Apress pro LINQ Language Integrated Query in C# 2008 phần 4 ppt

160 CHAPTER 5 ■ NONDEFERRED OPERATORS

This code is functionally equivalent to the previous example. Instead of calling the Where operator

to ensure a single element is in the sequence, I can provide the same sequence filtering operation in

the Single operator itself. This should return the only element in the input sequence whose id is 3.

Here are the results:

Anders Hejlsberg

Remember, if either prototype of the Single operator ends up with no element to return, an

InvalidOperationException is thrown. To avoid this, use the SingleOrDefault operator.

SingleOrDefault

The SingleOrDefault operator is similar to the Single operator except for how it behaves when an

element is not found.

Prototypes

There are two prototypes I cover.

The First SingleOrDefault Prototype

public static T SingleOrDefault<T>(

this IEnumerable<T> source);

This version of the prototype returns the only element found in the input sequence. If the sequence

is empty, default(T) is returned. For reference and nullable types, the default value is null. If more

than one element is found, an InvalidOperationException is thrown.

The second prototype of the SingleOrDefault operator allows you to pass a predicate to deter￾mine which element should be returned.

The Second SingleOrDefault Prototype

public static T SingleOrDefault<T>(

this IEnumerable<T> source,

Func<T, bool> predicate);

Exceptions

ArgumentNullException is thrown if any arguments are null.

InvalidOperationException is thrown if the operator finds more than one element for which the

predicate returns true.

Examples

Listing 5-30 is an example of the first SingleOrDefault prototype where no element is found. I have

to get an empty sequence to do this. I’ll use the Where operator and provide a key comparison for a

key that doesn’t exist for this purpose.

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CHAPTER 5 ■ NONDEFERRED OPERATORS 161

Listing 5-30. Calling the First SingleOrDefault Prototype Where an Element Is Not Found

Employee emp = Employee.GetEmployeesArray()

.Where(e => e.id == 5).SingleOrDefault();

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

I queried for the employee whose id is 5 since I know none exists, so an empty sequence will be

returned. Unlike the Single operator, the SingleOrDefault operator handles empty sequences just

fine. Here are the results:

NULL

Listing 5-31 is the same example where a single element is found. I use the Where operator to

provide a sequence with just one element.

Listing 5-31. Calling the First SingleOrDefault Prototype Where an Element Is Found

Employee emp = Employee.GetEmployeesArray()

.Where(e => e.id == 4).SingleOrDefault();

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

This time I specify an id I know exists. Here are the results for the code when an element is

found:

David Lightman

As you can see, the employee has been found. For the second SingleOrDefault prototype, shown

in Listing 5-32, I specify an id that I know exists. Instead of using the Where operator, I embed the filter

into the SingleOrDefault operator call.

Listing 5-32. Calling the Second SingleOrDefault Prototype Where an Element Is Found

Employee emp = Employee.GetEmployeesArray()

.SingleOrDefault(e => e.id == 4);

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

This example is functionally equivalent to the previous example except instead of filtering the

elements using the Where operator, I filter them by passing a predicate to the SingleOrDefault oper￾ator. Here are the results:

David Lightman

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162 CHAPTER 5 ■ NONDEFERRED OPERATORS

Now I will try that with a predicate that will not find a match, as shown in Listing 5-33.

Listing 5-33. Calling the Second LastOrDefault Prototype Where an Element Is Not Found

Employee emp = Employee.GetEmployeesArray()

.SingleOrDefault(e => e.id == 5);

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

Since there is no element whose id is 5, no elements are found. Here are the results:

NULL

While no elements were found in the sequence, the SingleOrDefault operator handled the situation

gracefully instead of throwing an exception.

ElementAt

The ElementAt operator returns the element from the source sequence at the specified index.

Prototypes

There is one prototype I cover.

The ElementAt Prototype

public static T ElementAt<T>(

this IEnumerable<T> source,

int index);

If the sequence implements IList<T>, the IList interface is used to retrieve the indexed element

directly. If the sequence does not implement IList<T>, the sequence is enumerated until the indexed

element is reached. An ArgumentOutOfRangeException is thrown if the index is less than zero or greater

than or equal to the number of elements in the sequence.

■Note In C#, indexes are zero-based. This means the first element’s index is zero. The last element’s index is

the sequence’s count minus one.

Exceptions

ArgumentNullException is thrown if the source argument is null.

ArgumentOutOfRangeException is thrown if the index is less than zero or greater than or equal to

the number of elements in the sequence.

Examples

Listing 5-34 is an example calling the only prototype of the ElementAt operator.

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Listing 5-34. Calling the ElementAt Operator

Employee emp = Employee.GetEmployeesArray()

.ElementAt(3);

Console.WriteLine("{0} {1}", emp.firstName, emp.lastName);

I specified that I want the element whose index is 3, which is the fourth element. Here are the

results of the query:

David Lightman

ElementAtOrDefault

The ElementAtOrDefault operator returns the element from the source sequence at the specified

index.

Prototypes

There is one prototype I cover.

The ElementAtOrDefault Prototype

public static T ElementAtOrDefault<T>(

this IEnumerable<T> source,

int index);

If the sequence implements IList<T>, the IList interface is used to retrieve the indexed element

directly. If the sequence does not implement IList<T>, the sequence will be enumerated until the

indexed element is reached.

If the index is less than zero or greater than or equal to the number of elements in the sequence,

default(T) is returned. For reference and nullable types, the default value is null. This is the behavior

that distinguishes it from the ElementAt operator.

Exceptions

ArgumentNullException is thrown if the source argument is null.

Examples

Listing 5-35 is an example calling the ElementAtOrDefault operator when the index is valid.

Listing 5-35. Calling the ElementAtOrDefault Operator with a Valid Index

Employee emp = Employee.GetEmployeesArray()

.ElementAtOrDefault(3);

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

Here are the results of the query:

David Lightman

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164 CHAPTER 5 ■ NONDEFERRED OPERATORS

Just as expected, the element at index 3 is retrieved. Now I will try a query with an invalid index

using the code in Listing 5-36.

Listing 5-36. Calling the ElementAtOrDefault Operator with an Invalid Index

Employee emp = Employee.GetEmployeesArray()

.ElementAtOrDefault(5);

Console.WriteLine(emp == null ? "NULL" :

string.Format("{0} {1}", emp.firstName, emp.lastName));

There is no element whose index is 5. Here are the results of the query:

NULL

Quantifiers

The following quantifier operators allow you to perform quantification type operations on input

sequences.

Any

The Any operator returns true if any element of an input sequence matches a condition.

Prototypes

There are two prototypes I cover.

The First Any Prototype

public static bool Any<T>(

this IEnumerable<T> source);

This prototype of the Any operator will return true if the source input sequence contains any

elements. The second prototype of the Any operator enumerates the source input sequence and

returns true if at least one element in the input sequence causes the predicate method delegate to

return true. The source input sequence enumeration halts once the predicate returns true.

The Second Any Prototype

public static bool Any<T>(

this IEnumerable<T> source,

Func<T, bool> predicate);

Exceptions

ArgumentNullException is thrown if any of the arguments are null.

Examples

First I will try the case of an empty sequence, as shown in Listing 5-37. I will use the Empty operator I

covered in the previous chapter.

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