Visual Basic WinForm 用循环实现直角三角形的类型转换错误解决

我曾尝试使用Dart完成这项Visual Basic作业，且已通过下述方案实现了效果：

不过我不确定该实现是否为最优方案，也不清楚Visual Studio中有哪些可用的命令或方法可帮助我完成该需求，目前我编写的VB代码如下：

Public Class Form1
    Dim input As Integer
    
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.Text = "Nested Loops"
    End Sub
    
    Private Sub Label1_Click(sender As Object, e As EventArgs) Handles Label.Click
    End Sub
    
    Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles InputTextBox.TextChanged
        input = Val(InputTextBox.Text)
    End Sub
    
    Private Sub ComputeButton_Click(sender As Object, e As EventArgs) Handles ComputeButton.Click
        For outer As Integer = 1 To input
            For inner As Integer = 1 To outer
                ListBox.Items.Add("#")
            Next
            ListBox.Items.Add("")
        Next
    End Sub
    
    Private Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListBox.SelectedIndexChanged
    End Sub
End Class

我尝试参照Dart的逻辑处理该问题时，出现了如下报错：

System.InvalidCastException: 'Conversion from string "#" to type 'Double' is not valid.'

现寻求该问题的解决方法。

内容的提问来源于stack exchange，提问作者Nana Kwame