vb.net 检查数据库值是否为空 [英] vb.net check if database value is null

问题描述

我正在 vb.net 中运行查询，我想测试该列是否为空.我试过了:

i am running a query in vb.net and i want to test if the column is null. i have tried:

If reader.GetString(2) IsNot Nothing Then

If IsDBNull(reader.GetString(2)) Then

If reader.GetString(2) IsNot NULL Then

If NOT reader.GetString(2) IS Nothing Then

If NOT reader.GetString(2) IS NULL Then

但他们都回来了:

Data is Null. This method or property cannot be called on Null values.

当我在 MySQL 中运行查询时，列显示 NULL

when i run my query in MySQL, the columns show NULL

推荐答案

问题在于 GetString 方法在内部将您的行值转换为字符串.如果行值为空，则会出现异常.

The problem is the GetString method that internally cast your row value to a string. If the row value is null you get the exception.

修复是使用 VB.NET 三元运算符

The fix is using the VB.NET ternary operator

Dim result = IF(reader.IsDbNull(2), "", reader.GetString(2))

或者如果您不想在第三个字段为空时分配默认值，您可以简单地编写

or if you don't want to assign a default value when the third field is null you could simply write

if Not reader.IsDbNull(2) Then
   .......
End if

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