像这样的怎么样?注意，如果myEnumerable为空，myDelimitedString可能为空。

IEnumerator enumerator = myEnumerable.GetEnumerator();
string myDelimitedString;
string current = null;

if( enumerator.MoveNext() )
    current = (string)enumerator.Current;

while( null != current)
{
    current = (string)enumerator.Current; }

    myDelimitedString += current;

    if( enumerator.MoveNext() )
        myDelimitedString += DELIMITER;
    else
        break;
}

我想更理论化地讨论这个问题(因为它已经有了足够多的实际答案)

.net为数据组(又称集合)提供了一个非常好的抽象模型。

在最顶端，也是最抽象的，你有一个IEnumerable它只是一组你可以枚举的数据。你如何枚举并不重要，重要的是你可以枚举一些数据。这个枚举是由一个完全不同的对象完成的，IEnumerator

这些接口定义如下:

//
// Summary:
//     Exposes an enumerator, which supports a simple iteration over a non-generic collection.
public interface IEnumerable
{
    //
    // Summary:
    //     Returns an enumerator that iterates through a collection.
    //
    // Returns:
    //     An System.Collections.IEnumerator object that can be used to iterate through
    //     the collection.
    IEnumerator GetEnumerator();
}

//
// Summary:
//     Supports a simple iteration over a non-generic collection.
public interface IEnumerator
{
    //
    // Summary:
    //     Gets the element in the collection at the current position of the enumerator.
    //
    // Returns:
    //     The element in the collection at the current position of the enumerator.
    object Current { get; }

    //
    // Summary:
    //     Advances the enumerator to the next element of the collection.
    //
    // Returns:
    //     true if the enumerator was successfully advanced to the next element; false if
    //     the enumerator has passed the end of the collection.
    //
    // Exceptions:
    //   T:System.InvalidOperationException:
    //     The collection was modified after the enumerator was created.
    bool MoveNext();
    //
    // Summary:
    //     Sets the enumerator to its initial position, which is before the first element
    //     in the collection.
    //
    // Exceptions:
    //   T:System.InvalidOperationException:
    //     The collection was modified after the enumerator was created.
    void Reset();
}

as you might have noticed, the IEnumerator interface doesn't "know" what an index is, it just knows what element it's currently pointing to, and how to move to the next one. now here is the trick: foreach considers every input collection an IEnumerable, even if it is a more concrete implementation like an IList<T> (which inherits from IEnumerable), it will only see the abstract interface IEnumerable. what foreach is actually doing, is calling GetEnumerator on the collection, and calling MoveNext until it returns false. so here is the problem, you want to define a concrete concept "Indices" on an abstract concept "Enumerables", the built in foreach construct doesn't give you that option, so your only way is to define it yourself, either by what you are doing originally (creating a counter manually) or just use an implementation of IEnumerator that recognizes indices AND implement a foreach construct that recognizes that custom implementation.

就我个人而言，我会创建一个这样的扩展方法

public static class Ext
{
    public static void FE<T>(this IEnumerable<T> l, Action<int, T> act)
    {
        int counter = 0;
        foreach (var item in l)
        {
            act(counter, item);
            counter++;
        }
    }
}

像这样使用它

var x = new List<string>() { "hello", "world" };
x.FE((ind, ele) =>
{
    Console.WriteLine($"{ind}: {ele}");
});

这也避免了在其他答案中看到的任何不必要的分配。

Ian Mercer在Phil Haack的博客上发表了一个类似的解决方案:

foreach (var item in Model.Select((value, i) => new { i, value }))
{
    var value = item.value;
    var index = item.i;
}

通过重载LINQ的Select函数，你可以得到item (item.value)和它的索引(item.i):

函数[在Select内部]的第二个参数表示源元素的索引。

新的{i, value}正在创建一个新的匿名对象。

如果你使用c# 7.0或更高版本，可以使用ValueTuple来避免堆分配:

foreach (var item in Model.Select((value, i) => ( value, i )))
{
    var value = item.value;
    var index = item.i;
}

你也可以删除这个项目。通过使用自动解构:

foreach (var (value, i) in Model.Select((value, i) => ( value, i )))
{
    // Access `value` and `i` directly here.
}

c# 7最终为我们提供了一种优雅的方式:

static class Extensions
{
    public static IEnumerable<(int, T)> Enumerate<T>(
        this IEnumerable<T> input,
        int start = 0
    )
    {
        int i = start;
        foreach (var t in input)
        {
            yield return (i++, t);
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        var s = new string[]
        {
            "Alpha",
            "Bravo",
            "Charlie",
            "Delta"
        };

        foreach (var (i, t) in s.Enumerate())
        {
            Console.WriteLine($"{i}: {t}");
        }
    }
}

可以用另一个包含索引信息的枚举数来包装原始枚举数。

foreach (var item in ForEachHelper.WithIndex(collection))
{
    Console.Write("Index=" + item.Index);
    Console.Write(";Value= " + item.Value);
    Console.Write(";IsLast=" + item.IsLast);
    Console.WriteLine();
}

下面是ForEachHelper类的代码。

public static class ForEachHelper
{
    public sealed class Item<T>
    {
        public int Index { get; set; }
        public T Value { get; set; }
        public bool IsLast { get; set; }
    }

    public static IEnumerable<Item<T>> WithIndex<T>(IEnumerable<T> enumerable)
    {
        Item<T> item = null;
        foreach (T value in enumerable)
        {
            Item<T> next = new Item<T>();
            next.Index = 0;
            next.Value = value;
            next.IsLast = false;
            if (item != null)
            {
                next.Index = item.Index + 1;
                yield return item;
            }
            item = next;
        }
        if (item != null)
        {
            item.IsLast = true;
            yield return item;
        }            
    }
}

除非你的集合可以通过某种方法返回对象的索引，否则唯一的方法就是使用像你的例子中那样的计数器。

然而，当使用索引时，唯一合理的答案是使用for循环。其他任何事情都会带来代码的复杂性，更不用说时间和空间的复杂性了。