简单的参数化选择查询在c#中需要花费时间来执行

我有一个表结构，如下所示:

CREATE TABLE [dbo].[MainTbl](
[RowNum] [int] IDENTITY(1,1) NOT NULL,
[TxtID] [varchar](20) NOT NULL,
[TxtKey] [varchar](20) NOT NULL,
[TrnDate] [datetime] NOT NULL,
[SrcID] [varchar](20) NOT NULL DEFAULT (''),
[ElemName] [varchar](20) NOT NULL,
[TblXml] [varchar](max) NOT NULL,
[ActiveStatus] [varchar](20) NOT NULL DEFAULT (''),
[DevLvl] [int] NOT NULL DEFAULT ((0)),
[Archive] [bit] NULL CONSTRAINT [DF_MainTbl_Archive]  DEFAULT ((0)),
CONSTRAINT [pkMainTbl] PRIMARY KEY CLUSTERED 
([RowNum] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

并执行下面的查询来获取XML:

select TblXml from MainTbl with (nolock) 
where TrnDate < @TrnDate and TxtID = @TxtID and TxtKey = @TxtKey 
and ActiveStatus != 'NonActive' and ElemName = 'xyz' order by TrnDate desc

这个查询在SSMS中执行了5秒，但是在通过c#代码执行时，大约需要5分钟以上。

我的c#代码接受查询大小写no。在"BuildQuery"中准备上述语句和返回一个sqlcommand，我传递参数并执行它，但是" sqlcommand . executereader();"需要花费很多时间。

private bool MainFunction(parameters)
{
    CheckXMLDate(4,'9/6/2016 1:00:00 PM','ABC','123' );
}
________________________________
public bool CheckXMLDate(bldqry,TrnDate,TxtID,TxtKey )
{
    SqlCommand sqlCommand = BuildQuery(bldqry);
    sqlCommand.Parameters.AddWithValue("@TrnDate", TrnDate);
    sqlCommand.Parameters.AddWithValue("@TxtID", TxtID);
    sqlCommand.Parameters.AddWithValue("@TxtKey", TxtKey);
    SqlDataReader dataReader = sqlCommand.ExecuteReader();
    //Some statements like below...
    if (check == success)
   {    dataReader.Close();
        return true;}
    dataReader.Close();
    return false;
}
_______________________________________
public SqlCommand BuildQuery(int caseNum)
{
string QryString = string.Empty;
SqlCommand sqlCommand = new SqlCommand(); 
sqlCommand.CommandType = CommandType.Text;
sqlCommand.Connection = con;
switch (caseNum)
{                
case 4: 
        {
            QryString = "select TblXml from MainTbl with (nolock) "+
                        "where TrnDate < @TrnDate "+
                        "and TxtID = @TxtID "+
                        "and TxtKey = @TxtKey " +
                        "and ActiveStatus != 'NonActive' and ElemName = 'xyz' order by TrnDate desc";
            break;
        }
 default:
        { break; }
}
sqlCommand.CommandText = QryString;
sqlCommand.CommandTimeout = 800;
return sqlCommand;
}

sqlCommand.Parameters.Add("@TrnDate", SqlDbType.VarChar).Value = TrnDate;
sqlCommand.Parameters.Add("@TxtID", SqlDbType.VarChar).Value = TxtID;
sqlCommand.Parameters.Add("@TxtKey", SqlDbType.VarChar).Value = TxtKey;