C# in Depth

Equality operations363A.4 Element operationsThis is another selection of query operators that are grouped in pairs (see table A.4).This time, the pairs all work the same way. There’s a simple version that picks a singleelement if it can or throws an exception if the specified element doesn’t exist, and aversion with OrDefault at the end of the name. The OrDefault version is exactly thesame except that it returns the default value for the result type instead of throwing anexception if it can’t find the element you’ve asked for. All of these operators useimmediate execution.The operator names are easily understood: First and Last return the first andlast elements of the sequence respectively (only defaulting if there are no elements),Single returns the only element in a sequence (defaulting if there isn’t exactly oneelement), and ElementAt returns a specific element by index (the fifth element, forexample). In addition, there’s an overload for all of the operators other thanElementAt to filter the sequence first—for example, First can return the first elementthat matches a given condition.Table A.4Single element selection examplesExpressionResultwords.ElementAt(2)words.ElementAtOrDefault(10)words.First()words.First(word => word.Length==3)words.First(word => word.Length==10)words.FirstOrDefault(word => word.Length==10)words.Last()words.Single()words.SingleOrDefault()words.Single(word => word.Length==5)words.Single(word => word.Length==10)"two"null"zero""one"Exception: No matching elementsnull"four"Exception: More than one elementnull"three"Exception: No matching elementsA.5 Equality operationsThere’s only one equality operation: SequenceEqual (see table A.5). This just comparestwo sequences for element-by-element equality, including order. For instance,the sequence 0, 1, 2, 3, 4 is not equal to 4, 3, 2, 1, 0. An overload allows a specificIEqualityComparer to be used when comparing elements. The return value is justa Boolean, and is computed with immediate execution.Licensed to Rhona Hadida