.net - VB.NET：输入字符串的格式不正确

Question

使用以下代码段

Foo = IIf(String.IsNullOrEmpty(txtFoo.Text), 0, Integer.Parse(txtFoo.Text.Trim))

当我提交没有值的字段时出现错误：“输入字符串的格式不正确。” 我没有任何空间或其他东西，并且 String.IsNullOrEmpty(txtFoo.Text) 返回 true。怎么了？谢谢你。

Answer 1

IIF will evaluate:

Integer.Parse(txtFoo.Text.Trim) 

irrespective of whether:

String.IsNullOrEmpty(txtFoo.Text) 

is true or not (as it is just a function with three arguments passed to it, so all arguments must be valid). So even if txtFoo.text is empty, it's still trying to Parse it to an Integer in this case.

If you're using VS2008, you can use the IF operator instead which will short-circuit as you're expecting IIF to do.

Answer 2

IIf is a function call rather than a true conditional operator, and that means both arguments have to be evaluated. Thus, it just attempts to call Integer.Parse() if your string is Null/Nothing.

If you're using Visual Studio 2008 or later, it's only a one character difference to fix your problem:

Foo = If(String.IsNullOrEmpty(txtFoo.Text), 0, Integer.Parse(txtFoo.Text.Trim())

This version of the If keyword is actually a true conditional operator that will do the short-circuit evaluation of arguments as expected.

If you're using Visual Studio 2005 or earlier, you fix it like this:

If String.IsNullOrEmpty(txtFoo.Text) Then Foo = 0 Else Foo = Integer.Parse(txtFoo.Text.Trim())

Answer 3

IIf is not a true ternary operator, it actually evaluates both parameter expressions. You probably want to use the If operator instead (VS 2008+).

You'd simply say

If(String.IsNullOrEmpty(txtFoo.Text), 0, Integer.Parse(txtFoo.Text.Trim()))

Answer 4

条件部分和“else”部分之间的一个区别是字符串的修剪。您可以在调用之前尝试修剪字符串IsNullOrEmpty。

Foo = IIf(String.IsNullOrEmpty(txtFoo.Text.Trim), 0, Integer.Parse(txtFoo.Text.Trim))