将数据行项从 DBNull 更改为字符串值

Question

我可能做错了，但这里是：

    OleDbDataAdapter adapter = new OleDbDataAdapter("SELECT * FROM [CRPRList$]", XLSConnect);
    //the XLSConnect is from a spreadsheet. I've confirmed the data is there.
    DataSet ds = new DataSet();
    adapter.Fill(ds);
    DataTable thisTable = ds.Tables[0];
    //The table exists and it does populate
    thisTable.Columns.Add("Summary");
    //The new column is added and I am able to populate it (below)

    for (int rowNum = 0; rowNum < thisTable.Rows.Count; rowNum++) { 
        //Here I'm cycling through the row items

        DataRow row = thisTable.Rows[rowNum];
        string pr = row["PR"].ToString();
        string cr = row["CR"].ToString();
        //The first two column values are grabbed and examined

        if ((pr != "" && pr != null)&&(cr == null || cr == "")) {
           //if the first column is empty then the second is examined and I am attempting
           //to populate the first column using the searchForCRNumber method
           //This method returns a string

            cr = this.searchForCRNumber(pr); //assignment works
            row[0] = cr; //Here is the problem, this assignment does not work (more explained below)
        }
        row["Summary"] = this.searchForSummary(cr);
        row.AcceptChanges();
    }
    this.showResults(thisTable);//works, except the changes above are not applied

row[0] = cr; 行自然是一个空值，因此它作为 DBNull 类型出现，并且不会接受 cr 字符串。

我得到的错误是System.FormatException: Input string was not in a correct format.

我已经搜索了将row[0] 转换为空字符串的方法或转换cr 的其他方法，但到目前为止我还没有找到方法。

编辑 我正在添加更多内容以帮助了解围绕此问题发生的情况。我认为我在上面已经足够彻底了，但也许不是......

XLSConnect 的数据来自：

private bool XSLConnection(string filePath, string fileType) { // returns false if XSL or XSLX is not the fileType
    string strConn;
    if (fileType.ToLower() == "xls" || fileType.ToLower() == "xlsx") {
        strConn = (fileType == "xlsx") ? string.Format("Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties=\"Excel 12.0\"", filePath) : string.Format("Microsoft.Jet.OLEDB.4.0;Data Source={0};Extended Properties=\"Excel 8.0\"", filePath);

        XLSConnect = new OleDbConnection(strConn);
        return true;
    } else return false;
}

这是searchForCRNumber 方法：

private string searchForCRNumber(string PRNumber) {
    return this.getProperty("MyReturnColumn", PRNumber, "MyMatchColumn");
}

这是getProperty 方法：

private string getProperty(string searchProperty, string searchVal, string rtrnProperty) {
    string undefined = "Undefined";
    string rtrn = null;
    string query = "SELECT " + rtrnProperty + " FROM MySQLTable WHERE " + searchProperty + " = '" + searchVal + "'";
    this.QCConn.Open();

    SqlDataReader reader = this.runQCConnect(query).ExecuteReader();
    reader.Read();
    rtrn = (!reader.HasRows)?
        undefined :
        reader[0].ToString();

    if (rtrn == "" || rtrn == null) rtrn = undefined;
    this.QCConn.Close();


    return rtrn;
}

更多编辑 我仍在研究这个，我开始怀疑我是否还没有发现错误。我发现了问题所在，并且仍在努力解决问题。上传的 xls(x) 可以有空单元格。我发现那些空单元格会导致 DataTable 有空单元格，但它们的类型是 DBNull ......当然，不能（自然）分配字符串值。

当我在 try/catch 中应用以下内容时：

throw new Exception("PR = " + pr + " and typeof = " + pr.GetType().ToString() + " && CR = " + cr + " and typeof = " + cr.GetType().ToString() + "\nCell typeof = " + row["CR"].GetType().ToString());

我收到以下回复：

PR = 7800 and typeof = System.String && CR = Undefined and typeof = System.String
Cell typeof = System.DBNull 

如您所见，该单元格是 System.DBNull 并且不接受字符串值。这是我错误的根源。但我仍然不知道如何使这项工作。有什么我想念的吗？无论 DBNull 值如何，如何将字符串值分配给这些单元格？

Answer 1

DBNull 不是空字符串，将其转换为字符串不会给您null。

如果这是您想要的，您可能应该在将其转换为字符串之前进行检查。执行以下操作：

string pr = (row["PR"] == System.DBNull.Value) ? null : row["PR"].ToString();
string cr = (row["CR"] == System.DBNull.Value) ? null : row["CR"].ToString();