blocks|key|3326798|text|下面是一个可以完成这项工作的扩展方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3326799|<System.Runtime.CompilerServices.Extension()>+_
Public+Function+Left(ByVal+str+As+String,+ByVal+length+As+Integer)+As+String
++++Return+str.Substring(0,+Math.Min(str.Length,+length))
End+Function|code-block|syntax|javascript|3326800|这意味着您可以像使用旧的VB+Left函数(即Left("foobar",+3)+)一样使用它，也可以使用较新的VB.NET语法。|offset|length|style|CODE|3326801|Dim+foo+=+"f".Left(3)+'+foo+=+"f"
Dim+bar+=+"bar123".Left(3)+'+bar+=+"bar"|3326802|entityMap^0|0|0|F|4|N|H|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|P|$]]

Here's an extension method that will do the job.

<pre class="lang-vb prettyprint-override"><code>&lt;System.Runtime.CompilerServices.Extension()&gt; _
Public Function Left(ByVal str As String, ByVal length As Integer) As String
 Return str.Substring(0, Math.Min(str.Length, length))
End Function
</code></pre>

This means you can use it just like the old VB <code>Left</code> function (i.e. <code>Left("foobar", 3)</code> ) or using the newer VB.NET syntax, i.e.

<pre class="lang-vb prettyprint-override"><code>Dim foo = "f".Left(3) ' foo = "f"
Dim bar = "bar123".Left(3) ' bar = "bar"
</code></pre>

blocks|key|3114104|text|另一行选项将类似于以下内容：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3114105|myString.Substring(0,+Math.Min(length,+myString.Length))|code-block|syntax|javascript|3114106|其中，myString是您尝试使用的字符串。|3114107|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Another one line option would be something like the following:

<pre><code>myString.Substring(0, Math.Min(length, myString.Length))
</code></pre>

Where myString is the string you are trying to work with.

blocks|key|345718|text|添加对Microsoft.VisualBasic库的引用，您就可以使用Strings.Left，这是完全相同的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|345719|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/microsoft.visualbasic.strings.left.aspx^0|Z|C|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Add a reference to the Microsoft.VisualBasic library and you can use the <a href="http://msdn.microsoft.com/en-us/library/microsoft.visualbasic.strings.left.aspx" rel="noreferrer">Strings.Left</a> which is exactly the same method.

blocks|key|3326909|text|别忘了null大小写：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3326910|public+static+string+Left(this+string+str,+int+count)
{
++++if+(string.IsNullOrEmpty(str)+%7C%7C+count+<+1)
++++++++return+string.Empty;
++++else
++++++++return+str.Substring(0,Math.Min(count,+str.Length));
}|code-block|syntax|javascript|3326911|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Don't forget the null case:
<pre><code>public static string Left(this string str, int count)
{
 if (string.IsNullOrEmpty(str) || count &lt; 1)
 return string.Empty;
 else
 return str.Substring(0,Math.Min(count, str.Length));
}
</code></pre>

blocks|key|3327042|text|使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3327043|using+System;

public+static+class+DataTypeExtensions
{
++++#region+Methods

++++public+static+string+Left(this+string+str,+int+length)
++++{
++++++++str+=+(str+??+string.Empty);
++++++++return+str.Substring(0,+Math.Min(length,+str.Length));
++++}

++++public+static+string+Right(this+string+str,+int+length)
++++{
++++++++str+=+(str+??+string.Empty);
++++++++return+(str.Length+>=+length)
++++++++++++?+str.Substring(str.Length+-+length,+length)
++++++++++++:+str;
++++}

++++#endregion
}|code-block|syntax|javascript|3327044|它不会出错，以空字符串的形式返回空值，并返回修剪过的值或基值。像"testx".Left(4)或str.Right(12)一样使用它；|3327045|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Use:
<pre><code>using System;

public static class DataTypeExtensions
{
 #region Methods

 public static string Left(this string str, int length)
 {
 str = (str ?? string.Empty);
 return str.Substring(0, Math.Min(length, str.Length));
 }

 public static string Right(this string str, int length)
 {
 str = (str ?? string.Empty);
 return (str.Length &gt;= length)
 ? str.Substring(str.Length - length, length)
 : str;
 }

