在.NET Framework中，System.Convert类中的ToString方法和ToInt32（ToInt64）方法都可以方便的实现各种进制间的相互转换。
 
1、Convert .ToString 方法
 
Convert .ToString 方法可以将整数值转换为其指定基数（必须是 2、8、10 或 16）的等效字符串表示形式。即可以将十进制整数转换成二进制、八进制、十进制（无实际意义）或十六进制数字的字符串。
 Convert .ToString（value，toBase）
 参数：value——整型，要转换的整数；toBase—— System .Int32 ，返回值的基数，必须是 2、8、10 或 16。
 返回值：System .String ，以 toBase 为基的 value 的字符串表示形式。
 如：
 
        Dim int As Integer = 256
        '十进制转二进制
        Dim str2 As String = Convert.ToString(int, 2)
        '十进制转八进制
        Dim str8 As String = Convert.ToString(int, 8)
        '十进制转十进制
        Dim str10 As String = Convert.ToString(int, 10)
        '十进制转十六进制
        Dim str16 As String = Convert.ToString(int, 16)
        Console.WriteLine("十进制转二进制：" & str2)
        Console.WriteLine("十进制转八进制：" & str8)
        Console.WriteLine("十进制转十进制：" & str10)
        Console.WriteLine("十进制转十六进制：" & str16)
 
测试结果如图：
 
 
2、Convert .ToInt32（ToInt64） 方法
 
将指定基数的数字的字符串表示形式转换为等效的整数。即可以将二进制、八进制、十进制（无实际意义）或十六进制整数的字符串表示形式转换为十进制的数字（整型）。
 Convert .ToUInt64（value，fromBase）
 参数：value—— System .String ，包含要转换的数字的字符串；fromBase——System .Int32 ，value 中数字的基数，它必须是 2、8、10 或 16。
 返回值：整型，与 value 中数字等效的 整数，如果 value 为 Nothing，则为 0（零）。
 如：
 
        '二进制
        Dim str2 As String = "100000000"
        '八进制
        Dim str8 As String = "400"
        '十六进制
        Dim str16 As String = "100"

        '二进制转十进制
        Dim int2 As Integer = Convert.ToInt32(str2, 2)
        '八进制转十进制
        Dim int8 As Integer = Convert.ToInt32(str8, 8)
        '十六进制转十进制
        Dim int16 As Integer = Convert.ToInt32(str16, 16)
 
3、二进制、八进制、十进制或十六进制间任意互转（输出字符串）
 
先把字符串转换成十进制数字，再把数字转换成其他进制的字符串。
 
    Sub Main()
        '二进制
        Dim str2 As String = "100000000"
        '八进制
        Dim str8 As String = "400"
        '十六进制
        Dim str16 As String = "100"

        '二进制转十六进制
        Dim str2to16 As String = ConvertString(str2, 2, 16)
        '八进制转十六进制
        Dim str8to16 As String = ConvertString(str8, 8, 16)
        '十六进制转二进制
        Dim str16to2 As String = ConvertString(str16, 16, 2)
    End Sub
    ''' <summary>
    ''' 进制转换
    ''' </summary>
    ''' <param name="value">二进制、八进制、十进制或十六进制整数的字符串表示形式</param>
    ''' <param name="fromBase">与value值中的相对应的基数（2、8、10或16）</param>
    ''' <param name="toBase">要转换成的进制基数（2、8、10或16）</param>
    ''' <returns></returns>
    Private Function ConvertString(ByVal value As String, ByVal fromBase As Int32, ByVal toBase As Int32) As String
        Dim intValue As Integer = Convert.ToInt32(value, fromBase)

        Return Convert.ToString(intValue, toBase)
    End Function
 
4、其他进制转换方法
 
Object.ToString方法或String.Format方法可以将十进制整数转换成十六进制的数字字符串表现形式，但没有提供其他进制的转换。
 如：
 
        Dim int As Integer = 256
        Dim strS As String = int.ToString("x")
        Dim strF As String = String.Format("{0:x}", int)
 
