Question

我正在处理一个Switch语句，并且我需要查看两个条件，看看值是否以特定值开头。 Switch语句是这样的。错误说“不能将bool类型转换为字符串”。

任何人都知道我是否可以在Switch中使用StartsWith或者我是否需要使用If ... Else语句？

switch(subArea)
            {
                case "4100":
                case "4101":
                case "4102":
                case "4200":
                    return "ABC";
                case "600A":
                    return "XWZ";
                case subArea.StartsWith("3*"):
                case subArea.StartsWith("03*"):
                    return "123";
                default:
                    return "ABCXYZ123";
            }

Answer 1

您正在切换String，而subArea.StartsWith()会返回Boolean，这就是您无法做到的原因。我建议你这样做：

if (subArea.StartsWith("3*") || subArea.StartsWith("03*"))
    return "123";

switch(subArea)
{
    case "4100":
    case "4101":
    case "4102":
    case "4200":
        return "ABC";
    case "600A":
        return "XWZ";
    default:
        return "ABCXYZ123";
}

结果将是相同的。

Answer 2

case标签必须是字符串，因为switch表达式是一个字符串;但是，StartsWith返回一个布尔值。我建议在default部分处理这些特殊情况。

switch(subArea)
{
    case "4100":
    case "4101":
    case "4102":
    case "4200":
        return "ABC";
    case "600A":
        return "XWZ";
    default:
        if (subArea.StartsWith("3") || subArea.StartsWith("03")) {
            return "123";
        }
        return "ABCXYZ123";
}

除非您希望subArea包含它，否则明星（*）可能是错误的。 StartWith不接受通配符。

或者你可以使用正则表达式：

if (Regex.IsMatch(subArea, "^3|^03")) { // or "^(3|03)"
    return "123";
}

其中^表示行首，|表示或。

Answer 3

Joe有点打败我，但这是另一种非切换方式，它基本上实现了一个带有规则集的模式匹配算法。

private static string GetSomeStringOrOther(string subArea)
{
    // Create a set of pattern matching functions...
    Func<string, string, bool> matchEquals = (a, b) => a.Equals(b);
    Func<string, string, bool> matchStarts = (a, b) => a.StartsWith(b);

    // Create a rule set...
    Tuple<string, string, Func<string, string, bool>>[] cases = new []
    {
        new Tuple<string, string, Func<string, string, bool>>("4100", "ABC", matchEquals),
        new Tuple<string, string, Func<string, string, bool>>("4101", "ABC", matchEquals),
        new Tuple<string, string, Func<string, string, bool>>("4102", "ABC", matchEquals),
        new Tuple<string, string, Func<string, string, bool>>("4200", "ABC", matchEquals),
        new Tuple<string, string, Func<string, string, bool>>("600A", "XWZ", matchEquals),
        new Tuple<string, string, Func<string, string, bool>>("3*", "123", matchStarts),
        new Tuple<string, string, Func<string, string, bool>>("03*", "123", matchStarts),
    };

    // Look for a match...
    foreach(var matchCase in cases)
    {
        if(matchCase.Item3(subArea, matchCase.Item1))
        {
            // Return if it matches...
            return matchCase.Item2;
        }
    }

    // Otherwise return the default...
    return "ABCXYZ123";
}

<强>优点

如果您需要新规则，则可以轻松添加到规则集中。
如果您需要新的模式匹配功能，请再次轻松添加。
如果规则发生变化，则无需进行大量返工。

<强>缺点

新手/初学者甚至一些中间开发者可能都不知道发生了什么。

<强>改进

将Tuple<string, string, Func<string, string, bool>>替换为代表Rule

Answer 4

自C＃7起，您可以执行以下操作：

switch(subArea)
{
    case "4100":
    case "4101":
    case "4102":
    case "4200":
       return "ABC";
    case "600A":
       return "XWZ";
    case string s when s.StartsWith("3*"):
       return "123";
    case string s when s.StartsWith("03*"):
       return "123";
    default:
       return "ABCXYZ123";
}

