Reverse each number in a list
You can improve the efficiency by reversing the numbers as integers instead of converting to strings and back.

Modifying the list and returning it is redundant since the list is passed by reference and your changes will persist to the original list.

Simplified your code could look like this:

public static void ReverseIntegerList_Array(List&lt;int&gt; intList)
{
    if(intList == null)
    {
        return;
    }
    int limit = intList.Count;
    for(int i = 0; i &lt; limit;++i)
    {
        intList[i] = ReverseNum(intList[i]);
    }
}

public static int ReverseNum(int num)
{
    int retVal = 0;
    while(num &gt; 0)
    {
        retVal = (retVal * 10) + num % 10;
        num /= 10;
    }
    return retVal;
}