创建指定数量的字符串

Question

我想在 VB.NET 控制台应用程序中创建指定数量的字符串。我试过这段代码，但它抛出了一个异常：

NullReferenceException：对象引用未设置到对象的实例中。

这是代码：

Module RandStrConsole

Dim r As New Random
Dim s As String
Dim result As System.Text.StringBuilder
Dim sb As System.Text.StringBuilder
Dim MaxChar As Integer


Sub Main()

    Console.Write("Enter String: ") : s = Console.ReadLine
    Console.Write("Maxchar: ") : MaxChar = Console.ReadLine

    For i As Integer = 1 To MaxChar

        Dim idx As Integer = r.Next(0, s.Count - 1)
        result = sb.Append(s.Substring(idx, 1)) 'NullReferenceException: Object Reference not set into an instance of an Object.
        result.ToString()

    Next

    Console.WriteLine(result)
    Console.ReadKey()


End Sub

End Module

注释部分指出异常发生的位置。

Answer 1

您忘记初始化StringBuilders。您必须使用New keyword。

Dim result As New System.Text.StringBuilder
Dim sb As New System.Text.StringBuilder