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16. Database Connectivity
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Configuring Commands
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Once IDbCommand is instantiated, it must be configured using the properties detailed in Table 16-6. If IDbCommand is created via the IDbConnection.CreateCommand method, the Connection property will be set to the correct value for the current IDbConnection. The key properties for any IDbCommand are CommandText and CommandType; the text contained in CommandText must be compatible with the CommandType value.
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Table 16-6. Properties of IDbCommand
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Comments Gets or sets the text of the command to execute against the data source. The syntax of this text depends on the CommandType property. CommandTimeout Gets or sets the time (in seconds) that the executing command waits for a response before terminating and throwing an error. The default value is 30 seconds. Gets or sets the command type represented by this IDbCommand instance. CommandType Specified using the following members of the System.Data.CommandType enumeration:
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Property CommandText
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StoredProcedure command text represents the name of a stored procedure. TableDirect supported only by OleDbCommand. Command text represents one or more table names for which the entire contents should be returned. Text command text represents an SQL command.
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Connection Transaction
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The default value is Text. Gets or sets the IDbConnection instance used by this command. Gets or sets the IDbTransaction instance in which this command executes.
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Configuring an IDbCommand to execute a direct SQL statement
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When executing a direct SQL statement such as a SELECT or a DELETE command, the CommandType must be set to System.Data.CommandType.Text (the default value), and the CommandText property must be set to the text of the SQL statement. The DBDemo.DeletePeople method demonstrates the configuration of an IDbCommand to execute a SQL DELETE statement:
private void DeletePeople() { // Create the SQL command to delete all records from // the "people" table IDbCommand sqlCmd = sqlCon.CreateCommand(); sqlCmd.CommandType = CommandType.Text; sqlCmd.CommandText = "DELETE FROM people"; // Execute the DELETE command sqlCmd.ExecuteNonQuery();
16. Database Connectivity
The CommandText can also include parameter identifiers that are substituted with values at run time. The syntax for parameter substitution is implementation specific. The SQL Data Provider supports named parameters prefixed with the @ symbol, as demonstrated in the following excerpt from the DBDemo.SetAge method:
IDbCommand sqlCmd = sqlCon.CreateCommand(); sqlCmd.CommandType = CommandType.Text; sqlCmd.CommandText = "UPDATE people SET age = @age" + " WHERE name = @name";
However, the OLE Data Provider supports only positional parameter substitution using the symbol, the same as the JDBC PreparedStatement. The preceding code fragment would appear as follows if implemented using an OLE Data Provider:
IDbCommand sqlCmd = sqlCon.CreateCommand(); sqlCmd.CommandType = CommandType.Text; sqlCmd.CommandText = "UPDATE people SET age = " + " WHERE name = ";
For both positional and named parameters, an IDbDataParameter instance must be provided for each parameter included in the command string. See the "Parameters" section coming up for details.
Configuring an IDbCommand to execute a stored procedure
When calling a stored procedure, you must set CommandType to System.Data.CommandType.StoredProcedure and the CommandText must be set to the name of the stored procedure. Later in the "Parameters" section we provide an alternative DBDemo.SetAge implementation that calls a stored procedure to update the age of a person. For now, the following code fragment demonstrates the configuration of an IDbCommand to execute a stored procedure named SetAge. IDbDataParameter instances must be provided for each input and output parameter specified by the stored procedure as well as the stored procedure's return value.
// Create a command to call the Stored Procedure SetAge IDbCommand sqlCmd = sqlCon.CreateCommand(); sqlCmd.CommandType = CommandType.StoredProcedure; sqlCmd.CommandText = "SetAge";
Parameters
Parameters for use with stored procedures or for substitution into text commands are represented by IDbDataParameter instances. An IDbDataParameter object must be instantiated and configured to satisfy each parameter specified in an IDbCommand. The IDbCommand.Parameters property contains an IDataParameterCollection in which the parameter objects are placed before the command is executed. IDataParameterCollection inherits from IList, ICollection, and IEnumerable and thus provides flexibility for adding, retrieving, and manipulating the contained IDbDataParameter instances.
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