Tài liệu Exporting the Results of a Query as a String docx

[ Team LiB ]Recipe 5.11 Exporting the Results of a Query as a String Problem You need to export the results of a query to a string in a manner similar to the GetString method of the A

[ Team LiB ]

Recipe 5.11 Exporting the Results of a Query as a String

Problem

You need to export the results of a query to a string in a manner similar to the GetString( ) method of the ADO Recordset

Solution

Write a routine to mimic the functionality of the ADO Recordset's GetString( ) method The sample code contains an event handler and a method:

Go Button.Click

Sets up the sample by creating a DataTable containing the Orders table from Northwind The GetString( ) method in this solution is called to convert the DataTable into a string similar to one that is generated by the GetString( ) method

of the ADO Recordset The string is displayed in a text box on the form

GetString( )

This method mimics the functionality of the GetString( ) method of the ADO Recordset The method iterates over the collection of rows and columns in the table appending the field values to a string Null values are replaced as specified and column and row delimiters are applied

The C# code is shown in Example 5-13

Example 5-13 File: AdoGetStringForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Text;

using System.Data;

using System.Data.SqlClient;

// Table name constants

private const String ORDERS_TABLE = "Orders";

//

private void goButton_Click(object sender, System.EventArgs e)

{

// Fill the Order table

SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Orders", ConfigurationSettings.AppSettings["Sql_ConnectString"]);

DataTable dt = new DataTable(ORDERS_TABLE);

da.Fill(dt);

// Call method to convert the DataTable to a string

resultTextBox.Text = GetString(dt, -1, null, null, null);

}

private String GetString(DataTable dt, int numRows,

String columnDelimiter, String rowDelimiter, String nullExpr)

{

if(numRows < 0)

// Negative number of rows returns all rows

numRows = dt.Rows.Count;

else

// Set number of rows to the lesser of the user entered

// number of rows and the number of rows in the table

numRows = Math.Max(numRows, dt.Rows.Count);

// Column delimiter defaults to TAB

if(columnDelimiter == null)

columnDelimiter = "\t";

// Row delimiter defaults to CARRIAGE RETURN

if(rowDelimiter == null)

rowDelimiter = "\r";

// Null expression defaults to empty string

if(nullExpr == null)

nullExpr = "";

StringBuilder sb = new StringBuilder( );

// Iterate over the collection of rows

for(int i = 0; i < numRows; i++)

{

// Iterate over the collection of columns

foreach (object col in dt.Rows[i].ItemArray)

{

// Replace null values as they occur

String colString = (col == System.DBNull.Value) ?

nullExpr : col.ToString( );

// Add the column value to the string

sb.Append(colString + columnDelimiter);

}

// Remove the column delimiter on last field

sb.Remove(sb.Length - columnDelimiter.Length,

columnDelimiter.Length);

// Append row delimiter

sb.Append(rowDelimiter);

}

return sb.ToString( );

}

Discussion

ADO.NET does not contain a method that is equivalent to the GetString( ) method of the ADO Recordset or a method that converts the Recordset to a string

This solution presents an ADO.NET method, which is also called GetString( ), that duplicates the functionality of the ADO GetString( ) method The prototype for the ADO.NET method is:

String tableString = GetString(DataTable dt, Integer numRows, String columnDelimiter, String rowDelimiter, String nullExpr);

Parameters

tableString

Returns a string corresponding to the rows selected from the table

dt

The DataTable to convert to a string

numRows

The number of rows in the table to convert If this number is negative, all rows in the table are converted If a number larger than the number of records in the table

is specified, all records are converted without error

columnDelimiter

The character or characters that separate columns The default value is the TAB character

rowDelimiter

The character or characters that separate rows The default value is the

CARRIAGE RETURN character

nullExpr

A string that is substituted for null column values in the table The default value is

an empty string

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