我无法重现的问题;它应该工作的罚款。有，但是，两个问题：

• 创建 SqlDataReader的错误


• 不使用 .ConnectionString 从配置项

以下编译罚款：

 使用（VAR CON =新的SqlConnection（
    ConfigurationManager.ConnectionStrings [DefaultConnection]。的ConnectionString））
使用（VAR CMD = con.CreateCommand（））
{
    常量字符串查询=选择[PDFFilePath]从[DBO] [AdmPDFManage]其中[PdfId] =（SELECT MAX（[PdfId]）FROM [DBO] [AdmPDFManage]）。
    cmd.CommandText =查询;
    con.Open（）;
    使用（VAR RD = cmd.ExecuteReader（））
    {
        而（rd.Read（））
        {
            字符串str = RD [0]的ToString（）;
            // .. 做一点事
        }
    }
}
 

请注意，如果你只希望1列，的ExecuteScalar 是简单的：

  cmd.CommandText =查询;
con.Open（）;
字符串str =（字符串）cmd.ExecuteScalar（）;
 

I get the following error Error 26 Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a cast?) ON THIS LINE string str = rd[0].ToString(); when try to get filepath in string can anyone sort out my problem

thanks in advance

try
{
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ToString()))
    {
        SqlCommand cmd = new SqlCommand();

        String query = "select [PDFFilePath] from [dbo].[AdmPDFManage] Where [PdfId] = (SELECT MAX([PdfId]) FROM [dbo].[AdmPDFManage]) ";
        cmd.Connection = con;
        cmd.CommandText = query;
        con.Open();
        SqlDataReader rd = new SqlDataReader();
        while (rd.Read())
        {
            string str = rd[0].ToString();

        }
    }
}
catch (Exception ex)
{
    throw;
}

I ALSO TRY THIS BUT GET THE SAME ERROR

using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ToString()))
            {
                SqlCommand cmd = new SqlCommand();

                String query = "select [PDFFilePath] from [dbo].[AdmPDFManage] Where [PdfId] = (SELECT MAX([PdfId]) FROM [dbo].[AdmPDFManage]) ";
                cmd.Connection = con;
                cmd.CommandText = query;
                con.Open();

                String PDFfilePath = (String)cmd.ExecuteScalar();              

            }

I can't reproduce the issue; it should work fine. There are, however, 2 problems:

• creating the SqlDataReader incorrectly
• not using the .ConnectionString from the configuration item

The following compiles fine:

using (var con = new SqlConnection(
    ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString))
using (var cmd = con.CreateCommand())
{
    const string query = "select [PDFFilePath] from [dbo].[AdmPDFManage] Where [PdfId] = (SELECT MAX([PdfId]) FROM [dbo].[AdmPDFManage]) ";
    cmd.CommandText = query;
    con.Open();
    using(var rd = cmd.ExecuteReader())
    {
        while (rd.Read())
        {
            string str = rd[0].ToString();
            // .. do something
        }
    }
}

Note that if you only expect 1 row, ExecuteScalar is simpler:

cmd.CommandText = query;
con.Open();
string str = (string) cmd.ExecuteScalar();