VB .NET

Here is a complete program sample using VISA in VB .NET and in C#.  The sample program is written for the Agilent PSA and ESA series spectrum analyzers.  It stores the current screen image on the instrument's flash as C:PICTURE.GIF.  It then transfers the image over GPIB or LAN and stores the image on your PC in the current directory as picture.gif.  The file C:PICTURE.GIF is then deleted from the instrument's flash.  

This sample demonstrates reading arrays, error handling, and basic session tasks.

VB .NET

Private Sub RunTutorial(ByVal instrAddress As String)
    ' Declare Variables used in the program
    Dim status As Integer      'VISA function status return code
    Dim defrm As Integer = 0   'Session to Default Resource Manager
    Dim vi As Integer = 0      'Session to instrument
    Dim x As Integer        'Loop Variable
    Dim ResultsArray(50000) As Byte 'results array, big enough to hold a GIF
    Dim length As Integer      'Number of bytes returned from instrument
    Dim headerlength As Integer 'length of header
    ' the file to write the picture
    Dim fs As System.IO.FileStream = Nothing
    'Set the default number of bytes that will be contained in the
    'ResultsArray to 50,000 (50kB)
    length = 50000
    Try
        If System.IO.File.Exists("picture.gif") Then
            System.IO.File.Delete("picture.gif")
        End If
        ' Open the default resource manager session
        status = visa32.viOpenDefaultRM(defrm)
        ' Open the session.  For GPIB, the address string looks like: 
        '       GPIB0::18::INSTR
        ' For PSA, to use LAN, change the string to
        ' "TCPIP0::xxx.xxx.xxx.xxx::inst0::INSTR" where 
        ' xxxxx is the IP address
        status = visa32.viOpen(defrm, "instrAddress", 0, 0, vi)
        CheckStatus(defrm, status)
        ' Set the I/O timeout to fifteen seconds
        status = visa32.viSetAttribute(vi, visa32.VI_ATTR_TMO_VALUE, 15000)
        CheckStatus(vi, status)
        'Store the current screen image on flash as C:PICTURE.GIF
        status = visa32.viPrintf(vi, ":MMEM:STOR:SCR 'C:PICTURE.GIF'" & vbLf)
        CheckStatus(vi, status)
        'Grab the screen image file from the instrument
        status = visa32.viPrintf(vi, ":MMEM:DATA? 'C:PICTURE.GIF'" & vbLf)
        CheckStatus(vi, status)
	' We're reading this as raw binary, although it is a IEEE 488.2
	' binary block containing byte data.  We could've used
	' "%#b" format string, and the byte array would not contain the
	' IEEE binary block header.
        status = visa32.viScanf(vi, "%#y", length, ResultsArray)
        CheckStatus(vi, status)
        'Delete the tempory file on the flash named C:PICTURE.GIF
        status = visa32.viPrintf(vi, ":MMEM:DEL 'C:PICTURE.GIF'" & vbLf)
        CheckStatus(vi, status)
        'Close the vi session and the resource manager session
        Call visa32.viClose(vi)
        vi = 0
        Call visa32.viClose(defrm)
        defrm = 0
        'Store the results in a text file
        fs = _
                    New System.IO.FileStream("picture.gif", _
                    IO.FileMode.OpenOrCreate)
        Dim zeroVal() As Char = {"0"}
        Dim zeroValByte() As Byte
        zeroValByte = System.Text.Encoding.ASCII.GetBytes(zeroVal)
        headerlength = ResultsArray(1) - zeroValByte(0) + 2
        fs.Write(ResultsArray, headerlength, length - 2 - headerlength)
    Catch err As System.ApplicationException
        MsgBox("*** Error : " & err.Message, vbExclamation, _
                                    "VISA Error Message")
        Exit Sub
    Catch err As System.SystemException
        MsgBox("*** Error : " & err.Message, vbExclamation, _
                                    "System Error Message")
        Exit Sub
    Catch err As System.Exception
        Debug.Fail("Unexpected Error")
        MsgBox("*** Error : " & err.Message, vbExclamation, _
                                    "Unexpected Error")
        Exit Sub
    Finally
        If Not fs Is Nothing Then fs.Close()
        If vi <> 0 Then
            Call visa32.viClose(vi)
        End If
        If defrm <> 0 Then
            Call visa32.viClose(defrm)
        End If
    End Try
End Sub
Private Sub CheckStatus(ByVal vi As Integer, ByVal status As Integer)
    If (status < visa32.VI_SUCCESS) Then
        Dim err As System.Text.StringBuilder = New System.Text.StringBuilder(256)
        visa32.viStatusDesc(vi, status, err)
        Throw New ApplicationException(err.ToString())
    End If
End Sub

