Pay_Type", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@Pay_Type", this.NamespaceManager).SetValue(string.Empty);
            string strPaymentBatch = xnPaymentLookupQuery.SelectSingleNode("@Payment_Batch", this.NamespaceManager).Value;
            xnPaymentLookupQuery.SelectSingleNode("@Payment_Batch", this.NamespaceManager).SetValue(string.Empty);

            //Add Pay_Date to the query so that Pay_Date can support wildcard characters.
            if (strPayDate != string.Empty)
            {
                if (strMySQLCommand != string.Empty)
                    strMySQLCommand += " AND ";

                //Check whether the user entered the wildcard character (%) as part of the title.
            if (strPayDate.Contains("%"))
                    strMySQLCommand += "[Pay Date] LIKE '" + strPayDate + "'";
                else
                    strMySQLCommand += "[Pay Date] LIKE '" + strPayDate + "%'";
            }

            //Construct the full query string.
            string strSQLQuery = strOrigSQLCommand;
            if (strMySQLCommand != string.Empty)
                strSQLQuery += " WHERE " + strMySQLCommand;

            //Set the command and run the query.
            cn.Command = strSQLQuery;
            cn.Execute();

            //Restore all the user entries to the Query fields so that the user entries will 
            //be available if you want to change and to rerun the query.
            xnPaymentLookupQuery.SelectSingleNode("@Portfolio_ID", this.NamespaceManager).SetValue(strPortfolioID);
            xnPaymentLookupQuery.SelectSingleNode("@Tax_Entity", this.NamespaceManager).SetValue(strTaxEntity);
            xnPaymentLookupQuery.SelectSingleNode("@File_Type", this.NamespaceManager).SetValue(strFileType);
            xnPaymentLookupQuery.SelectSingleNode("@Pay_Type", this.NamespaceManager).SetValue(strPayType);
            xnPaymentLookupQuery.SelectSingleNode("@Payment_Batch", this.NamespaceManager).SetValue(strPaymentBatch);
            xnPaymentLookupQuery.SelectSingleNode("@Pay_Date", this.NamespaceManager).SetValue(strPayDate);

            //Restore the default table command (for the next time).
            cn.Command = strOrigSQLCommand;

            //Clean up
            xnMain = null;
            cn = null;
            xnPaymentLookupQuery = null;
        }
    }
}

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很可能是你打电话给

xnPaymentLookupQuery。SelectSingleNode@Pay_Date，此为.NamespaceManager

正在返回null，因此当您尝试访问Value属性时，它将失败

您需要对该方法返回的内容进行空检查：

string strPayDate = string.Empty;
var strPayDateNode = xnPaymentLookupQuery.SelectSingleNode("@Pay_Date", this.NamespaceManager);
if(strPayDateNode != null)
{
    strPayDate = xnPaymentLookupQuery.SelectSingleNode("@Pay_Date", this.NamespaceManager).Value;
}

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