How to replace list item in best way

Question

if (listofelements.Contains(valueFieldValue.ToString()))
{
    listofelements[listofelements.IndexOf(valueFieldValue.ToString())] = value.ToString();
}

Answer 1

You could make it more readable and more efficient:

string oldValue = valueFieldValue.ToString();
string newValue = value.ToString();
int index = listofelements.IndexOf(oldValue);
if(index != -1)
    listofelements[index] = newValue;

This asks only once for the index. Your approach uses Contains first which needs to loop all items(in the worst case), then you're using IndexOf which needs to enumerate the items again .