{"id":1712,"date":"2023-05-04T20:31:02","date_gmt":"2023-05-04T13:31:02","guid":{"rendered":"https:\/\/csharptutorial.net\/?page_id=1712"},"modified":"2023-05-13T14:42:13","modified_gmt":"2023-05-13T07:42:13","slug":"csharp-state-pattern","status":"publish","type":"page","link":"https:\/\/www.csharptutorial.net\/csharp-design-patterns\/csharp-state-pattern\/","title":{"rendered":"C# State Pattern"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you will learn how to use the C# State pattern to allow an object to change its behavior when its internal state changes.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Introduction to the C# State pattern<\/h2>\n\n\n\n<p>The State pattern is a behavioral design pattern that allows an object to change its behavior when its internal state changes.<\/p>\n\n\n\n<p>The State pattern is useful when the behavior of an object depends on its state and needs to change dynamically when its state changes.<\/p>\n\n\n\n<p>The following UML diagram illustrates the State pattern:<\/p>\n\n\n<div class=\"wp-block-image\">\n<figure class=\"aligncenter size-full\"><img decoding=\"async\" src=\"https:\/\/csharptutorial.net\/wp-content\/uploads\/2023\/05\/CSharp-State-Pattern.svg\" alt=\"C# State Pattern\" class=\"wp-image-1718\"\/><\/figure>\n<\/div>\n\n\n<p>The State pattern typically consists of the following participants:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><code>Context<\/code> is an object that maintains the current state and provides an interface for the clients to interact with the object.<\/li>\n\n\n\n<li><code>IState<\/code> is an <a href=\"https:\/\/csharptutorial.net\/csharp-tutorial\/csharp-interface\/\">interface<\/a> that defines methods that the concrete states must implement.<\/li>\n\n\n\n<li><code><code>ConcreteState<\/code><\/code> is the concrete class that provides a specific implementation of the <code>IState<\/code> interface. Each <code><code>ConcreteState<\/code><\/code> class represents the different behavior of the object.<\/li>\n<\/ul>\n\n\n\n<p>When the state of the context changes, it delegates the behavior to the current <code><code>ConcreteState<\/code><\/code> object. The <code><code>ConcreteState<\/code><\/code> class implements the behavior associated with the state. It allows the object to change its behavior dynamically without changing its interface.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">C# State pattern example<\/h2>\n\n\n\n<p>Suppose you need to manage invoices with various states: unpaid, paid, canceled, and refunded:<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img decoding=\"async\" src=\"https:\/\/csharptutorial.net\/wp-content\/uploads\/2023\/05\/Csharp-State-Pattern-Example-Invoice-States.svg\" alt=\"C# State Pattern Example - Invoice States\" class=\"wp-image-1715\"\/><\/figure>\n\n\n\n<p>The following program illustrates how to use the State pattern to manage the behavior of invoices which changes dynamically based on their states:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-1\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">namespace<\/span> <span class=\"hljs-title\">StatePattern<\/span>;\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">interface<\/span> <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n}\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Invoice<\/span>\n{\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">int<\/span> Number\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">decimal<\/span> Amount\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">string<\/span> Description\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> IInvoiceState State\n    {\n        <span class=\"hljs-keyword\">get<\/span>; <span class=\"hljs-keyword\">set<\/span>;\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-title\">Invoice<\/span>(<span class=\"hljs-params\"><span class=\"hljs-keyword\">int<\/span> number, <span class=\"hljs-keyword\">decimal<\/span> amount, <span class=\"hljs-keyword\">string<\/span> description<\/span>)<\/span>\n    {\n        Number = number;\n        Amount = amount;\n        Description = description;\n\n        <span class=\"hljs-comment\">\/\/ Set the invoice as Unpaid<\/span>\n        State = <span class=\"hljs-keyword\">new<\/span> UnpaidState();\n    }\n\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Pay(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> PaidState();\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Cancel(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> CanceledState();\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Refund(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> RefundedState();\n    }\n}\n\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">UnpaidState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Paying invoice <span class=\"hljs-subst\">{invoice.Number}<\/span>...\"<\/span>);\n\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Canceling invoice <span class=\"hljs-subst\">{invoice.Number}<\/span>...\"<\/span>);\n\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> is unpaid and cannot be refunded.\"<\/span>);\n\n    }\n}\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">PaidState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has already been paid.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> cannot be cancelled.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been refunded.