VB.NET如何修改TextBox控件中单行文本的字体和颜色（密码强度校验场景）

核心结论 #

标准WinForm TextBox控件不支持单独修改某一行或部分文本的字体、颜色属性，所有格式配置只会作用于控件内的全部文本，无法实现局部差异化格式效果。

替代方案 #

直接将你当前的txtChcekPass控件从TextBox替换为RichTextBox控件即可，RichTextBox原生支持富文本编辑，可选中任意范围的文本单独设置字体、颜色、字号等格式，完全适配你的密码强度提示场景。

适配代码示例 #

替换控件后，你可以参考如下写法实现局部格式自定义，同时修复了你原代码中的两处逻辑bug：

Function Checker(password As String) As String
    Dim i, l, c, k As Byte
    Dim array() As Char = password.ToCharArray()
    ' 修复原代码逻辑问题：符号分支错误累加循环变量i，会导致跳过后续字符校验
    For idx = 0 To array.Length - 1
        If Char.IsUpper(array(idx)) Then
            c += 1
        ElseIf Char.IsLower(array(idx)) Then
            l += 1
        ElseIf Char.IsNumber(array(idx)) Then
            k += 1
        ElseIf Char.IsSymbol(array(idx)) Then
            i += 1
        End If
    Next

    ' 先清空控件内容
    pnlCheckPass.txtChcekPass.Clear()
    ' 若需要保留密码原文用默认格式展示，可先追加密码
    pnlCheckPass.txtChcekPass.AppendText(password)
    ' 记录强度提示文本的起始位置
    Dim tipStart As Integer = pnlCheckPass.txtChcekPass.TextLength

    Dim tipText As String = ""
    Dim tipColor As Color = Color.Black
    ' 修复原代码无效判断：将恒成立的2>=1改为小写字母校验l>=1
    If i >= 2 And k >= 3 And l >= 1 And c >= 2 And password.Length >= 6 Then
        tipColor = Color.DarkGreen
        tipText = " - Very strong password"
    ElseIf k >= 3 And l >= 2 And c >= 2 And i >= 2 And password.Length >= 6 Then
        tipColor = Color.Green
        tipText = " - Strong password"
    ElseIf k >= 2 And i >= 1 And c >= 1 And password.Length >= 6 Then
        tipColor = Color.Orange
        tipText = " - Medium password"
    ElseIf k = 1 And i >= 1 And password.Length = 4 Then
        tipColor = Color.Red
        tipText = " - Weak password"
    ElseIf k = 1 Then
        tipColor = Color.DarkRed
        tipText = " - Very weak password"
    Else
        tipColor = Color.IndianRed
        tipText = " - Blank password"
    End If

    ' 追加强度提示文本
    pnlCheckPass.txtChcekPass.AppendText(tipText)
    ' 选中提示文本部分单独设置格式
    pnlCheckPass.txtChcekPass.Select(tipStart, tipText.Length)
    pnlCheckPass.txtChcekPass.SelectionFont = New Font("Microsoft YaHei UI", 10)
    pnlCheckPass.txtChcekPass.SelectionColor = tipColor
    ' 取消选中状态
    pnlCheckPass.txtChcekPass.SelectionLength = 0

    Return pnlCheckPass.txtChcekPass.Text
End Function

补充说明 #

如果你不需要保留密码原文，仅需要展示强度提示，可跳过密码追加步骤，直接把强度提示文本赋值给RichTextBox后全选设置对应格式即可，改造成本极低。

内容的提问来源于stack exchange，提问作者Lasicak