5、进制基础知识
 
比如十进制的15，其二进制为1111，八进制为17，而16进制为F，各种进制之间是如何转换的呢？
 
（1）二进制、八进制、十六进制转十进制
 

 可见，在十进制中，每一位表示10的幂，要知道十进制位所代表的具体数字，可以将相应数字乘以10的幂，然后把结果相加。
 按照同样的方法，二进制中，每一位表示2的幂，则应将数字乘以2的幂，然后把结果相加即可得到相应的十进制数。
 依此类推，八进制和十六进制都可以按此方法转换为十进制。
 
（2）十进制转二进制、八进制、十六进制
 
方法：除相应进制的基数（如二进制的基数为2、八进制的基数为8、十六进制的基数为16）取余法，即每次将整数部分除以基数，余数为该位权上的数，而商继续除以基数，余数又为上一个位权上的数，这个步骤一直持续下去，直到商为0为止，最后读数时候，从最后一个余数读起，一直到最前面的一个余数。
 例：将十进制的(15)转换为二进制的步骤如下：
 
将商15除以2，商7余数为1；
将商7除以2，商3余数为1；
将商3除以2，商1余数为1；
将商1除以2，商0余数为1；
读数，读数字从最后的余数向前读，1111。
 
将十进制的(15)转换为八进制的步骤如下：
 1、 将商15除以8，商1余数为7；
 2、将商1除以8，商0余数为1；
 3、 读数，读数字从最后的余数向前读，17。
 
将十进制的(15)转换为十六进制的步骤如下：
 1、 将商15除以16，商0余数为15；
 2、读数，读数字从最后的余数向前读，15，十六进制用字母F表示。

 原文地址：
http://www.newxing.com/Tech/Program/VisualBasic/748.html
模块包含了下面的各种转换：


二进制转十进制
二进制转化为八进制
二进制转化为十六进制

八进制转化为十进制
八进制转化为二进制
八进制转化为十六进制

十进制转二进制
十进制转化为八进制
十进制转化为十六进制

十六进制转化为二进制
十六进制转化为八进制
十六进制转化为十进制

16进制转ASC
10进制长整数转4位16进制字符串
10进制长整数转2位16进制字符串
ASCII字符串转16进制字符串
反16进制数转10进制数，共8位
反16进制数转10进制数，共6位
反16进制数转10进制数，共4位
10进制数转反16进制数，共8位
0进制数转反16进制数，共6位
10进制数转反16进制数，共4位
记录集转二进制流
ASCII码转二进制流
二进制流转ASCII码

VBScript code复制代码'二进制转十进制
Public Function B2D(vBStr As String) As Long
     Dim vLen As Integer  '串长
     Dim vDec As Long     '结果
     Dim vG As Long       '权值
     Dim vI As Long       '位数
     Dim vTmp As String   '临时串
     Dim vN As Long       '中间值

    vLen = Len(vBStr)

    vG = 1 '初始权值
     vDec = 0   '结果初值
     B2D = vDec '返回初值

    For vI = vLen To 1 Step -1
         vTmp = Mid(vBStr, vI, 1) '取出当前位
         vN = Val(vTmp)

        If vN < 2 Then  '判断是不是合法二进制串,貌似不严谨,E文和符号会被判0而合法
             vDec = vDec + vG * vN '得到中间结果
             vG = vG + vG
         Else
             vDec = 0
             'msgbox "不是有效的二进制数",vbokonly
             Exit Function
         End If
    Next vI

    B2D = vDec
End Function

'十进制转二进制
Public Function D2B(Dec As Long) As String
     D2B = ""
     Do While Dec > 0
         D2B = Dec Mod 2 & D2B
         Dec = Dec \ 2
     Loop
End Function

' 用途：将十六进制转化为二进制
' 输入：Hex(十六进制数)
' 输入数据类型：String
' 输出：H2B(二进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function H2B(ByVal Hex As String) As String
     Dim i As Long
     Dim b As String
    