C#

private void RunTutorial(string instrAddress)
{
	// Declare Variables used in the program
	int status;		// VISA function status return code
	int defrm = 0;	// Session to Default Resource Manager
	int vi = 0;		// Session to instrument
	// results array, big enough to hold a GIF
	byte[] ResultsArray = new byte[50000]; 
	int length;		// Number of bytes returned from instrument
	int headerlength;	// length of header
	System.IO.FileStream fs = null;  // the file to write the picture
	// Set the default number of bytes that will be contained in the
	// ResultsArray to 50,000 (50kB)
	length = 50000;
	try
	{
		if (System.IO.File.Exists("picture.gif"))
			System.IO.File.Delete("picture.gif");
		// Open the default resource manager session
		status = visa32.viOpenDefaultRM(out defrm);
		// Open the session.  For GPIB, the address string looks like: 
		//       GPIB0::18::INSTR
		// For PSA, to use LAN, change the string to
		// "TCPIP0::xxx.xxx.xxx.xxx::inst0::INSTR" where 
		// xxxxx is the IP address
		status = visa32.viOpen(defrm, "instrAddress", 0, 0, out vi);
		CheckStatus(defrm, status);
		// Set the I/O timeout to fifteen seconds
		status = visa32.viSetAttribute(vi, 
					visa32.VI_ATTR_TMO_VALUE, 15000);
		CheckStatus(vi, status);
		//Store the current screen image on flash as C:PICTURE.GIF
		status = visa32.viPrintf(vi, 
					 ":MMEM:STOR:SCR 'C:PICTURE.GIF'\n");
		CheckStatus(vi, status);
		//Grab the screen image file from the instrument
		status = visa32.viPrintf(vi, ":MMEM:DATA? 'C:PICTURE.GIF'\n");
		CheckStatus(vi, status);
		// We're reading this as raw binary, although it is a IEEE 488.2
		// binary block containing byte data.  We could've used
		// "%#b" format string, and the byte array would not contain the
		// IEEE binary block header.
		status = visa32.viScanf(vi, "%#y", ref length, ResultsArray);
		CheckStatus(vi, status);
		//Delete the tempory file on the flash named C:PICTURE.GIF
		status = visa32.viPrintf(vi, ":MMEM:DEL 'C:PICTURE.GIF'\n");
		CheckStatus(vi, status);
		//Close the vi session and the resource manager session
		visa32.viClose(vi);
		vi = 0;
		visa32.viClose(defrm);
		defrm = 0;
		//Store the results in a text file
		fs = new System.IO.FileStream("picture.gif", 
			System.IO.FileMode.OpenOrCreate);
		char[] zeroVal = {'0'};
		byte[] zeroValByte;
		zeroValByte = System.Text.Encoding.ASCII.GetBytes(zeroVal);
		headerlength = ResultsArray[1] - zeroValByte[0] + 2;
		fs.Write(ResultsArray, headerlength, length - 2 - headerlength);
	}
	catch(System.ApplicationException err)
	{
		System.Windows.Forms.MessageBox.Show("*** Error : " + 
			err.Message, 
			"VISA Error Message",
			System.Windows.Forms.MessageBoxButtons.OK,
			System.Windows.Forms.MessageBoxIcon.Exclamation);
	}
	catch(System.SystemException err)
	{
		System.Windows.Forms.MessageBox.Show("*** Error : " + 
			err.Message, 
			"System Error Message",
			System.Windows.Forms.MessageBoxButtons.OK,
			System.Windows.Forms.MessageBoxIcon.Exclamation);
	}
	catch(System.Exception err)
	{
		System.Diagnostics.Debug.Fail("Unexpected Error");
		System.Windows.Forms.MessageBox.Show("*** Error : " + 
			err.Message, 
			"Unexpected Error",					
			System.Windows.Forms.MessageBoxButtons.OK,
			System.Windows.Forms.MessageBoxIcon.Exclamation);
	}
	finally
	{
		if ( fs != null )
			fs.Close();
		if (vi != 0)
			visa32.viClose(vi);
		if (defrm != 0)
			visa32.viClose(defrm);
	}
}
private void CheckStatus(int vi, int status)
{
	if (status < visa32.VI_SUCCESS)
	{
		System.Text.StringBuilder err = 
					new System.Text.StringBuilder(256);
		visa32.viStatusDesc(vi, status, err);
		throw new ApplicationException(err.ToString());
	}
} 

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