C# 向要显示的项目列表添加分隔符

我有一个要显示的项目列表，它们之间用C#分隔。使用普通迭代器，我将在开始或结束时使用一个额外的分隔符：

string[] sa = {"one", "two", "three", "four"};
string ns = "";
foreach(string s in sa)
{
    ns += s + " * ";
}
// ns has a trailing *:
// one * two * three * four * 

现在我可以使用for循环解决这个问题，如下所示：

ns = "";
for(int i=0; i<sa.Length; i++)
{
    ns += sa[i];
    if(i != sa.Length-1)
        ns += " * ";
}
// this works:
// one * two * three * four

ns=”“；
对于（int i=0；i您需要内置方法：

如果要对其他集合类型执行相同的操作，则如果首先使用LINQ的方法创建数组，则仍然可以使用
String.Join
：

这种方法的优点是可以在任何类型的序列上使用它，而不仅仅是字符串数组

var ns = sa.Aggregate( (c, s) => c + " * " + s);

我更喜欢卢克的解决方案

string ns = string.Join(" * ", sa);

或者，如果您的集合不是可索引的，而是可枚举的，则可以执行此操作：

string ns = "";
foreach(string s in sa)
{
    if (ns.Length != 0)
    {
        ns += " * ";
    }

    ns += s;
}

这与第二个示例类似，但它将测试放在循环的开始，并且不太可能出现第二个示例中可能出现的一次性错误。数组显然是可索引的，但在某些情况下会得到容器（即System.Collections.Generic.Dictionary.Values）这是不可索引的，你会想要这样的东西。
< P>除了优雅之外，你还需要考虑不同于字符串的类型的速度和可重用性。对于优雅，我建议使用扩展方法来抽象细节，以便普通用法看起来像：

ns = sa.Join(" * ");

<速度>考虑下面的变型测试，包括一些已经回答了问题的人提出的一些解决方案：

public void Test_variants()
{
    const string item = "a";
    const int numberOfTimes = 100000;
    const string delimiter = ", ";
    string[] items = new List<string>(Enumerable.Repeat(item, numberOfTimes)).ToArray();
    string expected = String.Join(delimiter, items);

    Time(StringJoin, items, delimiter, expected);
    Time(Aggregate, items, delimiter, expected);
    Time(CheckForEndInsideLoop_String, items, delimiter, expected);
    Time(CheckForBeginningInsideLoop_String, items, delimiter, expected);
    Time(RemoveFinalDelimiter_String, items, delimiter, expected);
    Time(CheckForEndInsideLoop_StringBuilder, items, delimiter, expected);
    Time(RemoveFinalDelimiter_StringBuilder, items, delimiter, expected);
}

private static void Time(Func<string[], string, string> func, string[] items, string delimiter, string expected)
{
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();
    string result = func(items, delimiter);
    stopwatch.Stop();
    bool isValid = result == expected;
    Console.WriteLine("{0}\t{1}\t{2}", stopwatch.Elapsed, isValid, func.Method.Name);
}

private static string CheckForEndInsideLoop_String(string[] items, string delimiter)
{
    string result = "";
    for (int i = 0; i < items.Length; i++)
    {
        result += items[i];
        if (i != items.Length - 1)
        {
            result += delimiter;
        }
    }
    return result;
}

private static string RemoveFinalDelimiter_String(string[] items, string delimiter)
{
    string result = "";
    for (int i = 0; i < items.Length; i++)
    {
        result += items[i] + delimiter;
    }
    return result.Substring(0, result.Length - delimiter.Length);
}

private static string CheckForBeginningInsideLoop_String(string[] items, string delimiter)
{
    string result = "";
    foreach (string s in items)
    {
        if (result.Length != 0)
        {
            result += delimiter;
        }
        result += s;
    }
    return result;
}

private static string CheckForEndInsideLoop_StringBuilder(string[] items, string delimiter)
{
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < items.Length; i++)
    {
        result.Append(items[i]);
        if (i != items.Length - 1)
        {
            result.Append(delimiter);
        }
    }
    return result.ToString();
}

private static string RemoveFinalDelimiter_StringBuilder(string[] items, string delimiter)
{
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < items.Length; i++)
    {
        result.Append(items[i]);
        result.Append(delimiter);
    }
    result.Length = result.Length - delimiter.Length;
    return result.ToString();
}

private static string StringJoin(string[] items, string delimiter)
{
    return String.Join(delimiter, items);
}

private static string Aggregate(string[] items, string delimiter)
{
    return items.Aggregate((c, s) => c + delimiter + s);
}

