参数在DBCommand中添加导致超时

Question

I have a Parameter add function like bellow:

public static void AddParameter(DbCommand comm, string ParamName, Object objValue, DbType Paramtype,int paramSize)
{
      //comm.Parameters.Clear();
      DbParameter param = comm.CreateParameter();
      param.ParameterName = ParamName;
      param.DbType = Paramtype;
      param.Size = paramSize;
      param.Direction = ParameterDirection.Input;
      param.Value = objValue;
      comm.Parameters.Add(param);
}

Now when do the following:

conn.Open();
DbCommand cmd = conn.CreateCommand();
cmd.CommandText = "SELECT * FROM tableName WHERE MemberID=@MemberID";
AddParameter(cmd, "@MemberID", "00000970-b1fd-49ad-89fc-1de066c0076c", DbType.String, 50);
cmd.ExecuteReader();

I get Timeout exception. But if I run the same command Like

SELECT * FROM tableName WHERE MemberID='00000970-b1fd-49ad-89fc-1de066c0076c'

I get my desired output... Can anybody help me find out where is my problem? conn here is a SqlConnection. Thanks.

Answer 1

You state that the column is varchar(50) , but you are using DbType.String ; however, DbType.String is nvarchar(...) - ie unicode. Now, sometimes the server will do the conversion the "right way around" (where "right" here means: the one that doesn't kill performance), but sometimes it will make the alternative choice (for example, it might decide that it can't use the index and do a table scan, or even worse - it might decide to convert every row in turn for the comparison). The best thing to do, then, is to specify the correct data-type in the first place: by using DbType.AnsiString . There's nothing "wrong" with either DbType.String or DbType.AnsiString - so long as they are correctly being used to represent nvarchar and varchar respectively.