How to declare a fixed-length string in VB.NET?

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醉酒成梦
醉酒成梦 2020-12-10 04:27

How do i Declare a string like this:

Dim strBuff As String * 256

in VB.NET?

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10条回答
  • 2020-12-10 04:59

    This hasn't been fully tested, but here's a class to solve this problem:

    ''' <summary>
    ''' Represents a <see cref="String" /> with a minimum
    ''' and maximum length.
    ''' </summary>
    Public Class BoundedString
    
        Private mstrValue As String
    
        ''' <summary>
        ''' The contents of this <see cref="BoundedString" />
        ''' </summary>
        Public Property Value() As String
            Get
                Return mstrValue
            End Get
    
            Set(value As String)
                If value.Length < MinLength Then
                    Throw New ArgumentException(String.Format("Provided string {0} of length {1} contains less " &
                                                              "characters than the minimum allowed length {2}.",
                                                              value, value.Length, MinLength))
                End If
    
                If value.Length > MaxLength Then
                    Throw New ArgumentException(String.Format("Provided string {0} of length {1} contains more " &
                                                              "characters than the maximum allowed length {2}.",
                                                              value, value.Length, MaxLength))
                End If
    
                If Not AllowNull AndAlso value Is Nothing Then
                    Throw New ArgumentNullException(String.Format("Provided string {0} is null, and null values " &
                                                                  "are not allowed.", value))
                End If
    
                mstrValue = value
            End Set
        End Property
    
        Private mintMinLength As Integer
        ''' <summary>
        ''' The minimum number of characters in this <see cref="BoundedString" />.
        ''' </summary>
        Public Property MinLength() As Integer
            Get
                Return mintMinLength
            End Get
    
            Private Set(value As Integer)
                mintMinLength = value
            End Set
    
        End Property
    
        Private mintMaxLength As Integer
        ''' <summary>
        ''' The maximum number of characters in this <see cref="BoundedString" />.
        ''' </summary>
        Public Property MaxLength As Integer
            Get
                Return mintMaxLength
            End Get
    
            Private Set(value As Integer)
                mintMaxLength = value
            End Set
        End Property
    
        Private mblnAllowNull As Boolean
        ''' <summary>
        ''' Whether or not this <see cref="BoundedString" /> can represent a null value.
        ''' </summary>
        Public Property AllowNull As Boolean
            Get
                Return mblnAllowNull
            End Get
    
            Private Set(value As Boolean)
                mblnAllowNull = value
            End Set
        End Property
    
        Public Sub New(ByVal strValue As String,
                       ByVal intMaxLength As Integer)
            MinLength = 0
            MaxLength = intMaxLength
            AllowNull = False
    
            Value = strValue
        End Sub
    
        Public Sub New(ByVal strValue As String,
                       ByVal intMinLength As Integer,
                       ByVal intMaxLength As Integer)
            MinLength = intMinLength
            MaxLength = intMaxLength
            AllowNull = False
    
            Value = strValue
        End Sub
    
        Public Sub New(ByVal strValue As String,
                       ByVal intMinLength As Integer,
                       ByVal intMaxLength As Integer,
                       ByVal blnAllowNull As Boolean)
            MinLength = intMinLength
            MaxLength = intMaxLength
            AllowNull = blnAllowNull
    
            Value = strValue
        End Sub
    End Class
    
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  • 2020-12-10 05:00

    Use the VBFixedString attribute. See the MSDN info here

    <VBFixedString(256)>Dim strBuff As String
    
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  • 2020-12-10 05:02

    Have you tried

    Dim strBuff as String
    

    Also see Working with Strings in .NET using VB.NET

    This tutorial explains how to represent strings in .NET using VB.NET and how to work with them with the help of .NET class library classes.

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  • 2020-12-10 05:04

    You can use Microsoft.VisualBasic.Compatibility:

    Imports Microsoft.VisualBasic.Compatibility
    
    Dim strBuff As New VB6.FixedLengthString(256)
    

    But it's marked as obsolete and specifically not supported for 64-bit processes, so write your own that replicates the functionality, which is to truncate on setting long values and padding right with spaces for short values. It also sets an "uninitialised" value, like above, to nulls.

    Sample code from LinqPad (which I can't get to allow Imports Microsoft.VisualBasic.Compatibility I think because it is marked obsolete, but I have no proof of that):

    Imports Microsoft.VisualBasic.Compatibility
    
    Dim U As New VB6.FixedLengthString(5)
    Dim S As New VB6.FixedLengthString(5, "Test")
    Dim L As New VB6.FixedLengthString(5, "Testing")
    Dim p0 As Func(Of String, String) = Function(st) """" & st.Replace(ChrW(0), "\0") & """"
    p0(U.Value).Dump()
    p0(S.Value).Dump()
    p0(L.Value).Dump()
    U.Value = "Test"
    p0(U.Value).Dump()
    U.Value = "Testing"
    p0(U.Value).Dump()
    

    which has this output:

    "\0\0\0\0\0"
    "Test "
    "Testi"
    "Test "
    "Testi"

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