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253 lines
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<li data-nav-id=/welcome/ title=Welcome class=dd-item><a href=/welcome/><b><i class="fas fa-home"></i> </b>Welcome</a><ul></ul></li>
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<li data-nav-id=/bcpatterns/ title="BC Patterns" class=dd-item><a href=/bcpatterns/><b><i class="fas fa-clone"></i> </b>BC Patterns</a><ul></ul></li>
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<li data-nav-id=/bcbestpractices/ title="BC Dev. Best Practices" class=dd-item><a href=/bcbestpractices/><b><i class="fas fa-medal"></i> </b>BC Dev. Best Practices</a><ul></ul></li>
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<li data-nav-id=/navpatterns/ title="NAV Patterns Archive" class="dd-item parent"><a href=/navpatterns/><b><i class="fas fa-save"></i> </b>NAV Patterns Archive</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/ title="1. Patterns" class="dd-item parent"><a href=/navpatterns/1-patterns/>1. Patterns</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/activity-log/ title="Activity Logs" class=dd-item><a href=/navpatterns/1-patterns/activity-log/>Activity Logs</a></li>
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<li data-nav-id=/navpatterns/1-patterns/argument-table/ title="Argument Table" class=dd-item><a href=/navpatterns/1-patterns/argument-table/>Argument Table</a></li>
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<li data-nav-id=/navpatterns/1-patterns/blocked-entity/ title="Blocked Entity" class=dd-item><a href=/navpatterns/1-patterns/blocked-entity/>Blocked Entity</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/blocked-entity/data-driven-blocked-entity/ title="Data Driven Blocked Entity" class=dd-item><a href=/navpatterns/1-patterns/blocked-entity/data-driven-blocked-entity/>Data Driven Blocked Entity</a></li></ul></li>
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<li data-nav-id=/navpatterns/1-patterns/cached-web-service-calls/ title="Cached Web Server Calls" class=dd-item><a href=/navpatterns/1-patterns/cached-web-service-calls/>Cached Web Server Calls</a></li>
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<li data-nav-id=/navpatterns/1-patterns/conditional-cascading-update/ title="Conditional Cascading Update" class=dd-item><a href=/navpatterns/1-patterns/conditional-cascading-update/>Conditional Cascading Update</a></li>
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<li data-nav-id=/navpatterns/1-patterns/copy-document/ title="Copy Document" class=dd-item><a href=/navpatterns/1-patterns/copy-document/>Copy Document</a></li>
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<li data-nav-id=/navpatterns/1-patterns/create-data-from-templates/ title="Create Data from Templates" class=dd-item><a href=/navpatterns/1-patterns/create-data-from-templates/>Create Data from Templates</a></li>
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<li data-nav-id=/navpatterns/1-patterns/create-urls-to-nav-clients/ title="Create URLs to NAV Clients" class=dd-item><a href=/navpatterns/1-patterns/create-urls-to-nav-clients/>Create URLs to NAV Clients</a></li>
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<li data-nav-id=/navpatterns/1-patterns/creating-custom-charts/ title="Creating Custom Charts" class=dd-item><a href=/navpatterns/1-patterns/creating-custom-charts/>Creating Custom Charts</a></li>
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<li data-nav-id=/navpatterns/1-patterns/cross-session-events/ title="Cross Session Events" class=dd-item><a href=/navpatterns/1-patterns/cross-session-events/>Cross Session Events</a></li>
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<li data-nav-id=/navpatterns/1-patterns/currently-active-record/ title="Currently Active Record" class=dd-item><a href=/navpatterns/1-patterns/currently-active-record/>Currently Active Record</a></li>
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<li data-nav-id=/navpatterns/1-patterns/data-migration-facade/ title="Data Migration Facade" class=dd-item><a href=/navpatterns/1-patterns/data-migration-facade/>Data Migration Facade</a></li>
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<li data-nav-id=/navpatterns/1-patterns/discovery-event/ title="Discovery Event" class=dd-item><a href=/navpatterns/1-patterns/discovery-event/>Discovery Event</a></li>
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<li data-nav-id=/navpatterns/1-patterns/document/ title=Document class=dd-item><a href=/navpatterns/1-patterns/document/>Document</a></li>
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<li data-nav-id=/navpatterns/1-patterns/easy-update-of-setup-or-supplementary-information/ title="Easy Update Of Setup Or Supplementary Information" class=dd-item><a href=/navpatterns/1-patterns/easy-update-of-setup-or-supplementary-information/>Easy Update Of Setup Or Supplementary Information</a></li>
