测试字符串是否为空 [英] Testing if a string is null

问题描述

我是 VBA 新手，还没有完全习惯语法，所以如果我的问题听起来很愚蠢，我很抱歉.

I am pretty new in VBA and I have not yet got used to the syntax completely, so I'm sorry if my question sounds stupid.

我正在 Word 2010 中使用 RequisitePro40 和 VBA 7.0.在我的一个模块中，我有以下循环和 If 条件:

I am working with RequisitePro40 and VBA 7.0 in Word 2010. In one of my modules I have the following loop and If conditions:

Dim rqRequirements As ReqPro40.Requirements
Dim rqRequirement As ReqPro40.Requirement
Const eAttrValueLookup_Label = 4
Dim a As Integer
...

For Each vReqKey In rqRequirements
    Set rqRequirement = rqRequirements(vReqKey)

    If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text <> Null Then
        a = 1
    End If

    If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text = Null Then
         a = 2
    End If

 Next

在循环的每次迭代中，a = 1 和 a = 2 都被执行！！

In each iteration of the loop, both a = 1 and a = 2 are executed!!

基于这个，相等和不等运算符是="和<>".因此，我希望 a = 1 或 a = 2 对字符串执行.我的语法有问题吗?还是 ReqPro 相关的问题?

Based on This, the equality and inequality operators are "=" and "<>". Therefore I would expect that either a = 1 or a = 2 execute for a string. Is there something wrong with my syntax? Or is it a ReqPro related Problem?

我也尝试使用Is"和IsNot"运算符，但它们导致编译器错误:类型不匹配

I also tried using "Is" and "IsNot" operators but they result in Compiler error: Type mismatch

有人可以帮我吗?

更新:实际目标是看看

rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text

rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text

是否为 Null.我添加了第二个 if 以表明该语句在某种程度上没有按照我期望的方式工作.

is Null or not. I added the second if to show the problem that the statement is somehow not working the way I expect it to work.

将Null"替换为vbNullString"没有做任何改变.

Replacing "Null" to "vbNullString" did not make any changes.

我也按照@Slai 的建议尝试了 IsNull 函数.结果几乎相同:

I also tried the IsNull function as @Slai suggested. the result is pretty much the same:

    If IsNull(rqRequirement.AttrValue(att, eAttrValueLookup_Label).text) Then
        a = 3
    End If

    If Not IsNull(rqRequirement.AttrValue(att, eAttrValueLookup_Label).text) Then
        a = 4
    End If

a = 3 和 a = 4 两个语句都为真并被执行.

Both statements a = 3 and a = 4 are true and executed.

推荐答案

VBA 不支持测试字符串是否为Null".VBA 不像 .NET 语言或 JavaScript(例如).基本变量类型都有一个默认值，从声明变量的那一刻起，字符串的长度为零 ("") - 它没有未实例化的状态.您还可以测试 vbNullString.

VBA doesn't support testing whether a string is "Null". VBA isn't like a .NET language or JavaScript (for example). The basic variable types all have a default value, a String is of zero length ("") from the moment the variable is declared - it has no uninstantiated state. You can also test for vbNullString.

如果你测试

Dim s as String
Debug.Print s = Null, s <> Null, s = "", s = "a", IsNull(s), s = vbNullString

回报是

Null  Null  True  False  False  True

因此，如果您要测试是否已将任何内容分配给 String 变量，那么您唯一可以做的就是:

So if you're trying to test whether anything has been assigned to a String variable the only things you can do are:

Debug.Print Len(s), s = "", Len(s) = 0, s = vbNullString

哪个返回

0  True  True True

请注意，这些可能性中最慢的是 s = ""，尽管它看起来最容易记住.

Note that the slowest of these possibilities is s = "", even though it seems the simplest to remember.

这篇关于测试字符串是否为空的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！