VB.NET 类似于 SQL 中可用的“ISNULL”函数？
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VB.NET similar function to "ISNULL" available in SQL?
提问by D. Caan
I have this in my VB.NET code:
我的 VB.NET 代码中有这个：
Dim myValue as String
myValue = ""
Dim myDblValue as Double
myDblValue CDbl(Val(myValue))
If myValuestring is null[like in this example], I'd like to set the value to a predefined value for example 999. I know in SQL I can use ISNULLfor that. Is there any equivalent in VB.NET? If not, how can it be done with available functions?
如果myValuestring 是null[like in this example]，我想将该值设置为预定义的值，例如999。我知道在 SQL 中我可以使用ISNULL它。VB.NET 中是否有任何等价物？如果没有，如何使用可用功能来完成？
回答by Gridly
To directly answer your question, IF operator is the nearest to ISNULL when used with the String.IsNull method.
为了直接回答您的问题，IF 运算符与 String.IsNull 方法一起使用时最接近 ISNULL。
Like this
像这样
Dim myValue As String
myValue = ""
Dim myDblValue As Double
Dim myDefaultValue As Double = 10.1 ' what ever you need
myDblValue = If(String.IsNullOrEmpty(myValue), myDefaultValue, CDbl(myValue))
If you need to worry about invalid text in myValue then you can use the Double.TryParse method like this.
如果您需要担心 myValue 中的无效文本，那么您可以像这样使用 Double.TryParse 方法。
Dim myValue As String
myValue = ""
Dim myDblValue As Double
Dim myDefaultValue As Double = 10.1 ' what ever you need
If String.IsNullOrEmpty(myValue) OrElse Not Double.TryParse(myValue, myDblValue) Then
myDblValue = myDefaultValue
End If
回答by Mark C.
If YourVariable.IsNullOrEmpty Then
myValue = DoWhateverYouWant
Else
DoOtherStuff
End If
回答by Mark Hall
You can use the String Methods IsNullOrEmptyor IsNullOrWhiteSpaceto create you own function something like this.
您可以使用字符串方法IsNullOrEmpty或IsNullOrWhiteSpace像这样创建自己的函数。
Public Function TestNull(value As String, defaultValue As String) as String
If String.IsNullOrEmpty(value) Then
Return defaultValue
Else
Return value
End If
End Function
Usuage:
用法：
myDblValue = CDbl(Val(TestNull(myValue, "999")))
Or since you are converting to a double you can just use the Double.TryParse Method it will fail if it can't be converted for any reason
或者，由于您要转换为双精度，因此您可以使用 Double.TryParse 方法，如果由于任何原因无法转换，它将失败
Public Function TestValidDecimal(value As String, defaultValue As Double) As Double
Dim temp As Double
If Double.TryParse(value, temp) Then
Return temp
Else
Return defaultValue
End If
End Function
回答by Tom
myDblValue = If(myValue = Nothing, 999, CDbl(Val(myValue)))
myDblValue = If(myValue = Nothing, 999, CDbl(Val(myValue)))
OR
或者
myDblValue = If(myValue = "", 999, CDbl(Val(myValue)))
myDblValue = If(myValue = "", 999, CDbl(Val(myValue)))
SEE:
看：
http://msdn.microsoft.com/en-us/library/bb513985(v=vs.90).aspx
http://msdn.microsoft.com/en-us/library/bb513985(v=vs.90).aspx