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<li data-nav-id=/navpatterns/1-patterns/error-message-processing/ title="Error Message Processing" class=dd-item><a href=/navpatterns/1-patterns/error-message-processing/>Error Message Processing</a></li>
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<li data-nav-id=/navpatterns/1-patterns/extending-the-role-center-headlines/ title="Extending the Role Center Headlines" class=dd-item><a href=/navpatterns/1-patterns/extending-the-role-center-headlines/>Extending the Role Center Headlines</a></li>
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<li data-nav-id=/navpatterns/1-patterns/feature-localization-for-data-structures/ title="Feature Localization For Data Structures" class=dd-item><a href=/navpatterns/1-patterns/feature-localization-for-data-structures/>Feature Localization For Data Structures</a></li>
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<li data-nav-id=/navpatterns/1-patterns/hooks/ title=Hooks class=dd-item><a href=/navpatterns/1-patterns/hooks/>Hooks</a></li>
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<li data-nav-id=/navpatterns/1-patterns/implementation-of-surrogate-keys-using-autoincrement-pattern/ title="Surrogate keys using Autoincrement Pattern" class=dd-item><a href=/navpatterns/1-patterns/implementation-of-surrogate-keys-using-autoincrement-pattern/>Surrogate keys using Autoincrement Pattern</a></li>
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<li data-nav-id=/navpatterns/1-patterns/instructions-in-the-ui/ title="Instructions in the UI" class=dd-item><a href=/navpatterns/1-patterns/instructions-in-the-ui/>Instructions in the UI</a></li>
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<li data-nav-id=/navpatterns/1-patterns/integration-of-addresses/ title="Integration of Addresses" class=dd-item><a href=/navpatterns/1-patterns/integration-of-addresses/>Integration of Addresses</a></li>
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<li data-nav-id=/navpatterns/1-patterns/journal-error-processing/ title="Journal Error Processing" class=dd-item><a href=/navpatterns/1-patterns/journal-error-processing/>Journal Error Processing</a></li>
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<li data-nav-id=/navpatterns/1-patterns/journal-template-batch-line/ title="Journal Template Batch Line" class=dd-item><a href=/navpatterns/1-patterns/journal-template-batch-line/>Journal Template Batch Line</a></li>
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<li data-nav-id=/navpatterns/1-patterns/master-data/ title="Master Data" class=dd-item><a href=/navpatterns/1-patterns/master-data/>Master Data</a></li>
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<li data-nav-id=/navpatterns/1-patterns/multi-file-download/ title="Multi-file Download" class=dd-item><a href=/navpatterns/1-patterns/multi-file-download/>Multi-file Download</a></li>
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<li data-nav-id=/navpatterns/1-patterns/multi-page-list/ title="Multi-Page List" class=dd-item><a href=/navpatterns/1-patterns/multi-page-list/>Multi-Page List</a></li>
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<li data-nav-id=/navpatterns/1-patterns/multilanguage-application-data/ title="Multilanguage Application Data" class=dd-item><a href=/navpatterns/1-patterns/multilanguage-application-data/>Multilanguage Application Data</a></li>
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<li data-nav-id=/navpatterns/1-patterns/net-exception-handling-in-cal/ title="NET Exception Handling in CAL" class=dd-item><a href=/navpatterns/1-patterns/net-exception-handling-in-cal/>NET Exception Handling in CAL</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/net-exception-handling-in-cal/tryfunction-net-exception-handling-in-cal/ title="TryFunction NET Exception Handling in CAL" class=dd-item><a href=/navpatterns/1-patterns/net-exception-handling-in-cal/tryfunction-net-exception-handling-in-cal/>TryFunction NET Exception Handling in CAL</a></li></ul></li>
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<li data-nav-id=/navpatterns/1-patterns/no-series/ title="No Series" class=dd-item><a href=/navpatterns/1-patterns/no-series/>No Series</a></li>
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<li data-nav-id=/navpatterns/1-patterns/notifications/ title=Notifications class=dd-item><a href=/navpatterns/1-patterns/notifications/>Notifications</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/notifications/in-context-notifications/ title="In-context Notifications" class=dd-item><a href=/navpatterns/1-patterns/notifications/in-context-notifications/>In-context Notifications</a></li>
