VB.NET Substring函数工作异常及ArgumentOutOfRangeException报错求助

• Substring方法参数理解错误：Substring(startIndex, length)的第二个参数是截取的长度，不是结束位置的索引。你直接把IndexOf("m")作为第二个参数，导致截取长度远大于实际需要，所以theLine2会读到多余内容；到theLine3时，计算出的长度超出字符串范围，直接触发ArgumentOutOfRangeException。
• 循环边界错误：File.ReadAllLines("this.txt").Length是文件总行数，比如有5行，索引范围是0-4，但你的循环写的是For t = 0 To lineCount，会多循环一次，最后一次sw2.ReadLine()会返回Nothing，对空字符串调用Substring必然报错。
• 重复读取文件：先用File.ReadAllLines读一次文件拿行数，再用StreamReader读第二次，既浪费资源，还可能因流位置问题导致读取异常。
• 未处理格式异常：如果某行缺少分隔符（比如没有"a"或"b"），IndexOf会返回-1，后续Substring操作也会报错。

方案1：修正原代码的参数与循环问题 #

Imports System.IO

Public Class Form1
    Dim array2(200) As String
    Dim theLine1 As String
    Dim theLine2 As String
    Dim theLine3 As String
    Dim theLine4 As String

    Private Sub btn_Read_Click(sender As Object, e As EventArgs) Handles btn_Read.Click
        ' 一次性读取所有行，避免重复IO操作
        Dim allLines = File.ReadAllLines("this.txt")
        ' 循环边界改为0到总行数-1，避免多循环一次
        For t = 0 To allLines.Length - 1
            Dim currentLine = allLines(t).Trim()
            ' 跳过空行
            If String.IsNullOrEmpty(currentLine) Then Continue For

            ' 预先获取所有分隔符的索引，避免重复计算
            Dim idxN = currentLine.IndexOf("n")
            Dim idxM = currentLine.IndexOf("m")
            Dim idxA = currentLine.IndexOf("a")
            Dim idxB = currentLine.IndexOf("b")

            ' 检查所有分隔符是否存在，避免索引为-1的情况
            If idxN = -1 Or idxM = -1 Or idxA = -1 Or idxB = -1 Then
                ' 可在此添加错误提示，比如跳过该行或弹窗提醒
                Continue For
            End If

            ' 正确计算Substring的长度：结束索引 - 起始索引
            theLine1 = currentLine.Substring(0, idxN).Trim()
            theLine2 = currentLine.Substring(idxN + 1, idxM - (idxN + 1)).Trim()
            theLine3 = currentLine.Substring(idxM + 1, idxA - (idxM + 1)).Trim()
            theLine4 = currentLine.Substring(idxA + 1, idxB - (idxA + 1)).Trim()

            array2(t) = currentLine
        Next
    End Sub
End Class

方案2：用Split方法简化提取（更推荐） #

既然每行格式固定为值1 n 值2 m 值3 a 值4 b，可以直接按空格分割后取对应位置的元素，代码更简洁可靠：

Imports System.IO

Public Class Form1
    Dim array2(200) As String
    Dim theLine1 As String
    Dim theLine2 As String
    Dim theLine3 As String
    Dim theLine4 As String

    Private Sub btn_Read_Click(sender As Object, e As EventArgs) Handles btn_Read.Click
        Dim allLines = File.ReadAllLines("this.txt")
        For t = 0 To allLines.Length - 1
            Dim currentLine = allLines(t).Trim()
            If String.IsNullOrEmpty(currentLine) Then Continue For

            ' 按空格分割，自动移除空元素
            Dim parts = currentLine.Split({" "}, StringSplitOptions.RemoveEmptyEntries)
            ' 检查分割后的元素数量是否符合预期（应为8个：值1,n,值2,m,值3,a,值4,b）
            If parts.Length <> 8 Then
                Continue For
            End If

            ' 直接取对应位置的元素
            theLine1 = parts(0)
            theLine2 = parts(2)
            theLine3 = parts(4)
            theLine4 = parts(6)

            array2(t) = currentLine
        Next
    End Sub
End Class

方案3：正则表达式提取（适配复杂格式） #

如果分隔符前后的空格不固定，或格式有轻微变化，用正则匹配更灵活：

Imports System.IO
Imports System.Text.RegularExpressions

Public Class Form1
    Dim array2(200) As String
    Dim theLine1 As String
    Dim theLine2 As String
    Dim theLine3 As String
    Dim theLine4 As String

    ' 预编译正则表达式，提升重复匹配的性能
    Private Shared ReadOnly linePattern As New Regex("^\s*(\S+)\s+n\s+(\S+)\s+m\s+(\S+)\s+a\s+(\S+)\s+b\s*$", RegexOptions.Compiled)

    Private Sub btn_Read_Click(sender As Object, e As EventArgs) Handles btn_Read.Click
        Dim allLines = File.ReadAllLines("this.txt")
        For t = 0 To allLines.Length - 1
            Dim currentLine = allLines(t)
            Dim match = linePattern.Match(currentLine)
            If match.Success Then
                theLine1 = match.Groups(1).Value
                theLine2 = match.Groups(2).Value
                theLine3 = match.Groups(3).Value
                theLine4 = match.Groups(4).Value

                array2(t) = currentLine
            End If
        Next
    End Sub
End Class

内容的提问来源于stack exchange，提问作者user22672706