 #endregion
}
</code></pre>
It shouldn't error, returns nulls as empty string, and returns trimmed or base values. Use it like &quot;testx&quot;.Left(4) or str.Right(12);

blocks|key|3326788|text|你可以自己制作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3326789|private+string+left(string+inString,+int+inInt)
{
++++if+(inInt+>+inString.Length)
++++++++inInt+=+inString.Length;
++++return+inString.Substring(0,+inInt);
}|code-block|syntax|javascript|3326790|我的是用C#编写的，您必须将其更改为Visual+Basic。|3326791|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You could make your own:
<pre><code>private string left(string inString, int inInt)
{
 if (inInt &gt; inString.Length)
 inInt = inString.Length;
 return inString.Substring(0, inInt);
}
</code></pre>
Mine is in C#, and you will have to change it for Visual Basic.

blocks|key|340764|text|您可以将对子字符串的调用包装在一个新函数中，按照其他答案中的建议测试它的长度(正确的方式)，或者使用Microsoft.VisualBasic命名空间并直接使用left+(通常被认为是错误的方式！)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|340765|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You can either wrap the call to substring in a new function that tests the length of it as suggested in other answers (the right way) or use the Microsoft.VisualBasic namespace and use left directly (generally considered the wrong way!)

blocks|key|345746|text|另一种技术是通过添加Left()方法来扩展string对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|345747|这是关于这项技术的源文章：|345748|http://msdn.microsoft.com/en-us/library/bb384936.aspx|offset|length|345749|下面是我的实现(用VB)：|345750|Module+StringExtensions

++++<Extension()>
++++Public+Function+Left(ByVal+aString+As+String,+Length+As+Integer)
++++++++Return+aString.Substring(0,+Math.Min(aString.Length,+Length))
++++End+Function

End+Module|code-block|syntax|javascript|345751|然后将此代码放在要使用扩展名的任何文件的顶部：|345752|Imports+MyProjectName.StringExtensions|345753|像这样使用它：|345754|MyVar+=+MyString.Left(30)|345755|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|1H|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|14|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|15|8|@]|9|@$F|16|G|17|1|18]]|A|$]]|$1|H|3|I|5|6|7|19|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|1A|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1B|8|@]|9|@]|A|$]]|$1|Q|3|R|5|L|7|1C|8|@]|9|@]|A|$M|N]]|$1|S|3|T|5|6|7|1D|8|@]|9|@]|A|$]]|$1|U|3|V|5|L|7|1E|8|@]|9|@]|A|$M|N]]|$1|W|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|X|$Y|$5|Z|10|11|A|$12|E]]]]

Another technique is to extend the string object by adding a Left() method.
Here is the source article on this technique:
<a href="http://msdn.microsoft.com/en-us/library/bb384936.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/bb384936.aspx</a>
Here is my implementation (in VB):
<pre><code>Module StringExtensions

 &lt;Extension()&gt;
 Public Function Left(ByVal aString As String, Length As Integer)
 Return aString.Substring(0, Math.Min(aString.Length, Length))
 End Function

End Module
</code></pre>
Then put this at the top of any file in which you want to use the extension:
<pre><code>Imports MyProjectName.StringExtensions
</code></pre>
Use it like this:
<pre><code>MyVar = MyString.Left(30)
</code></pre>

blocks|key|342255|text|我喜欢这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|342256|string+s+=+"hello+how+are+you";
s+=+s.PadRight(30).Substring(0,30).Trim();+//"hello+how+are+you"
s+=+s.PadRight(3).Substring(0,3).Trim();+//"hel"|code-block|syntax|javascript|342257|但是，如果你想要尾随空格或开始空格，那么你就不走运了。|342258|我真的很喜欢使用Math.Min，它似乎是一个更好的解决方案。|342259|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I like doing something like this:

<pre><code>string s = "hello how are you";
s = s.PadRight(30).Substring(0,30).Trim(); //"hello how are you"
s = s.PadRight(3).Substring(0,3).Trim(); //"hel"
</code></pre>

Though, if you want trailing or beginning spaces then you are out of luck.