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<li data-nav-id=/navpatterns/1-patterns/notifications/notification-lifecycle-management-pattern/ title="Notification Lifecycle Management Pattern" class=dd-item><a href=/navpatterns/1-patterns/notifications/notification-lifecycle-management-pattern/>Notification Lifecycle Management Pattern</a></li></ul></li>
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<li data-nav-id=/navpatterns/1-patterns/observer/ title=Observer class=dd-item><a href=/navpatterns/1-patterns/observer/>Observer</a></li>
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<li data-nav-id=/navpatterns/1-patterns/posting-routine-select-behavior/ title="Posting Routine - Select Behavior" class=dd-item><a href=/navpatterns/1-patterns/posting-routine-select-behavior/>Posting Routine - Select Behavior</a></li>
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<li data-nav-id=/navpatterns/1-patterns/product-name/ title="Product Name" class=dd-item><a href=/navpatterns/1-patterns/product-name/>Product Name</a></li>
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<li data-nav-id=/navpatterns/1-patterns/queries/ title=Queries class=dd-item><a href=/navpatterns/1-patterns/queries/>Queries</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/queries/select-distinct-with-queries/ title="SELECT DISTINCT with Queries" class=dd-item><a href=/navpatterns/1-patterns/queries/select-distinct-with-queries/>SELECT DISTINCT with Queries</a></li>
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<li data-nav-id=/navpatterns/1-patterns/queries/use-queries-to-detect-duplicate-records/ title="use Queries to Detect Duplicate Records" class=dd-item><a href=/navpatterns/1-patterns/queries/use-queries-to-detect-duplicate-records/>use Queries to Detect Duplicate Records</a></li>
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<li data-nav-id=/navpatterns/1-patterns/queries/use-queries-to-replace-nested-loops/ title="Use Queries to Replace Nested Loops" class=dd-item><a href=/navpatterns/1-patterns/queries/use-queries-to-replace-nested-loops/>Use Queries to Replace Nested Loops</a></li></ul></li>
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<li data-nav-id=/navpatterns/1-patterns/read-once-initialization-and-validation/ title="Read-once Initialization and Validation" class=dd-item><a href=/navpatterns/1-patterns/read-once-initialization-and-validation/>Read-once Initialization and Validation</a></li>
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<li data-nav-id=/navpatterns/1-patterns/released-entity/ title="Released Entity" class=dd-item><a href=/navpatterns/1-patterns/released-entity/>Released Entity</a></li>
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<li data-nav-id=/navpatterns/1-patterns/report-selection/ title="Report Selection" class=dd-item><a href=/navpatterns/1-patterns/report-selection/>Report Selection</a></li>
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<li data-nav-id=/navpatterns/1-patterns/security/ title=Security class=dd-item><a href=/navpatterns/1-patterns/security/>Security</a></li>
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<li data-nav-id=/navpatterns/1-patterns/setup-specificity-fallback/ title="Setup Specificity Fallback" class=dd-item><a href=/navpatterns/1-patterns/setup-specificity-fallback/>Setup Specificity Fallback</a></li>
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<li data-nav-id=/navpatterns/1-patterns/silent-file-upload-and-download/ title="Silent File Upload and Download" class=dd-item><a href=/navpatterns/1-patterns/silent-file-upload-and-download/>Silent File Upload and Download</a></li>
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<li data-nav-id=/navpatterns/1-patterns/singleton/ title=Singleton class="dd-item active parent"><a href=/navpatterns/1-patterns/singleton/>Singleton</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/singleton/singleton-codeunit/ title="Singleton Codeunit" class=dd-item><a href=/navpatterns/1-patterns/singleton/singleton-codeunit/>Singleton Codeunit</a></li>
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<li data-nav-id=/navpatterns/1-patterns/singleton/singleton-table/ title="Singleton Table" class=dd-item><a href=/navpatterns/1-patterns/singleton/singleton-table/>Singleton Table</a><ul>
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<li data-nav-id=/navpatterns/1-patterns/singleton/singleton-table/cue-table/ title="Cue Table" class=dd-item><a href=/navpatterns/1-patterns/singleton/singleton-table/cue-table/>Cue Table</a></li>
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<li data-nav-id=/navpatterns/1-patterns/singleton/singleton-table/setup-table/ title="Setup Table" class=dd-item><a href=/navpatterns/1-patterns/singleton/singleton-table/setup-table/>Setup Table</a></li></ul></li></ul></li>
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<li data-nav-id=/navpatterns/1-patterns/standard-journal/ title="Standard Journal" class=dd-item><a href=/navpatterns/1-patterns/standard-journal/>Standard Journal</a></li>