Answer 5

使用LINQ，● hadoop-datanode.service - Hadoop Datanode Loaded: loaded (/usr/lib/systemd/system/hadoop-datanode.service; enabled; vendor preset: disabled) Active: failed (Result: exit-code) since Fri 2017-12-29 05:29:15 JST; 53s ago Process: 30450 ExecStart=/usr/bin/hadoop datanode (code=exited, status=1/FAILURE) Main PID: 30450 (code=exited, status=1/FAILURE) Dec 29 05:29:14 <my user name> hadoop[30450]: at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:2728) Dec 29 05:29:14 <my user name> hadoop[30450]: at org.apache.hadoop.hdfs.server.datanode.DataNode.secureMain(DataNode.java:2872) Dec 29 05:29:14 <my user name> hadoop[30450]: at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:2896) Dec 29 05:29:14 <my user name> hadoop[30450]: 2017-12-29 05:29:14,859 INFO util.ExitUtil: Exiting with status 1: java.io.IOException: Incorrect configuration: namenode address dfs.namenode.servicerpc-address or dfs.namenode.rpc-address is not configured. Dec 29 05:29:14 <my user name> hadoop[30450]: 2017-12-29 05:29:14,865 INFO datanode.DataNode: SHUTDOWN_MSG: Dec 29 05:29:14 <my user name> hadoop[30450]: /************************************************************ Dec 29 05:29:14 <my user name> hadoop[30450]: SHUTDOWN_MSG: Shutting down DataNode at elect/10.0.2.15 Dec 29 05:29:14 <my user name> hadoop[30450]: ************************************************************/ Dec 29 05:29:15 elect systemd[1]: hadoop-datanode.service: Main process exited, code=exited, status=1/FAILURE Dec 29 05:29:15 <my user name> systemd[1]: hadoop-datanode.service: Failed with result 'exit-code'. ● hadoop-namenode.service - Hadoop Namenode Loaded: loaded (/usr/lib/systemd/system/hadoop-namenode.service; disabled; vendor preset: disabled) Active: failed (Result: exit-code) since Fri 2017-12-29 05:29:13 JST; 10min ago Process: 30449 ExecStart=/usr/bin/hadoop namenode (code=exited, status=1/FAILURE) Main PID: 30449 (code=exited, status=1/FAILURE) Dec 29 05:29:13 <my user name> hadoop[30449]: at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:929) Dec 29 05:29:13 <my user name> hadoop[30449]: at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1653) Dec 29 05:29:13 <my user name> hadoop[30449]: at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1720) Dec 29 05:29:13 <my user name> hadoop[30449]: 2017-12-29 05:29:13,366 INFO util.ExitUtil: Exiting with status 1: java.lang.IllegalArgumentException: Invalid URI for NameNode address (check fs.defaultFS): file:/// has no authority. Dec 29 05:29:13 <my user name> hadoop[30449]: 2017-12-29 05:29:13,368 INFO namenode.NameNode: SHUTDOWN_MSG: Dec 29 05:29:13 <my user name> hadoop[30449]: /************************************************************ Dec 29 05:29:13 <my user name> hadoop[30449]: SHUTDOWN_MSG: Shutting down NameNode at <my user name>/10.0.2.15 Dec 29 05:29:13 <my user name> hadoop[30449]: ************************************************************/ Dec 29 05:29:13 <my user name> systemd[1]: hadoop-namenode.service: Main process exited, code=exited, status=1/FAILURE Dec 29 05:29:13 <my user name> systemd[1]: hadoop-namenode.service: Failed with result 'exit-code'. ● hadoop-jobtracker.service - Hadoop Jobtracker Loaded: loaded (/usr/lib/systemd/system/hadoop-jobtracker.service; enabled; vendor preset: disabled) Active: failed (Result: exit-code) since Fri 2017-12-29 05:26:04 JST; 17min ago Process: 29726 ExecStart=/usr/bin/hadoop jobtracker (code=exited, status=1/FAILURE) Main PID: 29726 (code=exited, status=1/FAILURE) Dec 29 05:26:04 <my user name> hadoop[29726]: queue get information regarding JobQueues Dec 29 05:26:04 <my user name> hadoop[29726]: sampler sampler Dec 29 05:26:04 <my user name> hadoop[29726]: streaming launch a mapreduce streaming job Dec 29 05:26:04 <my user name> hadoop[29726]: version print the version Dec 29 05:26:04 <my user name> hadoop[29726]: Daemon Commands: Dec 29 05:26:04 <my user name> hadoop[29726]: tput: No value for $TERM and no -T specified Dec 29 05:26:04 <my user name> systemd[1]: hadoop-jobtracker.service: Main process exited, code=exited, status=1/FAILURE Dec 29 05:26:04 <my user name> hadoop[29726]: historyserver run job history servers as a standalone daemon Dec 29 05:26:04 <my user name> hadoop[29726]: SUBCOMMAND may print help when invoked w/o parameters or with -h. Dec 29 05:26:04 <my user name> systemd[1]: hadoop-jobtracker.service: Failed with result 'exit-code'. ● hadoop-secondarynamenode.service - Hadoop Secondary Namenode Loaded: loaded (/usr/lib/systemd/system/hadoop-secondarynamenode.service; disabled; vendor preset: disabled) Active: failed (Result: exit-code) since Fri 2017-12-29 05:25:59 JST; 20min ago Process: 29433 ExecStart=/usr/bin/hadoop secondarynamenode (code=exited, status=1/FAILURE) Main PID: 29433 (code=exited, status=1/FAILURE) Dec 29 05:25:59 <my user name> hadoop[29433]: at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.initialize(SecondaryNameNode.java:231) Dec 29 05:25:59 <my user name> hadoop[29433]: at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.<init>(SecondaryNameNode.java:194) Dec 29 05:25:59 <my user name> hadoop[29433]: at org.apache.hadoop.hdfs.server.namenode.SecondaryNameNode.main(SecondaryNameNode.java:690) Dec 29 05:25:59 <my user name> hadoop[29433]: 2017-12-29 05:25:59,387 INFO util.ExitUtil: Exiting with status 1: ExitException Dec 29 05:25:59 <my user name> hadoop[29433]: 2017-12-29 05:25:59,400 INFO namenode.SecondaryNameNode: SHUTDOWN_MSG: Dec 29 05:25:59 <my user name> hadoop[29433]: /************************************************************ Dec 29 05:25:59 <my user name> hadoop[29433]: SHUTDOWN_MSG: Shutting down SecondaryNameNode at <my user name>/10.0.2.15 Dec 29 05:25:59 <my user name> hadoop[29433]: ************************************************************/ Dec 29 05:25:59 <my user name> systemd[1]: hadoop-secondarynamenode.service: Main process exited, code=exited, status=1/FAILURE Dec 29 05:25:59 <my user name> systemd[1]: hadoop-secondarynamenode.service: Failed with result 'exit-code'. ● hadoop-tasktracker.service - Hadoop Tasktracker Loaded: loaded (/usr/lib/systemd/system/hadoop-tasktracker.service; disabled; vendor preset: disabled) Active: failed (Result: exit-code) since Fri 2017-12-29 05:26:04 JST; 23min ago Process: 29727 ExecStart=/usr/bin/hadoop tasktracker (code=exited, status=1/FAILURE) Main PID: 29727 (code=exited, status=1/FAILURE) Dec 29 05:26:04 <my user name> hadoop[29727]: queue get information regarding JobQueues Dec 29 05:26:04 <my user name> hadoop[29727]: sampler sampler Dec 29 05:26:04 <my user name> hadoop[29727]: streaming launch a mapreduce streaming job Dec 29 05:26:04 <my user name> hadoop[29727]: version print the version Dec 29 05:26:04 <my user name> hadoop[29727]: Daemon Commands: Dec 29 05:26:04 <my user name> hadoop[29727]: tput: No value for $TERM and no -T specified Dec 29 05:26:04 <my user name> hadoop[29727]: historyserver run job history servers as a standalone daemon Dec 29 05:26:04 <my user name> hadoop[29727]: SUBCOMMAND may print help when invoked w/o parameters or with -h. Dec 29 05:26:04 <my user name> systemd[1]: hadoop-tasktracker.service: Main process exited, code=exited, status=1/FAILURE Dec 29 05:26:04 <my user name> systemd[1]: hadoop-tasktracker.service: Failed with result 'exit-code'.可以通过将foreach和return语句替换为：{/ p>来简化the nice answer by @seriesOne

