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;
}

只需添加自己的索引。保持简单。

int i = -1;
foreach (var item in Collection)
{
    ++i;
    item.index = i;
}

我在LINQPad中构建了这个:

var listOfNames = new List<string>(){"John","Steve","Anna","Chris"};

var listCount = listOfNames.Count;

var NamesWithCommas = string.Empty;

foreach (var element in listOfNames)
{
    NamesWithCommas += element;
    if(listOfNames.IndexOf(element) != listCount -1)
    {
        NamesWithCommas += ", ";
    }
}

NamesWithCommas.Dump();  //LINQPad method to write to console.

你也可以使用string.join:

var joinResult = string.Join(",", listOfNames);

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。

最好像这样使用关键字继续安全构造

int i=-1;
foreach (Object o in collection)
{
    ++i;
    //...
    continue; //<--- safe to call, index will be increased
    //...
}

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;
}