VB.NET バイト単位でのLeft・Right・Mid・Len関数

VB.NETで文字列をバイト単位で扱う場合、は関数を自作する必要があります。以下に関数の説明をします。

バイト単位で文字列を扱う場合は、Encodingクラスを使用します。

以下にバイト単で文字列を扱う、Left・Right・Mid・Len関数を記載します。

LeftB関数

文字列の左側より指定したバイト数文字を取得します。

Public Shared Function LeftB(ByVal str As String, ByVal byteCount As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")
    Dim btBytes As Byte() = hEncode.GetBytes(str)
        
    If byteCount <= btBytes.Length Then
        Return hEncode.GetString(btBytes, 0, byteCount)
    End If
        
    Return str
End Function

Public Shared Function LeftB(ByVal str As String, ByVal byteCount As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim btBytes As Byte() = hEncode.GetBytes(str)

If byteCount <= btBytes.Length Then

Return hEncode.GetString(btBytes, 0, byteCount)

End If

Return str

End Function

RigthB関数

文字列の右側より指定したバイト数文字を取得します。

Public Shared Function RightB(ByVal str As String, ByVal byteCount As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")
    Dim btBytes As Byte() = hEncode.GetBytes(str)

    If byteCount <= btBytes.Length Then
        Return hEncode.GetString(btBytes, btBytes.Length - byteCount, byteCount)
    End If

    Return str
End Function

Public Shared Function RightB(ByVal str As String, ByVal byteCount As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim btBytes As Byte() = hEncode.GetBytes(str)

If byteCount <= btBytes.Length Then

Return hEncode.GetString(btBytes, btBytes.Length - byteCount, byteCount)

End If

Return str

End Function

MidB関数

文字列の指定位置より指定バイト数文字を取得します。

Public Shared Overloads Function MidB(ByVal str As String, ByVal startindex As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")
    Dim btBytes As Byte() = hEncode.GetBytes(str)

    If startindex <= btBytes.Length Then
        Dim byteCount As Integer = btBytes.Length - startindex + 1
        Return hEncode.GetString(btBytes, startindex - 1, byteCount)
    End If

    Return String.Empty
End Function

Public Shared Overloads Function MidB(ByVal str As String, ByVal startindex As Integer, ByVal byteCount As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")
    Dim btBytes As Byte() = hEncode.GetBytes(str)

    If startindex <= btBytes.Length Then
        If (btBytes.Length - startindex) < byteCount Then
            byteCount = btBytes.Length - startindex + 1
        End If
        Return hEncode.GetString(btBytes, startindex - 1, byteCount)
    End If

    Return String.Empty
End Function

Public Shared Overloads Function MidB(ByVal str As String, ByVal startindex As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim btBytes As Byte() = hEncode.GetBytes(str)

If startindex <= btBytes.Length Then

Dim byteCount As Integer = btBytes.Length - startindex + 1

Return hEncode.GetString(btBytes, startindex - 1, byteCount)

End If

Return String.Empty

End Function

Public Shared Overloads Function MidB(ByVal str As String, ByVal startindex As Integer, ByVal byteCount As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim btBytes As Byte() = hEncode.GetBytes(str)

If startindex <= btBytes.Length Then

If (btBytes.Length - startindex) < byteCount Then

byteCount = btBytes.Length - startindex + 1

End If

Return hEncode.GetString(btBytes, startindex - 1, byteCount)

End If

Return String.Empty

End Function

LenB関数

文字列のバイト数を取得します。

Public Shared Function LenB(ByVal str As String) As Integer
    Return System.Text.Encoding.GetEncoding("Shift_JIS").GetByteCount(str)
End Function

Public Shared Function LenB(ByVal str As String) As Integer

Return System.Text.Encoding.GetEncoding("Shift_JIS").GetByteCount(str)

End Function

以上の関数では、最後に取り出す文字が、全角の場合半分に切れたるするので全角がある文字列では使えない。

全角半角の混じった文字列で使用するためには、以下の関数のように、文字を数えてじみちに取り出します。

・以下の関数は、最終文字が全角の前・後半分にかかった場合、指定バイト数より１バイト少ない文字列を戻しています。

LeftBC関数

Public Shared Function LeftBC(ByVal str As String, ByVal byteCount As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

    Dim n As Integer = 0
    Dim m As Integer = 0

    Dim l As Integer = str.Length
    Dim i As Integer = 0
    Do While (i < l)
        n += hEncode.GetByteCount(str(i))
        If byteCount < n Then Exit Do
        m += 1
        i += 1
    Loop

    Return Strings.Left(str, m)
End Function

Public Shared Function LeftBC(ByVal str As String, ByVal byteCount As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim n As Integer = 0

Dim m As Integer = 0

Dim l As Integer = str.Length

Dim i As Integer = 0

Do While (i < l)

n += hEncode.GetByteCount(str(i))

If byteCount < n Then Exit Do

m += 1

i += 1

Loop

Return Strings.Left(str, m)

End Function

RightBC関数

Public Shared Function RightBC(ByVal str As String, ByVal byteCount As Integer) As String
    Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

    Dim n As Integer = 0
    Dim m As Integer = 0

    Dim i As Integer = str.Length - 1
    Do While (0 <= i)
        n += hEncode.GetByteCount(str(i))
        If byteCount < n Then Exit Do
        m += 1
        i -= 1
    Loop

    Return Strings.Right(str, m)
End Function

Public Shared Function RightBC(ByVal str As String, ByVal byteCount As Integer) As String

Dim hEncode As System.Text.Encoding = System.Text.Encoding.GetEncoding("Shift_JIS")

Dim n As Integer = 0

Dim m As Integer = 0

Dim i As Integer = str.Length - 1

Do While (0 <= i)

n += hEncode.GetByteCount(str(i))

If byteCount < n Then Exit Do

m += 1

i -= 1

Loop

Return Strings.Right(str, m)

End Function