When to use Cast() and Oftype() in Linq

It should be noted that Cast(Of T) can be used on IEnumerable unlike other LINQ functions, so if there's ever a case where you need to use LINQ on a non-generic collection or list such as an ArrayList, you can use Cast(Of T) to cast to an IEnumerable(Of T) where LINQ can work.

You should call Cast<string>() if you know that all of the items are strings.
If some of them aren't strings, you'll get an exception.

You should call OfType<string>() if you know that some of the items aren't strings and you don't want those items.
If some of them aren't strings, they won't be in the new IEnumerable<string>.

OfType - return only the elements that can safely be cast to type x.
Cast - will try to cast all the elements into type x. if some of them are not from this type you will get InvalidCastException

EDIT
for example:

object[] objs = new object[] { "12345", 12 };
objs.Cast<string>().ToArray(); //throws InvalidCastException
objs.OfType<string>().ToArray(); //return { "12345" }

Source: LINQ Tip: Enumerable.OfType - Solutionizing .NET

Fundamentally, Cast<T>() is implemented like this:

public IEnumerable<T> Cast<T>(this IEnumerable source)
{
  foreach(object o in source)
    yield return (T) o;
}

Using an explicit cast performs well, but will result in an InvalidCastException if the cast fails. A less efficient yet useful variation on this idea is OfType<T>():

public IEnumerable<T> OfType<T>(this IEnumerable source)
{
  foreach(object o in source)
    if(o is T t)
      yield return t;
}

The returned enumeration will only include elements that can safely be cast to the specified type.