DbConnection, DbCommand, dan DbException
Setelah Anda membuat DbProviderFactory dan DbConnection, Anda kemudian dapat menggunakan perintah dan pembaca data untuk mengambil data dari sumber data.
Mengambil Contoh Data
Contoh ini mengambil objek DbConnection sebagai argumen. DbCommand dibuat untuk memilih data dari tabel Kategori dengan mengatur CommandText ke pernyataan SQL SELECT. Kode mengasumsikan bahwa tabel Kategori ada di sumber data. Koneksi dibuka dan data diambil menggunakan DbDataReader.
// Takes a DbConnection and creates a DbCommand to retrieve data
// from the Categories table by executing a DbDataReader.
static void DbCommandSelect(DbConnection connection)
{
const string queryString =
"SELECT CategoryID, CategoryName FROM Categories";
// Check for valid DbConnection.
if (connection != null)
{
using (connection)
{
try
{
// Create the command.
DbCommand command = connection.CreateCommand();
command.CommandText = queryString;
command.CommandType = CommandType.Text;
// Open the connection.
connection.Open();
// Retrieve the data.
DbDataReader reader = command.ExecuteReader();
while (reader.Read())
{
Console.WriteLine("{0}. {1}", reader[0], reader[1]);
}
}
catch (Exception ex)
{
Console.WriteLine("Exception.Message: {0}", ex.Message);
}
}
}
else
{
Console.WriteLine("Failed: DbConnection is null.");
}
}
' Takes a DbConnection and creates a DbCommand to retrieve data
' from the Categories table by executing a DbDataReader.
Private Shared Sub DbCommandSelect(ByVal connection As DbConnection)
Dim queryString As String = _
"SELECT CategoryID, CategoryName FROM Categories"
' Check for valid DbConnection.
If Not connection Is Nothing Then
Using connection
Try
' Create the command.
Dim command As DbCommand = connection.CreateCommand()
command.CommandText = queryString
command.CommandType = CommandType.Text
' Open the connection.
connection.Open()
' Retrieve the data.
Dim reader As DbDataReader = command.ExecuteReader()
Do While reader.Read()
Console.WriteLine("{0}. {1}", reader(0), reader(1))
Loop
Catch ex As Exception
Console.WriteLine("Exception.Message: {0}", ex.Message)
End Try
End Using
Else
Console.WriteLine("Failed: DbConnection is Nothing.")
End If
End Sub
Menjalankan Contoh Perintah
Contoh ini mengambil objek DbConnection sebagai argumen. Jika DbConnection valid, koneksi dibuka dan DbCommand dibuat dan dijalankan. CommandText diatur ke pernyataan SQL INSERT yang melakukan penyisipan ke tabel Kategori di database Northwind. Kode mengasumsikan bahwa database Northwind ada di sumber data, dan sintaks SQL yang digunakan dalam pernyataan INSERT valid untuk penyedia yang ditentukan. Kesalahan yang terjadi pada sumber data ditangani oleh blok kode DbException, dan semua pengecualian lainnya ditangani di blok Exception.
// Takes a DbConnection, creates and executes a DbCommand.
// Assumes SQL INSERT syntax is supported by provider.
static void ExecuteDbCommand(DbConnection connection)
{
// Check for valid DbConnection object.
if (connection != null)
{
using (connection)
{
try
{
// Open the connection.
connection.Open();
// Create and execute the DbCommand.
DbCommand command = connection.CreateCommand();
command.CommandText =
"INSERT INTO Categories (CategoryName) VALUES ('Low Carb')";
var rows = command.ExecuteNonQuery();
// Display number of rows inserted.
Console.WriteLine("Inserted {0} rows.", rows);
}
// Handle data errors.
catch (DbException exDb)
{
Console.WriteLine("DbException.GetType: {0}", exDb.GetType());
Console.WriteLine("DbException.Source: {0}", exDb.Source);
Console.WriteLine("DbException.ErrorCode: {0}", exDb.ErrorCode);
Console.WriteLine("DbException.Message: {0}", exDb.Message);
}
// Handle all other exceptions.
catch (Exception ex)
{
Console.WriteLine("Exception.Message: {0}", ex.Message);
}
}
}
else
{
Console.WriteLine("Failed: DbConnection is null.");
}
}
' Takes a DbConnection and executes an INSERT statement.
' Assumes SQL INSERT syntax is supported by provider.
Private Shared Sub ExecuteDbCommand(ByVal connection As DbConnection)
' Check for valid DbConnection object.
If Not connection Is Nothing Then
Using connection
Try
' Open the connection.
connection.Open()
' Create and execute the DbCommand.
Dim command As DbCommand = connection.CreateCommand()
command.CommandText = _
"INSERT INTO Categories (CategoryName) VALUES ('Low Carb')"
Dim rows As Integer = command.ExecuteNonQuery()
' Display number of rows inserted.
Console.WriteLine("Inserted {0} rows.", rows)
' Handle data errors.
Catch exDb As DbException
Console.WriteLine("DbException.GetType: {0}", exDb.GetType())
Console.WriteLine("DbException.Source: {0}", exDb.Source)
Console.WriteLine("DbException.ErrorCode: {0}", exDb.ErrorCode)
Console.WriteLine("DbException.Message: {0}", exDb.Message)
' Handle all other exceptions.
Catch ex As Exception
Console.WriteLine("Exception.Message: {0}", ex.Message)
End Try
End Using
Else
Console.WriteLine("Failed: DbConnection is Nothing.")
End If
End Sub
Menangani Kesalahan Data dengan DbException
Kelas DbException adalah kelas dasar untuk semua pengecualian yang diberikan atas nama sumber data. Anda dapat menggunakannya dalam kode penanganan pengecualian untuk menangani pengecualian yang diberikan oleh penyedia yang berbeda tanpa harus merujuk kelas pengecualian tertentu. Fragmen kode berikut menunjukkan cara menggunakan DbException untuk menampilkan informasi kesalahan yang dikembalikan oleh sumber data menggunakan properti GetType, Source, ErrorCode, dan Message. Output akan menampilkan jenis kesalahan, sumber yang menunjukkan nama penyedia, kode kesalahan, dan pesan yang terkait dengan kesalahan.
Try
' Do work here.
Catch ex As DbException
' Display information about the exception.
Console.WriteLine("GetType: {0}", ex.GetType())
Console.WriteLine("Source: {0}", ex.Source)
Console.WriteLine("ErrorCode: {0}", ex.ErrorCode)
Console.WriteLine("Message: {0}", ex.Message)
Finally
' Perform cleanup here.
End Try
try
{
// Do work here.
}
catch (DbException ex)
{
// Display information about the exception.
Console.WriteLine("GetType: {0}", ex.GetType());
Console.WriteLine("Source: {0}", ex.Source);
Console.WriteLine("ErrorCode: {0}", ex.ErrorCode);
Console.WriteLine("Message: {0}", ex.Message);
}
finally
{
// Perform cleanup here.
}