    Hex = UCase(Hex)
     For i = 1 To Len(Hex)
         Select Case Mid(Hex, i, 1)
             Case "0": b = b & "0000"
             Case "1": b = b & "0001"
             Case "2": b = b & "0010"
             Case "3": b = b & "0011"
             Case "4": b = b & "0100"
             Case "5": b = b & "0101"
             Case "6": b = b & "0110"
             Case "7": b = b & "0111"
             Case "8": b = b & "1000"
             Case "9": b = b & "1001"
             Case "A": b = b & "1010"
             Case "B": b = b & "1011"
             Case "C": b = b & "1100"
             Case "D": b = b & "1101"
             Case "E": b = b & "1110"
             Case "F": b = b & "1111"
         End Select
     Next i
     While Left(b, 1) = "0"
         b = Right(b, Len(b) - 1)
     Wend
     H2B = b
End Function

' 用途：将二进制转化为十六进制
' 输入：Bin(二进制数)
' 输入数据类型：String
' 输出：B2H(十六进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function B2H(ByVal Bin As String) As String
     Dim i As Long
     Dim H As String
     If Len(Bin) Mod 4 <> 0 Then
         Bin = String(4 - Len(Bin) Mod 4, "0") & Bin
     End If
    
    For i = 1 To Len(Bin) Step 4
         Select Case Mid(Bin, i, 4)
             Case "0000": H = H & "0"
             Case "0001": H = H & "1"
             Case "0010": H = H & "2"
             Case "0011": H = H & "3"
             Case "0100": H = H & "4"
             Case "0101": H = H & "5"
             Case "0110": H = H & "6"
             Case "0111": H = H & "7"
             Case "1000": H = H & "8"
             Case "1001": H = H & "9"
             Case "1010": H = H & "A"
             Case "1011": H = H & "B"
             Case "1100": H = H & "C"
             Case "1101": H = H & "D"
             Case "1110": H = H & "E"
             Case "1111": H = H & "F"
         End Select
     Next i
     While Left(H, 1) = "0"
         H = Right(H, Len(H) - 1)
     Wend
     B2H = H
End Function

' 用途：将十六进制转化为十进制
' 输入：Hex(十六进制数)
' 输入数据类型：String
' 输出：H2D(十进制数)
' 输出数据类型：Long
' 输入的最大数为7FFFFFFF,输出的最大数为2147483647
Public Function H2D(ByVal Hex As String) As Long
     Dim i As Long
     Dim b As Long
    
    Hex = UCase(Hex)
     For i = 1 To Len(Hex)
         Select Case Mid(Hex, Len(Hex) - i + 1, 1)
             Case "0": b = b + 16 ^ (i - 1) * 0
             Case "1": b = b + 16 ^ (i - 1) * 1
             Case "2": b = b + 16 ^ (i - 1) * 2
             Case "3": b = b + 16 ^ (i - 1) * 3
             Case "4": b = b + 16 ^ (i - 1) * 4
             Case "5": b = b + 16 ^ (i - 1) * 5
             Case "6": b = b + 16 ^ (i - 1) * 6
             Case "7": b = b + 16 ^ (i - 1) * 7
             Case "8": b = b + 16 ^ (i - 1) * 8
             Case "9": b = b + 16 ^ (i - 1) * 9
             Case "A": b = b + 16 ^ (i - 1) * 10
             Case "B": b = b + 16 ^ (i - 1) * 11
             Case "C": b = b + 16 ^ (i - 1) * 12
             Case "D": b = b + 16 ^ (i - 1) * 13
             Case "E": b = b + 16 ^ (i - 1) * 14
             Case "F": b = b + 16 ^ (i - 1) * 15
         End Select
     Next i
     H2D = b
End Function

