C# in Depth

Conversion361Cast and OfType convert an untyped sequence into a typed one, either throwingan exception (for Cast) or ignoring (for OfType) elements of the input sequence thataren’t implicitly convertible to the output sequence element type. This may also beused to convert typed sequences into more specifically typed sequences, such as convertingIEnumerable to IEnumerable. The conversions are performedin a streaming manner with deferred execution.ToDictionary and ToLookup both take delegates to obtain the key for any particularelement; ToDictionary returns a dictionary mapping the key to the element type,whereas ToLookup returns an appropriately typed ILookup. A lookup is like a dictionarywhere the value associated with a key isn’t one element but a sequence of elements.Lookups are generally used when duplicate keys are expected as part of normal operation,whereas a duplicate key will cause ToDictionary to throw an exception. Morecomplicated overloads of both methods allow a custom IEqualityComparer to beused to compare keys, and a conversion delegate to be applied to each element beforeit is put into the dictionary or lookup.The examples in table A.3 use two additional sequences to demonstrate Cast andOfType:object[] allStrings = {"These", "are", "all", "strings"};object[] notAllStrings = {"Number", "at", "the", "end", 5};Table A.3Conversion examplesExpressionallStrings.Cast()allStrings.OfType()notAllStrings.Cast()notAllStrings.OfType()Result"These", "are", "all", "strings"(as IEnumerable)"These", "are", "all", "strings"(as IEnumerable)Exception is thrown while iterating, at point of failingconversion"Number", "at", "the", "end"(as IEnumerable)numbers.ToArray() 0, 1, 2, 3, 4(as int[])numbers.ToList() 0, 1, 2, 3, 4(as List)words.ToDictionary(word =>word.Substring(0, 2))Dictionary contents:"ze": "zero""on": "one""tw": "two""th": "three""fo": "four"Licensed to Rhona Hadida