Unless I misunderstand the question, the following seems to work fine for me in C# 4:
public static class Defines
{
public static T Identity<T>(T pValue)
{
return pValue;
}
...
You can then do the following in your example:
var result =
enumerableOfEnumerables
.SelectMany(Defines.Identity);
As well as use Defines.Identity
anywhere you would use a lambda that looks like x => x
.