// using System.Linq;

// Look for a match...
var result = cases
    .Where(c => c.Item3(subArea, c.Item1))
    .FirstOrDefault();

// Return the match or the default.
return result == null ? "ABCXYZ123" : result.Item2;

Answer 6

只是为了好玩，这是另一个避免切换声明的解决方案。

var map = new[] {
    new { Value = "4100", StartsWith = false, Result="ABC" },
    new { Value = "4101", StartsWith = false, Result="ABC" },
    new { Value = "4102", StartsWith = false, Result="ABC" },
    new { Value = "4200", StartsWith = false, Result="ABC" },
    new { Value = "600A", StartsWith = false, Result="XWZ" },
    new { Value = "3*", StartsWith = true, Result="123" },
    new { Value = "03*", StartsWith = true, Result="123" },
};

var subarea = ... whatever ...;

var result = map.Where(e =>
        {
            if (e.StartsWith)
            {
                return subarea.StartsWith(e.Value);
            }
            else
            {
                return subarea == e.Value;
            }
        }
    )
    .Select(e => e.Result)
    .FirstOrDefault() ?? "ABCXZ123";

数组map中的顺序决定了优先级，因此，例如，您可以在＆＃34; 3 * 11＆＃34;以及{{{ 1}}匹配＆＃34; 3 *＆＃34;，例如：

StartsWith

Answer 7

借助when clause，您现在可以执行以下操作：

Run 1:MAPE=20.99
Run 2:MAPE=30.02
Run 3:MAPE=50.22

Answer 8

从 C# 9 开始，您可以执行以下操作：

switch(subArea)
{
    case "4100":
    case "4101":
    case "4102":
    case "4200":
        return "ABC";
    case "600A":
        return "XWZ";
    case { } when subArea.StartsWith("3*"):
    case { } when subArea.StartsWith("03*"):
        return "123";
    default:
        return "ABCXYZ123";
}

在Switch语句中使用.StartsWith？

8 个答案: