VB.NET使用For Each循环遍历集合时如何处理Nothing空引用异常

解决方案 #

当然可以，你只需要在外层遍历到Person对象后、遍历其Names集合前，先加一层Names属性的非空校验即可，修正后的For Each写法如下：

Dim Counter As Integer = 0
Dim AllNames As String = String.Empty

For Each individual As Person In PeopleContacts
    ' 先检查当前Person的Names集合是否为空，避免遍历空集合抛出空引用异常
    If individual.Names IsNot Nothing Then
        For Each indName As Name In individual.Names
            If indName IsNot Nothing Then
                Counter += 1
                AllNames &= Counter.ToString & ": " & indName.DisplayName & vbCrLf             
            End If
        Next
    End If
Next

逻辑说明 #

• 你之前的异常是因为尝试遍历individual.Names这个空引用对象，而非内部的Name元素为空，原来的校验逻辑位置不对才没有生效
• 保留For Each结构的同时，只需要把非空校验提前到遍历Names集合之前，就能达到和下标遍历完全一致的效果，可读性和可维护性也更高

可选优化建议 #

如果Name集合的元素数量较多，直接拼接String会产生大量临时对象，性能较差，可以改用StringBuilder优化：

Dim Counter As Integer = 0
Dim AllNamesBuilder As New System.Text.StringBuilder()

For Each individual As Person In PeopleContacts
    If individual.Names IsNot Nothing Then
        For Each indName As Name In individual.Names
            If indName IsNot Nothing Then
                Counter += 1
                AllNamesBuilder.AppendLine($"{Counter}: {indName.DisplayName}")
            End If
        Next
    End If
Next

' 最后统一转换为字符串
Dim AllNames As String = AllNamesBuilder.ToString()

内容的提问来源于stack exchange，提问作者Rob