I really like the use of Math.Min, it seems to be a better solution.

blocks|key|3114239|text|只是在一个非常特殊的情况下：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3114240|如果您在左侧执行此操作，并且您将使用某些部分字符串检查数据，例如：|3114241|if(Strings.Left(str,+1)=="*")+...;|offset|length|style|CODE|3114242|然后，您还可以使用StartsWith和EndsWith等C#实例方法来执行这些任务。|3114243|if(str.StartsWith("*"))...;|3114244|entityMap^0|0|0|0|Y|0|9|A|K|8|0|0|R|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|R|8|@$F|S|G|T|H|I]]|9|@]|A|$]]|$1|J|3|K|5|6|7|U|8|@$F|V|G|W|H|I]|$F|X|G|Y|H|I]]|9|@]|A|$]]|$1|L|3|M|5|6|7|Z|8|@$F|10|G|11|H|I]]|9|@]|A|$]]|$1|N|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|O|$]]

Just in a very special case:
If you are doing this left and you will check the data with some partial string, for example:
<code>if(Strings.Left(str, 1)==&quot;*&quot;) ...;</code>
Then you can also use C# instance methods, such as <code>StartsWith</code> and <code>EndsWith</code> to perform these tasks.
<code>if(str.StartsWith(&quot;*&quot;))...;</code>

blocks|key|345786|text|如果您希望避免使用扩展方法并防止长度不足错误，请尝试此方法|type|unstyled|depth|inlineStyleRanges|entityRanges|data|345787|string+partial_string+=+text.Substring(0,+Math.Min(15,+text.Length))+
//+example+of+15+character+max|code-block|syntax|javascript|345788|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you want to avoid using an extension method and prevent an under-length error, try this

<pre><code>string partial_string = text.Substring(0, Math.Min(15, text.Length)) 
// example of 15 character max
</code></pre>

As a non-.NET programmer I'm looking for the .NET equivalent of the old Visual Basic function <code>left(string, length)</code>. It was lazy in that it worked for any length string. As expected, <code>left(&quot;foobar&quot;, 3) = &quot;foo&quot;</code> while, most helpfully, <code>left(&quot;f&quot;, 3) = &quot;f&quot;</code>.
In .NET <code>string.Substring(index, length)</code> throws exceptions for everything out of range. In Java I always had the Apache-Commons lang.StringUtils handy. In Google I don't get very far searching for string functions.
<hr/>
@Noldorin - Wow, thank you for your VB.NET extensions! My first encounter, although it took me several seconds to do the same in C#:
<pre><code>public static class Utils
{
 public static string Left(this string str, int length)
 {
 return str.Substring(0, Math.Min(length, str.Length));
 }
}
</code></pre>
Note the static class and method as well as the <code>this</code> keyword. Yes, they are as simple to invoke as <code>&quot;foobar&quot;.Left(3)</code>. See also <a href="http://msdn.microsoft.com/en-us/library/bb383977.aspx" rel="noreferrer">C# extensions on MSDN</a>.

.NET equivalent of the old vb left(string, length) function

Java

作为一名非.NET程序员，我正在寻找旧的Visual Basic function left(string, length)的.NET等效项。它是懒惰的，因为它对任何长度的字符串都有效。正如预期的那样，left("foobar", 3) = "foo"，而最有用的是，left("f", 3) = "f"。在.NET中，string.Substring(index, length)会对超出范围的所有

问与旧的vb .NET (string，length)函数等效
EN

回答 11

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问与旧的vb .NET (string，length)函数等效
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