Here is a example of using yield:

public static IEnumerable<int> Unique(IEnumerable<int> nums)
{
    Dictionary<int, int> uniqueVals = new
        Dictionary<int, int>();
    foreach (int num in nums)
    {
        if (!uniqueVals.ContainsKey(num))
        {
            uniqueVals.Add(num, num);
            yield return num;
        }
    }
}

Dompare the typical way and the "yield" to get IEnumerable

# IList<string> FindBobs(IEnumerable<string> names)  
# {  
#     var bobs = new List<string>();  
#   
#     foreach(var currName in names)  
#     {  
#         if(currName == "Bob")  
#             bobs.Add(currName);  
#     }  
#   
#     return bobs;  
# }
# IEnumerable<string> FindBobs(IEnumerable<string> names)  
# {  
#     foreach(var currName in names)  
#     {  
#         if(currName == "Bob")  
#             yield return currName;  
#     }  
# }