' 用途：将十进制转化为十六进制
' 输入：Dec(十进制数)
' 输入数据类型：Long
' 输出：D2H(十六进制数)
' 输出数据类型：String
' 输入的最大数为2147483647,输出最大数为7FFFFFFF
Public Function D2H(Dec As Long) As String
     Dim a As String
     D2H = ""
     Do While Dec > 0
         a = CStr(Dec Mod 16)
         Select Case a
             Case "10": a = "A"
             Case "11": a = "B"
             Case "12": a = "C"
             Case "13": a = "D"
             Case "14": a = "E"
             Case "15": a = "F"
         End Select
         D2H = a & D2H
         Dec = Dec \ 16
     Loop
End Function

' 用途：将十进制转化为八进制
' 输入：Dec(十进制数)
' 输入数据类型：Long
' 输出：D2O(八进制数)
' 输出数据类型：String
' 输入的最大数为2147483647,输出最大数为17777777777
Public Function D2O(Dec As Long) As String
     D2O = ""
     Do While Dec > 0
         D2O = Dec Mod 8 & D2O
         Dec = Dec \ 8
     Loop
End Function

' 用途：将八进制转化为十进制
' 输入：Oct(八进制数)
' 输入数据类型：String
' 输出：O2D(十进制数)
' 输出数据类型：Long
' 输入的最大数为17777777777,输出的最大数为2147483647
Public Function O2D(ByVal Oct As String) As Long
     Dim i As Long
     Dim b As Long
    
    For i = 1 To Len(Oct)
         Select Case Mid(Oct, Len(Oct) - i + 1, 1)
             Case "0": b = b + 8 ^ (i - 1) * 0
             Case "1": b = b + 8 ^ (i - 1) * 1
             Case "2": b = b + 8 ^ (i - 1) * 2
             Case "3": b = b + 8 ^ (i - 1) * 3
             Case "4": b = b + 8 ^ (i - 1) * 4
             Case "5": b = b + 8 ^ (i - 1) * 5
             Case "6": b = b + 8 ^ (i - 1) * 6
             Case "7": b = b + 8 ^ (i - 1) * 7
         End Select
     Next i
     O2D = b
End Function

' 用途：将二进制转化为八进制
' 输入：Bin(二进制数)
' 输入数据类型：String
' 输出：B2O(八进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function B2O(ByVal Bin As String) As String
     Dim i As Long
     Dim H As String
     If Len(Bin) Mod 3 <> 0 Then
         Bin = String(3 - Len(Bin) Mod 3, "0") & Bin
     End If
    
    For i = 1 To Len(Bin) Step 3
         Select Case Mid(Bin, i, 3)
             Case "000": H = H & "0"
             Case "001": H = H & "1"
             Case "010": H = H & "2"
             Case "011": H = H & "3"
             Case "100": H = H & "4"
             Case "101": H = H & "5"
             Case "110": H = H & "6"
             Case "111": H = H & "7"
         End Select
     Next i
     While Left(H, 1) = "0"
         H = Right(H, Len(H) - 1)
     Wend
     B2O = H
End Function

' 用途：将八进制转化为二进制
' 输入：Oct(八进制数)
' 输入数据类型：String
' 输出：O2B(二进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function O2B(ByVal Oct As String) As String
     Dim i As Long
     Dim b As String
    
    For i = 1 To Len(Oct)
         Select Case Mid(Oct, i, 1)
             Case "0": b = b & "000"
             Case "1": b = b & "001"
             Case "2": b = b & "010"
             Case "3": b = b & "011"
             Case "4": b = b & "100"
             Case "5": b = b & "101"
             Case "6": b = b & "110"
             Case "7": b = b & "111"
         End Select
     Next i
     While Left(b, 1) = "0"
         b = Right(b, Len(b) - 1)
     Wend
     O2B = b
End Function

' 用途：将八进制转化为十六进制
' 输入：Oct(八进制数)
' 输入数据类型：String
' 输出：O2H(十六进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function O2H(ByVal Oct As String) As String
     Dim Bin As String
     Bin = O2B(Oct)
     O2H = B2H(Bin)
End Function

' 用途：将十六进制转化为八进制
' 输入：Hex(十六进制数)
' 输入数据类型：String
' 输出：H2O(八进制数)
' 输出数据类型：String
' 输入的最大数为2147483647个字符
Public Function H2O(ByVal Hex As String) As String
     Dim Bin As String
     Hex = UCase(Hex)
     Bin = H2B(Hex)
     H2O = B2O(Bin)
End Function

