ImmutableList<T>.FindIndex Method (Int32, Predicate<T>)
Searches for an element that matches the conditions defined by the specified predicate, and returns the zero-based index of the first occurrence within the range of elements in the immutable list that extends from the specified index to the last element.
Assembly: System.Collections.Immutable (in System.Collections.Immutable.dll)
The zero-based starting index of the search.
Return ValueType: System.Int32
The zero-based index of the first occurrence of an element that matches the conditions defined by match, if found; otherwise, ?1.