The vbCrC Help Page
The vbCrC Custom control has a very simple usage. It contains three methods which are used to expose the CRC calculation routines to Visual Basic. The methods and there usage are as follows.
CrcCalc...
Usage...
Dim lRetCrc as long
lRetCrc = VbCrC1.CrcCalc("This is my String", 1)
This will assign the calculated CRC 16 value of the string 'This is my String' to the variable lRetCrc. The method returns a long value containing a calculated CRC. It accepts 2 - 3 arguments. The first being the string that you wish to calculate a CRC on, the second argument is an integer value from 0 to 3 which indicates the type of CRC calculation routine to use. The control supports four standard routines.
0 = CRC32
1 = CRC16
2 = CRC-CCITT
3 = CRC-CCITT Reversed
The third argument is optional and may contain an initial value for the CRC. If it is left blank the CRC will be initialized with the default value for the CRC routine that is specified.

CrcFileCalc...
Usage...
Dim lRetCrc as long
lRetCrc = vbCrC1.CrcFileCalc("c:\autoexec.bat", 1)
The syntax of the CrcFileCalc method is basically identical to that of the CrcCalc method except that the string that you pass the method is actually the path and name of a file to calculate a CRC on. The above example will return the CRC16 of the file c:\autoexec.bat.
CrcCustomCalc...
Usage...
Dim lRetCrc as long
lRetCrc = vbCrC1.CrcCustomCalc("This is my string", &h8408&, &hFFFF&, False, True)
The syntax of the CrcCustomCalc method is similar to that of the other two methods, however it takes some additional arguments. As was the case of the other methods it returns a long value containing the calculated CRC value. All of it's arguments are required parameters, and they are in order, the string to calculate, the polynomial to use, the seed to use, A true/false parameter setting whether you want a 32 bit return value, and a true/false parameter setting whether to use the forward, or reverse algorithm.
In the above example the string that is passed is 'This is my string'. Passing it false as the fourth parameter indicates that we will be returning a 16 bit value as the CRC, and passing it true as the fifth parameter indicates that we want to use the reversed CRC algorithm. So therefore the value returned to lRetCrc would be the reversed 16 bit CRC calculation on the string 'This is my string', using the polynomial 0x8408, and the seed of 0xFFFF.
In actuallity the sample above will return the same value as using CrcCalc with the CCITT Reversed algorithm, because the that is actually what I defined with the options I passed to CrcCustomCalc. Now if this all seems about as clear as mud to you, you may want to read the brief explanation of crc calculation on this page.

NEW The installation now also includes a dll file which can be used for applications that do not wish to have a visual representation of the control. An example of this would be an application which does not contain a Form. The DLL is still very ßeta and has not been fully tested, however it should generate the correct CRC's. The DLL can be used simply by adding a reference to it from the Project | References menu. The file should be located in the windows system directory and will be named vbcrc.dll. Once you have added the reference to the DLL it exposes the two functions in the DLL as global functions to Visual Basic, meaning that they can be used exactly as you would use Visual Basic's built in functions. The two functions included in the DLL are the CrcCalc function and the CrcFileCalc function. At this time the CrcCustomCalc has not been implemented in the DLL version of vbCrC. If this is something you would like to see please email me to let me know I can be reached via email at jhaase@iland.net