'====================================================

'16进制转ASC
Function H2A(InputData As String) As String
  Dim mydata
  mydata = Chr(Val("&H" & InputData))
  H2A = mydata
  Exit Function
End Function

'10进制长整数转4位16进制字符串
Function S2H(Num As Long) As String
Dim mynum As String
mynum = Hex(Num)
If Len(mynum) = 1 Then mynum = "000" + mynum
If Len(mynum) = 2 Then mynum = "00" + mynum
If Len(mynum) = 3 Then mynum = "0" + Left(mynum, 2) + Right(mynum, 1)
If Len(mynum) = 4 Then mynum = Right(mynum, 2) + Left(mynum, 2)
S2H = mynum
End Function

'10进制长整数转2位16进制字符串
Function S2H2(Num As Long) As String
Dim mynum As String
mynum = Hex(Num)
If Len(mynum) = 1 Then mynum = "0" + mynum
S2H2 = mynum
End Function

'ASCII字符串转16进制字符串
Public Function A2H(str As String) As String
Dim strlen As Integer
Dim i As Integer
Dim mystr As String
mystr = ""
strlen = Len(str)
For i = 1 To strlen Step 1
mystr = mystr + Hex$(Asc(Mid(str, i, 1)))
Next i
A2H = mystr
End Function

'=====================================================
'进制反转
'=====================================================

'反16进制数转10进制数，共8位
Function FHexToInt(ByVal str As String) As String
    Dim text1 As String
    text1 = str
    Dim text2 As String
    text2 = Mid(text1, 7, 2)
    Dim text3 As String
    text3 = Mid(text1, 5, 2)
    Dim text4 As String
    text4 = Mid(text1, 3, 2)
    Dim text5 As String
    text5 = Mid(text1, 1, 2)
    FHexToInt = Val("&H" & text2 & text3 & text4 & text5)
    Exit Function
End Function
'反16进制数转10进制数，共6位
Function FHexToInt6(ByVal str As String) As String
    Dim text1 As String
    text1 = str
    Dim text2 As String
    text2 = Mid(text1, 5, 2)
    Dim text4 As String
    text3 = Mid(text1, 3, 2)
    Dim text5 As String
    text4 = Mid(text1, 1, 2)
    FHexToInt6 = Val("&H" & text2 & text3 & text4)
    Exit Function
End Function

'反16进制数转10进制数，共4位
Function FHexToInt4(ByVal str As String) As String
    Dim text1 As String
    text1 = str
    Dim text2 As String
    text2 = Mid(text1, 3, 2)
    Dim text4 As String
    text3 = Mid(text1, 1, 2)
    FHexToInt4 = Val("&H" & text2 & text3)
    Exit Function
End Function

'10进制数转反16进制数，共8位
Function IntToFHex(ByVal nums As Long) As String
    Dim text1 As String
    'text1 = Convert.ToString(nums, &H10)
    text1 = O2H(nums)
    If (Len(text1) = 1) Then
        text1 = ("0000000" & text1)
    End If
    If (Len(text1) = 2) Then
        text1 = ("000000" & text1)
    End If
    If (Len(text1) = 3) Then
        text1 = ("00000" & text1)
    End If
    If (Len(text1) = 4) Then
        text1 = ("0000" & text1)
    End If
    If (Len(text1) = 5) Then
        text1 = ("000" & text1)
    End If
    If (Len(text1) = 6) Then
        text1 = ("00" & text1)
    End If
    If (Len(text1) = 7) Then
        text1 = ("0" & text1)
    End If
    Dim text2 As String
    text2 = Mid(text1, 7, 2)
    Dim text3 As String
    text3 = Mid(text1, 5, 2)
    Dim text4 As String
    text4 = Mid(text1, 3, 2)
    Dim text5 As String
    text5 = Mid(text1, 1, 2)
    IntToFHex = text2 & text3 & text4 & text5
    Exit Function
End Function
'10进制数转反16进制数，共6位
Function IntToFHex6(ByVal nums As Long) As String
    Dim text1 As String
    text1 = O2H(nums)
    If (Len(text1) = 1) Then
        text1 = ("00000" & text1)
    End If
    If (Len(text1) = 2) Then
        text1 = ("0000" & text1)
    End If
    If (Len(text1) = 3) Then
        text1 = ("000" & text1)
    End If
    If (Len(text1) = 4) Then
        text1 = ("00" & text1)
    End If
    If (Len(text1) = 5) Then
        text1 = ("0" & text1)
    End If
    Dim text2 As String
    text2 = Mid(text1, 5, 2)
    Dim text3 As String
    text3 = Mid(text1, 3, 2)
    Dim text4 As String
    text4 = Mid(text1, 1, 2)
    IntToFHex6 = text2 & text3 & text4
    Exit Function
End Function