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<li data-nav-id=/navpatterns/1-patterns/temporary-dataset-report/ title="Temporary Dataset Report" class=dd-item><a href=/navpatterns/1-patterns/temporary-dataset-report/>Temporary Dataset Report</a></li>
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<li data-nav-id=/navpatterns/1-patterns/totals-and-discounts-on-subpages-sales-and-purchases/ title="Totals and Discounts on Subpages Sales and Purchases" class=dd-item><a href=/navpatterns/1-patterns/totals-and-discounts-on-subpages-sales-and-purchases/>Totals and Discounts on Subpages Sales and Purchases</a></li>
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<li data-nav-id=/navpatterns/1-patterns/transfer-custom-fields/ title="Transfer Custom Fields" class=dd-item><a href=/navpatterns/1-patterns/transfer-custom-fields/>Transfer Custom Fields</a></li>
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<li data-nav-id=/navpatterns/1-patterns/variant-facade/ title="Variant Facade" class=dd-item><a href=/navpatterns/1-patterns/variant-facade/>Variant Facade</a></li></ul></li>
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<li data-nav-id=/navpatterns/2-anti-patterns/ title="2. Anti-Patterns" class=dd-item><a href=/navpatterns/2-anti-patterns/>2. Anti-Patterns</a><ul>
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<li data-nav-id=/navpatterns/2-anti-patterns/nav-upgrade/ title="Nav Upgrade" class=dd-item><a href=/navpatterns/2-anti-patterns/nav-upgrade/>Nav Upgrade</a></li>
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<li data-nav-id=/navpatterns/2-anti-patterns/reusable-bugs/ title="Reusable Bugs" class=dd-item><a href=/navpatterns/2-anti-patterns/reusable-bugs/>Reusable Bugs</a></li></ul></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/ title="3. CAL Coding Guidelines" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/>3. CAL Coding Guidelines</a><ul>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/design/ title=Design class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/design/>Design</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/internally-used-dot-net-types/ title="Internally-used DotNet Types" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/internally-used-dot-net-types/>Internally-used DotNet Types</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/internationalization/ title=Internationalization class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/internationalization/>Internationalization</a><ul>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/internationalization/using-calcdate/ title="Using Calcdate" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/internationalization/using-calcdate/>Using Calcdate</a></li></ul></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/ title=Localizability class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/>Localizability</a><ul>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/captionml-for-system-tables/ title="CaptionML on System Pages" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/captionml-for-system-tables/>CaptionML on System Pages</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/fieldcaption-and-tablecaption/ title="FIELDCAPTION and TABLECAPTION" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/fieldcaption-and-tablecaption/>FIELDCAPTION and TABLECAPTION</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/global-text-constants/ title="Global Text Constants" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/global-text-constants/>Global Text Constants</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/use-text-constants/ title="Use Text Constants" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/use-text-constants/>Use Text Constants</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/localizability/using-optioncaptionml/ title="Using OptionCaptionML" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/localizability/using-optioncaptionml/>Using OptionCaptionML</a></li></ul></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/ title=Readability class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/>Readability</a><ul>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/begin-as-an-afterword/ title="Begin as an 'After Word'" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/begin-as-an-afterword/>Begin as an 'After Word'</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/begin-end/ title="Begin-End - Compound Only" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/begin-end/>Begin-End - Compound Only</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/binary-operator-line-start/ title="Binary Operator to Start Line" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/binary-operator-line-start/>Binary Operator to Start