Joe，既然您使用的是LINQ，那么如果您从序列中创建一个数组，然后使用String.Join，您将获得更好的性能和更可读的代码。例如：String.Join（“*”，test.ToArray（））；是的，可以从一个对象序列将某个字段投影到一个字符串序列，然后使用您的方法。我不知道字符串。Join before-一个说明LINQ不是万能药的完美示例：）+1，扩展方法非常适合于此。在回答另一个问题时，我发布了一个类似的例子，它也将转换器函数作为参数：
ns = sa.Join(" * ");

public void Test_variants()
{
    const string item = "a";
    const int numberOfTimes = 100000;
    const string delimiter = ", ";
    string[] items = new List<string>(Enumerable.Repeat(item, numberOfTimes)).ToArray();
    string expected = String.Join(delimiter, items);

    Time(StringJoin, items, delimiter, expected);
    Time(Aggregate, items, delimiter, expected);
    Time(CheckForEndInsideLoop_String, items, delimiter, expected);
    Time(CheckForBeginningInsideLoop_String, items, delimiter, expected);
    Time(RemoveFinalDelimiter_String, items, delimiter, expected);
    Time(CheckForEndInsideLoop_StringBuilder, items, delimiter, expected);
    Time(RemoveFinalDelimiter_StringBuilder, items, delimiter, expected);
}

private static void Time(Func<string[], string, string> func, string[] items, string delimiter, string expected)
{
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();
    string result = func(items, delimiter);
    stopwatch.Stop();
    bool isValid = result == expected;
    Console.WriteLine("{0}\t{1}\t{2}", stopwatch.Elapsed, isValid, func.Method.Name);
}

private static string CheckForEndInsideLoop_String(string[] items, string delimiter)
{
    string result = "";
    for (int i = 0; i < items.Length; i++)
    {
        result += items[i];
        if (i != items.Length - 1)
        {
            result += delimiter;
        }
    }
    return result;
}

private static string RemoveFinalDelimiter_String(string[] items, string delimiter)
{
    string result = "";
    for (int i = 0; i < items.Length; i++)
    {
        result += items[i] + delimiter;
    }
    return result.Substring(0, result.Length - delimiter.Length);
}

private static string CheckForBeginningInsideLoop_String(string[] items, string delimiter)
{
    string result = "";
    foreach (string s in items)
    {
        if (result.Length != 0)
        {
            result += delimiter;
        }
        result += s;
    }
    return result;
}

private static string CheckForEndInsideLoop_StringBuilder(string[] items, string delimiter)
{
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < items.Length; i++)
    {
        result.Append(items[i]);
        if (i != items.Length - 1)
        {
            result.Append(delimiter);
        }
    }
    return result.ToString();
}

private static string RemoveFinalDelimiter_StringBuilder(string[] items, string delimiter)
{
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < items.Length; i++)
    {
        result.Append(items[i]);
        result.Append(delimiter);
    }
    result.Length = result.Length - delimiter.Length;
    return result.ToString();
}

private static string StringJoin(string[] items, string delimiter)
{
    return String.Join(delimiter, items);
}

private static string Aggregate(string[] items, string delimiter)
{
    return items.Aggregate((c, s) => c + delimiter + s);
}

00:00:00.0027745    True    StringJoin
00:00:24.5523967    True    Aggregate
00:00:47.8091632    True    CheckForEndInsideLoop_String
00:00:47.4682981    True    CheckForBeginningInsideLoop_String
00:00:23.7972864    True    RemoveFinalDelimiter_String
00:00:00.0076439    True    CheckForEndInsideLoop_StringBuilder
00:00:00.0052803    True    RemoveFinalDelimiter_StringBuilder
00:00:00.0001788    True    StringJoin
00:00:00.0014983    True    Aggregate
00:00:00.0001666    True    CheckForEndInsideLoop_String
00:00:00.0002202    True    CheckForBeginningInsideLoop_String
00:00:00.0002061    True    RemoveFinalDelimiter_String
00:00:00.0002663    True    CheckForEndInsideLoop_StringBuilder
00:00:00.0002278    True    RemoveFinalDelimiter_StringBuilder
public static string Join<T>([CanBeNull] this IEnumerable<T> items, [CanBeNull] string delimiter)
{
    StringBuilder result = new StringBuilder();
    if (items != null && items.Any())
    {
        delimiter = delimiter ?? "";
        foreach (T item in items)
        {
            result.Append(item);
            result.Append(delimiter);
        }
        result.Length = result.Length - delimiter.Length;
    }
    return result.ToString();
}

ns = sa.Join(" * ");