VB.NET报错：从类型'DBNull'到'String'转换无效的解决方法

错误原因 #

报错conversion from type 'DBNull' to type 'string' is not valid的核心原因是：DataGridView绑定的数据源中存在数据库空值（即DBNull.Value），直接将单元格的DBNull类型值赋值给String类型的文本框Text属性时，VB.NET无法完成隐式类型转换，直接抛出运行时异常。
原始代码未对单元格空值做任何校验，只要某行某列存在空值，对应位置的赋值语句就会触发报错。另外原始代码没有判断点击位置是否为有效数据行，点击列头时还可能触发空引用错误。

修复方案 #

基础修复（直接替换原有事件代码） #

核心逻辑是对每个单元格取值前先判断是否为DBNull，空值则赋值空字符串，非空值再转成字符串赋值给文本框，同时增加行索引有效性判断：

Private Sub DataGridView1_CellContentClick(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellContentClick
    ' 点击列头等非数据行区域时直接退出，避免空引用报错
    If e.RowIndex < 0 Then Exit Sub

    ' 逐字段判断DBNull后赋值
    txt_ID.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(0).Value), "", DataGridView1.CurrentRow.Cells(0).Value.ToString())
    txt_DATE.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(1).Value), "", DataGridView1.CurrentRow.Cells(1).Value.ToString())
    txt_FNAME.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(2).Value), "", DataGridView1.CurrentRow.Cells(2).Value.ToString())
    txt_LNAME.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(3).Value), "", DataGridView1.CurrentRow.Cells(3).Value.ToString())
    txt_AGE.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(4).Value), "", DataGridView1.CurrentRow.Cells(4).Value.ToString())
    txt_SEX.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(5).Value), "", DataGridView1.CurrentRow.Cells(5).Value.ToString())
    txt_REGNO.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(6).Value), "", DataGridView1.CurrentRow.Cells(6).Value.ToString())
    txt_ROOMNO.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(7).Value), "", DataGridView1.CurrentRow.Cells(7).Value.ToString())
    txt_HEIGHT.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(8).Value), "", DataGridView1.CurrentRow.Cells(8).Value.ToString())
    txt_WEIGHT.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(9).Value), "", DataGridView1.CurrentRow.Cells(9).Value.ToString())
    txt_CELLNO.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(10).Value), "", DataGridView1.CurrentRow.Cells(10).Value.ToString())
    txt_ADDRESS.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(11).Value), "", DataGridView1.CurrentRow.Cells(11).Value.ToString())
    txt_TELLNO.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(12).Value), "", DataGridView1.CurrentRow.Cells(12).Value.ToString())
    txt_FAXNO.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(13).Value), "", DataGridView1.CurrentRow.Cells(13).Value.ToString())
    txt_EMAIL.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(14).Value), "", DataGridView1.CurrentRow.Cells(14).Value.ToString())
    txt_REFERMD.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(15).Value), "", DataGridView1.CurrentRow.Cells(15).Value.ToString())
    txt_MD.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(16).Value), "", DataGridView1.CurrentRow.Cells(16).Value.ToString())
    txt_ANES.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(17).Value), "", DataGridView1.CurrentRow.Cells(17).Value.ToString())
    txt_PROC.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(18).Value), "", DataGridView1.CurrentRow.Cells(18).Value.ToString())
    txt_INDICATION.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(19).Value), "", DataGridView1.CurrentRow.Cells(19).Value.ToString())
    txt_XRAY.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(20).Value), "", DataGridView1.CurrentRow.Cells(20).Value.ToString())
    txt_TARGET.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(21).Value), "", DataGridView1.CurrentRow.Cells(21).Value.ToString())
    txt_PREP1.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(22).Value), "", DataGridView1.CurrentRow.Cells(22).Value.ToString())
    txt_PREP2.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(23).Value), "", DataGridView1.CurrentRow.Cells(23).Value.ToString())
    txt_PREP3.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(24).Value), "", DataGridView1.CurrentRow.Cells(24).Value.ToString())
    txt_QUALITY.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(25).Value), "", DataGridView1.CurrentRow.Cells(25).Value.ToString())
    txt_SEGMENT.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(26).Value), "", DataGridView1.CurrentRow.Cells(26).Value.ToString())
    txt_BBP.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(27).Value), "", DataGridView1.CurrentRow.Cells(27).Value.ToString())
    txt_PREMED.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(28).Value), "", DataGridView1.CurrentRow.Cells(28).Value.ToString())
    txt_SCOPE.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(29).Value), "", DataGridView1.CurrentRow.Cells(29).Value.ToString())
    txt_PROCTIME.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(30).Value), "", DataGridView1.CurrentRow.Cells(30).Value.ToString())
    txt_FINDINGS.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(31).Value), "", DataGridView1.CurrentRow.Cells(31).Value.ToString())
    txt_BIOPSY.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(32).Value), "", DataGridView1.CurrentRow.Cells(32).Value.ToString())
    txt_LOC.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(33).Value), "", DataGridView1.CurrentRow.Cells(33).Value.ToString())
    txt_CYTOLOGY.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(34).Value), "", DataGridView1.CurrentRow.Cells(34).Value.ToString())
    txt_SIZE.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(35).Value), "", DataGridView1.CurrentRow.Cells(35).Value.ToString())
    txt_ENDODESC.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(36).Value), "", DataGridView1.CurrentRow.Cells(36).Value.ToString())
    txt_THERAPEUTIC.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(37).Value), "", DataGridView1.CurrentRow.Cells(37).Value.ToString())
    txt_COMPLICATIONS.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(38).Value), "", DataGridView1.CurrentRow.Cells(38).Value.ToString())
    txt_DIAGNOSIS.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(39).Value), "", DataGridView1.CurrentRow.Cells(39).Value.ToString())
    txt_OUTCOME.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(40).Value), "", DataGridView1.CurrentRow.Cells(40).Value.ToString())
    txt_LATECOMP.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(41).Value), "", DataGridView1.CurrentRow.Cells(41).Value.ToString())
    txt_HISTOPATH.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(42).Value), "", DataGridView1.CurrentRow.Cells(42).Value.ToString())
    txt_ENDOSPCOPIST.Text = If(IsDBNull(DataGridView1.CurrentRow.Cells(43).Value), "", DataGridView1.CurrentRow.Cells(43).Value.ToString())
End Sub

优化写法（减少重复代码） #

逐行写判断逻辑重复度高，可以封装一个通用的单元格取值函数，统一处理空值、格式问题：

''' <summary>
''' 安全获取DataGridView单元格的字符串值
''' </summary>
''' <param name="cell">目标单元格对象</param>
''' <returns>空值返回空字符串，非空值返回去除首尾空格的字符串内容</returns>
Private Function GetSafeCellText(cell As DataGridViewCell) As String
    If cell Is Nothing OrElse cell.Value Is Nothing OrElse IsDBNull(cell.Value) Then
        Return String.Empty
    End If
    Return cell.Value.ToString().Trim()
End Function

封装后赋值语句可以大幅简化，示例：

txt_ID.Text = GetSafeCellText(DataGridView1.CurrentRow.Cells(0))
txt_DATE.Text = GetSafeCellText(DataGridView1.CurrentRow.Cells(1))
' 其余字段按相同格式调用即可

后续如果需要统一增加文本格式化、特殊字符处理逻辑，只需要修改GetSafeCellText函数内部，不需要逐行调整赋值代码。

额外交互优化 #

当前使用的CellContentClick事件只有点击到单元格内的实际内容时才会触发，点击单元格空白区域、用键盘上下切换选中行时不会执行加载逻辑。如果需要行选中状态变化就自动加载数据，可以把逻辑迁移到SelectionChanged事件中，适配更多操作场景。

内容的提问来源于stack exchange，提问作者Lloyd Merwyn Mandanas