\"<\/span>);\n    }\n}\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">CanceledState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been canceled and cannot be paid.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has already been canceled and cannot be canceled again.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been canceled and cannot be refunded.\"<\/span>);\n    }\n}\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">RefundedState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> was refunded and cannot be cancelled.\"<\/span>);\n\n    }\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> was refunded and cannot be paid.\"<\/span>);\n\n    }\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> cannot be refunded again.\"<\/span>);\n    }\n}\n\n<span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Program<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">static<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Main<\/span>(<span class=\"hljs-params\"><span class=\"hljs-keyword\">string<\/span>&#91;] args<\/span>)<\/span>\n    {\n        <span class=\"hljs-keyword\">var<\/span> invoice = <span class=\"hljs-keyword\">new<\/span> Invoice(<span class=\"hljs-number\">123<\/span>, <span class=\"hljs-number\">1000<\/span>m, <span class=\"hljs-string\">\"Software Dev Services\"<\/span>);\n\n        invoice.Pay();\n        invoice.Refund();\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-1\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-2\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\">Paying invoice <span class=\"hljs-number\">123.<\/span>..\nInvoice <span class=\"hljs-number\">123<\/span> has been refunded.<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-2\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>How it works.<\/p>\n\n\n\n<p>First, define the <code>IInvoiceState<\/code> interface that has three methods <code>Pay<\/code>, <code>Cancel<\/code>, and <code>Refund<\/code>:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-3\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">interface<\/span> <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>;\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Second, define the <code>Invoice<\/code> class that has four properties <code>Number<\/code>, <code>Amount<\/code>, <code>Description<\/code>, and <code>State<\/code>. Note that this is for simplicity purposes. In the real-world application, an invoice may contain more information. The constructor of the <code>Invoice<\/code> class initializes the <code>State<\/code> property to the <code>UnpaidState<\/code> object:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Invoice<\/span>\n{\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">int<\/span> Number\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">decimal<\/span> Amount\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">string<\/span> Description\n    {\n        <span class=\"hljs-keyword\">get<\/span>;\n    }\n    <span class=\"hljs-keyword\">public<\/span> IInvoiceState State\n    {\n        <span class=\"hljs-keyword\">get<\/span>; <span class=\"hljs-keyword\">set<\/span>;\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-title\">Invoice<\/span>(<span class=\"hljs-params\"><span class=\"hljs-keyword\">int<\/span> number, <span class=\"hljs-keyword\">decimal<\/span> amount, <span class=\"hljs-keyword\">string<\/span> description<\/span>)<\/span>\n    {\n        Number = number;\n        Amount = amount;\n        Description = description;\n\n        <span class=\"hljs-comment\">\/\/ Set the invoice as Unpaid<\/span>\n        State = <span class=\"hljs-keyword\">new<\/span> UnpaidState();\n    }\n\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Pay(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> PaidState();\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Cancel(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> CanceledState();\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\"><\/span>)<\/span>\n    {\n        State.Refund(<span class=\"hljs-keyword\">this<\/span>);\n        State = <span class=\"hljs-keyword\">new<\/span> RefundedState();\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Third, define the <code>UnpaidState<\/code> class that implements the <code>IInvoiceState<\/code> interface:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-5\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">UnpaidState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Paying invoice <span class=\"hljs-subst\">{invoice.Number}<\/span>...\"<\/span>);\n\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Canceling invoice <span class=\"hljs-subst\">{invoice.Number}<\/span>...\"<\/span>);\n\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> is unpaid and cannot be refunded.\"<\/span>);\n\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>An unpaid invoice can be paid or canceled but cannot be refunded.<\/p>\n\n\n\n<p>Fourth, define the <code>PaidState<\/code> class that also implements the <code>IInvoiceState<\/code> interface:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">PaidState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has already been paid.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> cannot be cancelled.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been refunded.\"<\/span>);\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>A paid invoice cannot be paid again or canceled but can be refunded.<\/p>\n\n\n\n<p>Fifth define the <code>CanceledState<\/code> class that implements the <code>IInvoiceState<\/code> interface:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-7\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">CanceledState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been canceled and cannot be paid.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has already been canceled and cannot be canceled again.\"<\/span>);\n    }\n\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> has been canceled and cannot be refunded.\"<\/span>);\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-7\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>A canceled invoice cannot be canceled, refunded, or paid.<\/p>\n\n\n\n<p>Similarly, a refunded invoice cannot be canceled, refunded, or paid:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-8\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">RefundedState<\/span> : <span class=\"hljs-title\">IInvoiceState<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Cancel<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> was refunded and cannot be cancelled.\"<\/span>);\n\n    }\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Pay<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> was refunded and cannot be paid.\"<\/span>);\n\n    }\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Refund<\/span>(<span class=\"hljs-params\">Invoice invoice<\/span>)<\/span>\n    {\n        Console.WriteLine(<span class=\"hljs-string\">$\"Invoice <span class=\"hljs-subst\">{invoice.Number}<\/span> cannot be refunded again.\"<\/span>);\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-8\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Finally, create an invoice object and call the <code>Pay()<\/code> and <code>Refund()<\/code> method sequentially:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-9\" data-shcb-language-name=\"C#\" data-shcb-language-slug=\"cs\"><span><code class=\"hljs language-cs\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title\">Program<\/span>\n{\n    <span class=\"hljs-function\"><span class=\"hljs-keyword\">public<\/span> <span class=\"hljs-keyword\">static<\/span> <span class=\"hljs-keyword\">void<\/span> <span class=\"hljs-title\">Main<\/span>(<span class=\"hljs-params\"><span class=\"hljs-keyword\">string<\/span>&#91;] args<\/span>)<\/span>\n    {\n        <span class=\"hljs-keyword\">var<\/span> invoice = <span class=\"hljs-keyword\">new<\/span> Invoice(<span class=\"hljs-number\">123<\/span>, <span class=\"hljs-number\">1000<\/span>m, <span class=\"hljs-string\">\"Software Dev Services\"<\/span>);\n\n        invoice.Pay();\n        invoice.Refund();\n    }\n}<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-9\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">C#<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">cs<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<h2 class=\"wp-block-heading\">Summary<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Use the State pattern to let an object change its behavior when its internal state changes.<\/li>\n<\/ul>\n<div class=\"helpful-block-content\" data-title=\"\">\n\t<header>\n\t\t<div class=\"wth-question\">Was this tutorial helpful ?<\/div>\n\t\t<div class=\"wth-thumbs\">\n\t\t\t<button\n\t\t\t\tdata-post=\"1712\"\n\t\t\t\tdata-post-url=\"https:\/\/www.csharptutorial.net\/csharp-design-patterns\/csharp-state-pattern\/\"\n\t\t\t\tdata-post-title=\"C# State Pattern\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded wth-yes-btn\"\n\t\t\t>\n\t\t\t\t<svg\n\t\t\t\t\txmlns=\"http:\/\/www.w3.org\/2000\/svg\"\n\t\t\t\t\tviewBox=\"0 0 24 24\"\n\t\t\t\t\tfill=\"none\"\n\t\t\t\t\tstroke=\"currentColor\"\n\t\t\t\t\tstroke-width=\"2\"\n\t\t\t\t\tstroke-linecap=\"round\"\n\t\t\t\t\tstroke-linejoin=\"round\"\n\t\t\t\t\tclass=\"feather feather-thumbs-up block w-full 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2H17\"\n\t\t\t\t\t><\/path>\n\t\t\t\t<\/svg>\n\t\t\t\t<span class=\"sr-only\"> No <\/span>\n\t\t\t<\/button>\n\t\t<\/div>\n\t<\/header>\n\n\t<div class=\"wth-form hidden\">\n\t\t<div class=\"wth-form-wrapper\">\n\t\t\t<div class=\"wth-title\"><\/div>\n\t\t\t\n\t\t\t<textarea class=\"wth-message\"><\/textarea>\n\n\t\t\t<button class=\"btn btn-primary wth-btn-submit\">Send<\/button>\n\t\t\t<button class=\"btn wth-btn-cancel\">Cancel<\/button>\n\t\t\n\t\t<\/div>\n\t<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>In this tutorial, you will learn how to use the C# State pattern to allow an object to change its behavior when its internal state changes.<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1441,"menu_order":25,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-1712","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/pages\/1712","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/comments?post=1712"}],"version-history":[{"count":5,"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/pages\/1712\/revisions"}],"predecessor-version":[{"id":1815,"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/pages\/1712\/revisions\/1815"}],"up":[{"embeddable":true,"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/pages\/1441"}],"wp:attachment":[{"href":"https:\/\/www.csharptutorial.net\/wp-json\/wp\/v2\/media?parent=1712"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}