'10进制数转反16进制数，共4位
Function IntToFHex4(ByVal nums As Long) As String
    Dim text1 As String
    text1 = O2H(nums)
    If (Len(text1) = 1) Then
        text1 = ("000" & text1)
    End If
    If (Len(text1) = 2) Then
        text1 = ("00" & text1)
    End If
    If (Len(text1) = 3) Then
        text1 = ("0" & text1)
    End If
    Dim text2 As String
    text2 = Mid(text1, 3, 2)
    Dim text3 As String
    text3 = Mid(text1, 1, 2)
    IntToFHex4 = text2 & text3
    Exit Function
End Function

'==========================================

Public Function B2S(ByVal str As Byte)
    strto = ""
    For i = 1 To LenB(str)
       If AscB(MidB(str, i, 1)) > 127 Then
           strto = strto & Chr(AscB(MidB(str, i, 1)) * 256 + AscB(MidB(str, i + 1, 1)))
           i = i + 1
       Else
           strto = strto & Chr(AscB(MidB(str, i, 1)))
       End If
    Next
    B2S = strto
End Function

Public Function V2H(ByVal sHex As String, Optional bUnicode As Boolean)
    Dim sByte As Variant
    Dim byChar() As Byte
    Dim i As Long
    sHex = Replace(sHex, vbCrLf, "")
    sByte = Split(sHex, " ")
    ReDim byChar(0 To UBound(sByte)) As Byte
    For i = 0 To UBound(sByte)
        byChar(i) = Val("&h" & sByte(i))
    Next
    If bUnicode Then
        V2H = byChar
    Else
        V2H = StrConv(byChar, vbUnicode)
    End If
End Function

'记录集转二进制流

Public Function R2B(rs As Recordset) As Variant              '记录集转换为二进制数据
    Dim objStream As Stream
    Set objStream = New Stream
    objStream.Open
    objStream.Type = adTypeBinary
    rs.Save objStream, adPersistADTG
    objStream.Position = 0
    R2B = objStream.Read()
    Set objStream = Nothing
End Function

'ASCII码转二进制流

Public Function A2B(str As String) As Variant
   Dim a() As Byte, s As String
   s = str
   a = StrConv(s, vbFromUnicode) '字符串转换为byte型 'a 是byte数组，你可以在程序中调用 ，但不能在textbox中显示。
   A2B = a
End Function

'二进制流转ASCII码

Public Function B2A(vData As Variant) As String
   Dim s As String
   s = StrConv(vData, vbUnicode) 'byte型转换为字符串
   B2A = s
End Function
 

#include <cstdio>
#include <cstring>
using namespace std;
typedef long long ll;

int main() {
    char change[20] = {'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'};
    ll x;
    scanf("%lld",&x);
    char str[110];
    int cnt = 0;
    do {
        str[cnt++] = change[x % 16];
        x /= 16;
    } while (x != 0);

    for(int i=cnt-1; i>=0; i--) {
        printf("%c",str[i]);
    }

    return 0;
}

循环把十进制转换成16进制（四位 不够四位高位补0）