How Do I Handle Nullreferenceexception Unhandled In The Following Code

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VB.NET

Imports System.Data.SqlClient
Imports System.Linq
Imports System.Data




Public Class Form

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim conn As SqlConnection
        Dim cmd As SqlCommand
        Dim rd As SqlDataReader
        conn.ConnectionString = "Data Source=localhost;Initial Catalog=user;Integrated security=true"
        cmd.Connection = conn
        conn.Open()
        cmd.CommandText = "Select login,password from user where login='" & TextBox1.Text & "'and password='" & TextBox2.Text & "'"
        rd = cmd.ExecuteReader
        If rd.HasRows Then
            Welcome.Show()
        Else
            MsgBox("Invalid Login or Password")
        End If

    End Sub
End Class

Posted 23-Jan-16 23:33pm

Member 11910983

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Kornfeld Eliyahu Peter 24-Jan-16 5:37am

Use debugger to find what is null, than decide what to do with that...

Member 11910983 24-Jan-16 5:39am

it is showing the error at connection string parameters "integrated security=true"

3 solutions

Solution 2

You have to start initializing your objects...

VB.NET

Dim conn As SqlConnection

This line creates a 'placeholder' for an object of type SqlConnection, but no real object created so the 'placeholder' is empty - null!!!

VB.NET

Dim conn As New SqlConnection

For more info: SqlConnection Constructor (String) (System.Data.SqlClient)[^]

Posted 23-Jan-16 23:44pm

Kornfeld Eliyahu Peter

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Sascha Lefèvre · Accepted Answer · 2016-01-23T23:43:00

You declared conn and cmd as variables but you didn't initialize them with instances of SqlConnection / SqlCommand. You need to either create an instance with the keyword New or use the ProviderFactory[^]-class to get an instance of them.

The latter (using the ProviderFactory) is recommended because then, if you also use either the abstract Db******-base-classes (like DbConnection, DbCommand, ..) or the interfaces (IDbConnection, IDbCommand, ..) instead of the concrete and SQL-Server-dependent classes (SqlConnection, SqlCommand, ..) you'll make your code independent of a concrete database system and will be able to change from SQL Server to some other DB system easily or at least more easily.

Also, you shouldn't use string concatenation for building SQL-statements. Your code becomes susceptible to SQL-injection and less readable. Use Parameters instead: SqlCommand.Parameters Property (System.Data.SqlClient)[^]

Thomas Daniels · Accepted Answer · 2016-01-23T23:45:00

Solution 3

After the Dim conn As SqlConnection statement, conn is still null (same for cmd and rd). You need to give conn and cmd a value by calling their constructor.

VB.NET

Dim conn As New SqlConnection()
Dim cmd As New SqlCommand()

Note: you do not have to do this for rd, because SqlDataReader has no constructor and you get the reader from ExecuteReader anyway.

Posted 23-Jan-16 23:45pm

Thomas Daniels

Comments

Member 11910983 24-Jan-16 6:14am

now i'm getting a new exception "SQLException" .But, i've already added a connection to the database

Thomas Daniels 24-Jan-16 6:15am

What's the full exception message? Note that this exception is actually not related to the NullReferenceException but to the ways you communicate with the database. I'll take a look at the error, but I cannot promise that I can find it.

Member 11910983 24-Jan-16 6:19am

Exception message:
Cannot open the database "user" requested by the login.The login failed.

Thomas Daniels 24-Jan-16 6:22am

Looks like you have to fix your connection string.

Also (unrelated), I saw in your code that you use string concatenation to create queries. Don't do this. You are vulnerable to SQL Injection. Use parameterized queries instead: http://www.dotnetperls.com/sqlparameter.

Sergey Alexandrovich Kryukov 24-Jan-16 14:15pm

5ed.
—SA

Thomas Daniels 24-Jan-16 14:27pm

Thank you.

How Do I Handle Nullreferenceexception Unhandled In The Following Code

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