Tài liệu Passing Null Values to Parameters ppt

The sample code, as shown in Example 2-18, uses a single stored procedure: SP0214_NullParameter Accepts a single parameter and a returns one-row result set containing a single value ind

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Recipe 2.14 Passing Null Values to Parameters

Problem

You need to pass a null value to a parameter

Solution

Use the System.DbNull.Value static value

The sample code, as shown in Example 2-18, uses a single stored procedure:

SP0214_NullParameter

Accepts a single parameter and a returns one-row result set containing a single value indicating whether that input parameter was null

Example 2-18 Stored procedure: SP0214_NullParameter

CREATE PROCEDURE SP0214_NullParameter

@ValueIn int

AS

if @ValueIn is null

select 1 as IsParameterNull

else

select 0 as IsParameterNull

return 0

The sample code contains two event handlers:

Go Button.Click

Creates a stored procedure command for SP0214_NullParameter and defines its

single input parameter The input parameter is set to System.DbNull.Value if the user has checked the Null Parameter check box on the form; otherwise it is set to the value that the user has entered in the Parameter text box The stored procedure

is executed using the ExecuteScalar( ) method and the value returned indicates whether the input parameter for the stored procedure is null

Null Parameter CheckBox.CheckedChange

Clears the value of the Parameter text box if checked and enables the Parameter text box if not checked

The C# code is shown in Example 2-19

Example 2-19 File: NullParameterForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Data;

using System.Data.SqlClient;

//

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

{

// Create the connection

SqlConnection conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

// Create the stored procedure command

SqlCommand cmd = new SqlCommand("SP0214_NullParameter", conn);

cmd.CommandType = CommandType.StoredProcedure;

// Define the parameter

cmd.Parameters.Add("@ValueIn", SqlDbType.Int);

// Set the value of the paramter

if (isNullCheckBox.Checked)

cmd.Parameters[0].Value = System.DBNull.Value;

else

cmd.Parameters[0].Value = parameterTextBox.Text;

// Retrieve whether parameter null from stored procedure

try

{

conn.Open( );

bool isNullParm = Convert.ToBoolean(cmd.ExecuteScalar( ));

resultTextBox.Text = "Null Parameter = " + isNullParm;

}

catch (Exception ex)

{

resultTextBox.Text = ex.Message;

}

finally

{

conn.Close( );

}

}

private void isNullCheckBox_CheckedChanged(object sender,

System.EventArgs e)

{

if(isNullCheckBox.Checked)

parameterTextBox.Text = "";

parameterTextBox.Enabled = !isNullCheckBox.Checked;

}

Discussion

System.DBNull is not the same as null in C# or Nothing in VB.NET System.DBNull indicates that the object represents missing or nonexistent data, typically in a database, while null and Nothing indicate that an object or variable has not yet been initialized Passing a null parameter value into a stored procedure is not very different than passing any other parameter value Construct the parameter using either the Parameter constructor

or the Add( ) method of the ParameterCollection for the Command object The value is set by passing System.DBNull.Value for the value argument when constructing the

parameter or by setting the Value property of the Parameter object to

System.DBNull.Value

To test for a null value, the IsDBNull( ) method returns a Boolean value that indicates whether an object expression evaluates to System.DBNull This is the same as comparing

an object to the System.DBNull.Value using an equality operator

If the stored procedure accepts optional parameters—parameters for which a default value is specified—you can set these parameters to null in C# or Nothing in VB.NET This is not the same as setting the parameter value to System.DBNull.Value The

parameter value is set to the default value specified for the parameter

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