/// <summary>
/// Initializes a new instance of the <see cref="NestedEnumerableExpression"/> class.
/// </summary>
/// <param name="baseEnumerable">The base enumerable.</param>
/// <param name="getNestedEnumerable">
/// Expression that takes elements from the base enumerable and returns nested enumerables.
/// </param>
/// <param name="elementType">The type of the element of the enumerable.</param>
public NestedEnumerableExpression(
LinearEnumerableExpression baseEnumerable,
Func <ParameterExpression, EnumerableExpression> getNestedEnumerable,
Type elementType)
{
this.BaseEnumerable = baseEnumerable ?? throw new ArgumentNullException(nameof(baseEnumerable));
this.GetNestedEnumerable = getNestedEnumerable ?? throw new ArgumentNullException(nameof(getNestedEnumerable));
this.ElementType = elementType ?? throw new ArgumentNullException(nameof(elementType));
}
/// <summary>
/// Takes a selector and uses it to map the elements of an enumerable.
/// expression : Expression{IQueryable{T}}.
/// selector : ((Expression{U} -> Expression{void}) -> (Expression{T} -> Expression{void})).
/// returns : Expression{IQueryable{U}}.
///
/// The selector takes a continuation that handles new elements and returns a continuation
/// that takes old elements, gets the new element, and passes it into that continuation.
/// </summary>
/// <param name="expression">The enumerable expression to map.</param>
/// <param name="selector">The mapping function.</param>
/// <param name="newElementType">The new type of the enumerable.</param>
/// <returns>The new mapped enumerable.</returns>
private static EnumerableExpression SelectRaw(
this EnumerableExpression expression,
Func <Func <ParameterExpression, Expression>, Func <ParameterExpression, Expression> > selector,
Type newElementType)
{
return(expression switch
{
LinearEnumerableExpression linear =>
new LinearEnumerableExpression(
linear.Initialize,
linear.HasNext,
c => linear.MoveNext(selector(c)),
newElementType),
NestedEnumerableExpression nested =>
new NestedEnumerableExpression(
nested.BaseEnumerable,
e => nested.GetNestedEnumerable(e).SelectRaw(selector, newElementType),
newElementType),
_ => throw new ArgumentException("Must be a linear or nested enumerable.", nameof(expression)),
});
