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cm.Parameters.Add( new SqlParameter( "@dayToCalc", SqlDbType.DateTime) ).Value = day; cm.Parameters.Add( new SqlParameter( "@total", SqlDbType.Money) ). Direction = ParameterDirection.Output; cm.Connection.Open(); cm.ExecuteNonQuery(); cm.Connection.Close(); return Convert.ToDouble(cm.Parameters["@total"].Value); } The advantage to this is that your process doesn t incur the overhead of preparing to retrieve a result set that you re not actually planning on getting back. Tabular data is a memory hog. Aside from the actual size of the data, there s a lot of metadata describing the result set that gets created in preparation for the set. This involves many allocations to the managed heap; therefore, it should be avoided when results aren t going to be processed as tabular data. Even when retrieving a single row from the database, output parameters will perform better than a select statement. Here s a stored procedure that queries on the primary key of the publishers table; given this, it will always only return a single row of data (or none). (A script for this procedure is in usp_GetPubDetails.sql in the Code10 project.) create procedure usp_GetPubDetails @pub_id char(4), @pub_name varchar(40) OUTPUT, @city varchar(20) OUTPUT, @state char(2) OUTPUT, @country varchar(30) OUTPUT as SELECT @pub_name = pub_name, @city = city, @state = state, @country = country FROM publishers WHERE (pub_id = @pub_id) Instead of incurring the overhead of using a data reader, this procedure is executed with ExecuteNonQuery, and the data is retrieved from the values of the output parameters. Here, the stored procedure is used from the custom constructor of a business object. (You ll find this business object in the app_Code\PublisherBO.cs file of the Web10 project.)
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public PublisherBO(string pubId) { SqlCommand cm = new SqlCommand( "usp_GetPubDetails", new SqlConnection(ConfigurationManager.ConnectionStrings ["localPubs"].ToString()) ); cm.CommandType = CommandType.StoredProcedure; cm.Parameters.Add( new SqlParameter( "@pub_id", SqlDbType.Char,4) ).Value = pubId; cm.Parameters.Add( new SqlParameter( "@pub_name", SqlDbType.VarChar,10) ).Direction = ParameterDirection.Output; cm.Parameters.Add( new SqlParameter( "@city", SqlDbType.VarChar,10) ).Direction = ParameterDirection.Output; cm.Parameters.Add( new SqlParameter( "@state", SqlDbType.VarChar,10) ).Direction = ParameterDirection.Output; cm.Parameters.Add( new SqlParameter( "@country", SqlDbType.VarChar,10) ).Direction = ParameterDirection.Output; cm.Connection.Open(); cm.ExecuteNonQuery(); cm.Connection.Close();
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this.pubId = this.pubName this.city = this.state = this.country }
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cm.Parameters["@pub_id"].Value.ToString(); = cm.Parameters["@pub_name"].Value.ToString(); cm.Parameters["@city"].Value.ToString(); cm.Parameters["@state"].Value.ToString(); = cm.Parameters["@country"].Value.ToString();
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When you need to retrieve a single value from a single row, an execute method for an even more specific type of query exists. This is basically a database lookup, where the select statement being used has a single column in the column list and is querying on a primary or unique column in the where clause. You can use ExecuteScalar as a programming convenience in this instance. It returns an instance of a System.Object, and, therefore, must always be cast into the return type expected. (You can find this code in app_Code\DataReaderIE.cs of the Web10 project.) public int GetBookCount(string pubid) { string sql = "select count(title_id) from titles " + "where pub_id = @pubid"; SqlCommand cm = new SqlCommand( sql, new SqlConnection(ConfigurationManager.ConnectionStrings ["localPubs"].ToString()) ); cm.Parameters.Add( new SqlParameter( "@pubid", SqlDbType.Char, 4) ).Value = pubid; cm.Connection.Open(); int count = Convert.ToInt32(cm.ExecuteScalar()); cm.Connection.Close(); return count; } A data reader should always be closed after the result set (or sets) are processed. Close the data reader before closing the Connection the reader is on. Also, be sure to understand the role and function of each of the different execute commands in the previous section, and use the one appropriate to the command being executed.
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CommandBehavior
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The ExecuteReader method has an overloaded footprint that accepts an instance of a CommandBehavior enumeration value. CommandBehavior enables you to influence the reader in many different ways. We list the different values of the CommandBehavior enumeration in Table 10-5. Table 10-5. Values of the CommandBehavior Enumeration and Their Effect on a Data Reader
Enumeration Value
CloseConnection Default KeyInfo SchemaOnly SequentialAccess SingleResult SingleRow
Meaning in Life
Causes the connection used for the data reader to be closed when the data reader is closed. The default value used when no argument is specified and does not affect the behavior of the data reader. Returns only column and primary key information. Gets only schema information from the database for the command, but executes the command against the database. Causes a forward-only cursor to be placed over the columns, so that data not accessed by the program is not marshaled to the caller. Use to inform the reader you re expecting only a single result set. Use to inform the reader you re expecting only a single row. Managed Providers may use this to optimize the performance for returning a single row.
Let s talk about a scenario where you would use one of these behaviors: SequentialAccess. By default, a data reader loads an entire row into memory at once. It doesn t expose random access to rows of data, but random access is available for the columns within a single row. When you re working with a result set that has columns that can contain a lot of data (usually because of having a column of binary information in the result set), you can access only those large chunks of data on demand by setting the CommandBehavior to SequentialAccess. When using SequentialAccess, the data reader grants forward-only access to the columns within a single row of data. If some criteria in the first few columns determine whether or not the large column will be accessed, you can use this option to avoid marshaling all of the data to the client for each and every row. There may not be many situations where this is actually necessary (hopefully you can apply the criteria at the database level and return only the binarylarge objects you need), but it s a good technique to be aware of should the situation arise.
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