What is an easy way to append or prepend a single value to an IEnumerable<T>?

I wrote custom extension methods to do this:

public static IEnumerable<T> Append<T>(this IEnumerable<T> source, T item)
{
    foreach (T i in source)
        yield return i;

    yield return item;
}

public static IEnumerable<T> Prepend<T>(this IEnumerable<T> source, T item)
{
    yield return item;

    foreach (T i in source)
        yield return i;
}

In your scenario, you would write:

var all = GetData().Prepend(GetHeaders());

As chilltemp commented, this does not mutate the original collection. In true Linq fashion, it generates a new IEnumerable<T>.

Note: An eager null argument check is recommended for source, but not shown for brevity.

Use the Enumerable.Concat extension method. For appending values instead of prepending, you'd just call Concat in reverse. (ie: GetData().Concat(GetHeaders());)

If GetHeaders() returns a single string array, I'd personally probably wrap it in a single element array instead of a list:

 var all = (new[] {GetHeaders()}).Concat(GetData());