Reading and Writing Registry Values with Visual Basic
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Retrieving Unknown Registry Values
Very frequently, we're even less sure of the basic items of information that we need to access Registry values. Possibly we don't know the precise number and names of the values we want, or we don't know their precise data type, or we have no real idea of how long they are. This is frequently the case, for instance, when working with most recently used (MRU) lists, which often are stored in the Registry. Although there are a number of implementations for MRUs, most commonly the name of a file is assigned a simple, one-letter value name (
c, and the like), and an
MRUList named value keeps track of the entries by simply listing their names in order. For instance, the string stored to
MRUList might be
dcabe, indicating that the value stored to d was most recently used, followed by the value stored to
c, and so on. In our case, we'll build a routine that reads the MRU list for the Windows Run dialog, which is accessed by selecting the Run option from the Start menu.
When our uncertainly about Registry values extends to most of the values of a key, as it does in this case, we can begin by calling
RegQueryInfoKey to collect three useful items of information about the key's values. Its syntax is:
Public Declare Function RegQueryInfoKey Lib "advapi32.dll" Alias "RegQueryInfoKeyA" ( _ ByVal hKey As Long, _ ByVal lpClass As String, _ lpcbClass As Long, _ ByVal lpReserved As Long, _ lpcSubKeys As Long, _ lpcbMaxSubKeyLen As Long, _ lpcbMaxClassLen As Long, _ lpcValues As Long, _ ' Number of values lpcbMaxValueNameLen As Long, _ ' Length of longest value name lpcbMaxValueLen As Long, _ ' Length of longest value data lpcbSecurityDescriptor As Long, _ lpftLastWriteTime As FILETIME _ ) As Long
We can substitute a null or a null string (depending on the data type) for arguments in which we're not interested. The function tells us the following:
- How many values the key contains, including the unnamed value
- The number of characters in the longest named value, not including a terminating null character
- The number of bytes of data, not including a possible terminating null character
In the case of the MRU list, calling
RegQueryInfoKey once can save us from having to call
RegQueryValueEx twice for each value, since we don't know the amount of data that each value contains.
In building our MRU list routine, once we know the longest value name and the size of the buffer we need to create, we can begin to read values in either of two ways. One way (which we won't use here) is to retrieve all of the values in the key by calling
RegEnumValue. Its syntax is:
Public Declare Function RegEnumValue Lib "advapi32.dll" Alias "RegEnumValueA" ( _ ByVal hKey As Long, _ ' Handle to open key ByVal dwIndex As Long, _ ' Index (0 to total - 1 ByVal lpValueName As String, _ ' name of value lpcbValueName As Long, _ ' kength of value name ByVal lpReserved As Long, _ ' reserved, must be 0 lpType As Long, _ ' data type lpData As Byte, _ ' value data lpcbData As Long _ ' length of value data ) As Long
The major disadvantage of
RegEnumValue is that it takes a byte array as the
lpdata argument, and so requires that we reassemble our data from the byte array. While this is easy to do, it offers horrendous performance for keys with large numbers of values or for values with large amounts of data. A better alternative is to call
RegEnumValue to get the value's name and, if necessary, its data type, and then to use this information to call
RegQueryValueEx. The following code fragment illustrates this:
' Open registry key RegOpenKeyEx HKEY_CURRENT_USER, sPath, 0, KEY_READ, hKey ' Collect information on key's values RegQueryInfoKey hKey, "", 0, 0, 0, 0, 0, nValues, lValueName, lValueData, 0, ft ' Enumerate values For nCtr = 0 To nValues - 1 ' Reinitialize buffer on each iteration lName = lValueName + 1 sName = Space(lName) ' Get enumerated value RegEnumValue hKey, nCtr, sName, lName, 0, lType, ByVal 0, 0 ' Trim value name sName = Left(sName, lName) ' If necessary, handle data types ldata = lValueData + 1 sData = Space(ldata) ' Get data RegQueryValueEx hKey, sName, 0, 0, ByVal sData, ldata ' Trim string data sData = Left(sData, ldata) ' Process value name and data nCtr = nCtr + 1 Next RegCloseKey hKey
However, let's take a look at what we know about the values we want to retrieve. We know that they are all strings (of type
REG_SZ). Once we call
RegQueryInfoKey, we can determine how large a buffer must be allocated to accommodate the data. And we know the name of one value,
MRUList, whose data is an ordered list of the names of the values that we want to retrieve. This means that we can omit enumerating the values and implement our routine by calling
RegQueryValueEx for each key that we want to retrieve. The code appears as follows:
Public Sub ShowRunMRU() Dim iCtr As Integer Dim hKey As Long Dim nValues As Long, lValueData As Long Dim lData As Long Dim sPath As String Dim sList As String Dim sMsg As String Dim sData As String Dim sName As String Dim ft As FILETIME sPath = "Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU" ' Open registry key RegOpenKeyEx HKEY_CURRENT_USER, sPath, 0, KEY_READ, hKey ' Collect information on key's values If RegQueryInfoKey(hKey, "", 0, 0, 0, 0, 0, nValues, 0, lValueData, _ 0, ft) <> ERROR_SUCCESS Then sMsg = "Run MRU list not found" Else ' initialize data buffer lData = lValueData + 1 sData = Space(lData) ' Retrieve MRUList Value If RegQueryValueEx(hKey, "MRUList", 0, 0, ByVal sData, lData) <> ERROR_SUCCESS Then sMsg = "Run MRU list not found" Else ' Trim MRU list data sList = Left(sData, lData - 1) For iCtr = 1 To Len(sList) ' Get value name sName = Mid(sList, iCtr, 1) ' Reinitialize buffer on each iteration lData = lValueData + 1 sData = Space(lData) ' Get data RegQueryValueEx hKey, sName, 0, 0, ByVal sData, lData ' Trim string data sData = Left(sData, lData - 1) sMsg = sMsg & sData & vbCrLf Next End If RegCloseKey hKey End If ' Display MRU list MsgBox sMsg, vbOKOnly, "Run MRU List" End Sub
Setting a Registry Value
To write a value to the Registry, you call the
RegSetValueEx function. Its syntax is:
Public Declare Function RegSetValueEx Lib "advapi32.dll" Alias "RegSetValueExA" ( _ ByVal hKey As Long, _ ' Handle to registry key ByVal lpValueName As String, _ ' Name of value ByVal Reserved As Long, _ ' Reserved, must be 0 ByVal dwType As Long, _ ' Registry data type constant lpData As Any, _ ' Value's data ByVal cbData As Long _ ' Length in bytes of data ) As Long
The function is fairly straightforward, as far as Registry functions go. A few comments are in order, though:
If you are writing an unnamed value to the Registry,
lpValueNameshould be a null string. You also have to be careful about platform: On Windows 95/98/ME, the unnamed value must be of type
REG_SZ. On Windows NT/2000/XP, it can be of any valid type.
REG_EXPAND_SZ) must be null-terminated. In string arrays (
REG_MULTI_SZ), each array element must be null-terminated, and the array as a whole must be terminated with two nulls (one for the last element of the array, and one for the array itself). Visual Basic, however, takes care of adding the final terminating null when we pass the string by value.
cbData, the count of bytes in the data, must include the terminating null character or characters for Registry string data.
The function will create a new value if one does not already exist. If it does exist, it will overwrite the old value with the new one. Because of the possibility of accidentally overwriting data, in some cases you may want to call
RegQueryValueExto check whether a Registry value exists.
The following code illustrates the use of the major Registry value types used by developers.
' Defined to save as REG_BINARY data Public Type AppFontInfo Name As String * 15 Size As Integer Bold As Boolean Italic As Boolean End Type Public Sub WriteRegistryValues() Dim hKey As Long Dim lDisp As Long Dim sPath As String Dim sa As SECURITY_ATTRIBUTES Dim sValueName As String sPath = "Software\MyCompany\MyApp\Settings" If RegCreateKeyEx(HKEY_LOCAL_MACHINE, sPath, 0, vbNullString, REG_OPTION_NON_VOLATILE, _ KEY_ALL_ACCESS, sa, hKey, lDisp) <> ERROR_SUCCESS Then MsgBox "Unable to create registry key." Exit Sub End If ' Create a REG_DWORD value Dim dword As Long sValueName = "MRUItems" dword = 5 RegSetValueEx hKey, sValueName, 0, REG_DWORD, dword, Len(dword) ' Create a REG_BINARY value Dim fi As AppFontInfo fi.Name = "Arial" fi.Size = 12 fi.Bold = False fi.Italic = False sValueName = "DefaultFont" RegSetValueEx hKey, sValueName, 0, REG_BINARY, fi, Len(fi) ' Create a REG_SZ value Dim sValue As String sValue = "GraphicsViewer" sValueName = "AddIn" RegSetValueEx hKey, sValueName, 0, REG_SZ, ByVal sValue, Len(sValue) + 1 ' Create a REG_EXPAND_SZ value Dim sLibPath As String sLibPath = "%windir%\twain_32.dll" sValueName = "Library" RegSetValueEx hKey, sValueName, 0, REG_EXPAND_SZ, ByVal sLibPath, Len(sLibPath) + 1 'Create a REG_MULTI_SZ value Dim ColorString As String Dim Color As Variant, Colors As Variant Colors = Array("Yellow", "Black", "Purple") ' Form array string For Each Color In Colors ColorString = ColorString & Color & Chr(0) Next sValueName = "Colors" RegSetValueEx hKey, sValueName, 0, REG_MULTI_SZ, ByVal ColorString, Len(ColorString) + 1 ' Close registry key RegCloseKey hKey End Sub
Ron Petrusha is the author and coauthor of many books, including "VBScript in a Nutshell."
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