Tutorial by Examples



The yield keyword is used to define a function which returns an IEnumerable or IEnumerator (as well as their derived generic variants) whose values are generated lazily as a caller iterates over the returned collection. Read more about the purpose in the remarks section. The following example has a...
public IEnumerable<User> SelectUsers() { // Execute an SQL query on a database. using (IDataReader reader = this.Database.ExecuteReader(CommandType.Text, "SELECT Id, Name FROM Users")) { while (reader.Read()) { int id = reader.GetInt32(0...
You can extend the functionality of existing yield methods by passing in one or more values or elements that could define a terminating condition within the function by calling a yield break to stop the inner loop from executing. public static IEnumerable<int> CountUntilAny(int start, HashSet...
An iterator method is not executed until the return value is enumerated. It's therefore advantageous to assert preconditions outside of the iterator. public static IEnumerable<int> Count(int start, int count) { // The exception will throw when the method is called, not when the result i...
public IEnumerable<int> F1() { for (int i = 0; i < 3; i++) yield return i; //return F2(); // Compile Error!! foreach (var element in F2()) yield return element; } public int[] F2() { return new[] { 3, 4, 5 }; }
Only when the foreach statement moves to the next item does the iterator block evaluate up to the next yield statement. Consider the following example: private IEnumerable<int> Integers() { var i = 0; while(true) { Console.WriteLine("Inside iterator: " + i...
If an iterator method has a yield inside a try...finally, then the returned IEnumerator will execute the finally statement when Dispose is called on it, as long as the current point of evaluation is inside the try block. Given the function: private IEnumerable<int> Numbers() { yield re...
The IEnumerable<T> interface has a single method, GetEnumerator(), which returns an IEnumerator<T>. While the yield keyword can be used to directly create an IEnumerable<T>, it can also be used in exactly the same way to create an IEnumerator<T>. The only thing that changes ...
The yield keyword allows lazy-evaluation of the collection. Forcibly loading the whole collection into memory is called eager evaluation. The following code shows this: IEnumerable<int> myMethod() { for(int i=0; i <= 8675309; i++) { yield return i; } } ... // ...
using System; using System.Collections.Generic; using System.Linq; using System.Numerics; // also add reference to System.Numberics namespace ConsoleApplication33 { class Program { private static IEnumerable<BigInteger> Fibonacci() { BigInteger p...
Using yield break as opposed to break might not be as obvious as one may think. There are lot of bad examples on the Internet where the usage of the two is interchangeable and doesn't really demonstrate the difference. The confusing part is that both of the keywords (or key phrases) make sense only...

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