Line</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/blank-lines/ title="Blank Lines.md" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/blank-lines/>Blank Lines.md</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/case-actions/ title="CASE Action" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/case-actions/>CASE Action</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/colon-usage-in-case/ title="Colon usage in CASE" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/colon-usage-in-case/>Colon usage in CASE</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/comments-curly-brackets/ title="Comments inside Curly Brackets" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/comments-curly-brackets/>Comments inside Curly Brackets</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/comments-spacing/ title="Comment Spacing" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/comments-spacing/>Comment Spacing</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/end-else-pair/ title="END ELSE Pair" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/end-else-pair/>END ELSE Pair</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/indentation/ title=Indentation class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/indentation/>Indentation</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/keyword-pairs-indentation/ title="Keyword Pairs - Indentation" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/keyword-pairs-indentation/>Keyword Pairs - Indentation</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/line-start-keywords/ title="Line Start Keywords" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/line-start-keywords/>Line Start Keywords</a></li>
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<li data-nav-id=/navpatterns/3-cal-coding-guidelines/readability/lonely-repeat/ title="Lonely Repeat" class=dd-item><a href=/navpatterns/3-cal-coding-guidelines/readability/lonely-repeat/>Lonely Repeat</a></li>
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<h1>Singleton</h1>
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<h2 id=singleton>Singleton</h2>
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<p><em>By Bogdana Botez at Microsoft Development Center Copenhagen</em><br>
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<a href=0535.Singleton.png><img src=0535.Singleton.png alt=" "></a></p>
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<p>**Context: <strong>The pattern described in this article applies to Dynamics NAV only. For the general definition of the <strong>Singleton</strong> pattern, see for example <a href=https://en.wikipedia.org/wiki/Singleton_pattern>this link</a>.</strong><br>
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**</p>
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<p><strong>Problem</strong>: As a C/AL developer, you need to coordinate action (through a codeunit) or store information (in a table) that is unique across the system.</p>
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<p><strong>Forces:</strong></p>
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<ul>
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<li><strong>Lost reference to centralizer:</strong> An instance of a relevant object could attempt to centralize control or data in the system. However, once this object is no longer in scope, the reference to it is lost and cannot be retrieved for later coordination</li>
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<li>**Cannot rebuild the initial state: **A new instance of the same object could be created, however the last known state of the lost instance cannot be known anymore.</li>
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</ul>
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<p><strong>Solution:</strong> create an object which resides in memory in a single copy (instance). Have a way to retrieve this unique object from code. This object can either be a <a href=/nav/w/designpatterns/151.singleton-table><strong>Singleton Table</strong></a>, or a <a href=/nav/w/designpatterns/283.singleton-codeunit><strong>Singleton Codeunit</strong></a>.</p>
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<p>The implementation varies depending on the technology and language used. In object-oriented (OO) languages like C#, C++ or Java, the <strong>Singleton</strong> uses static classes and class members to instantiate. There is a mechanism to ensure only one instance of the object can exist. This mechanism is many times explicit (like in OO languages) and sometimes implicit as part of the compiler or platform (as it is in NAV).</p>
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<p><strong>Benefits:</strong></p>
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<ul>
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<li><strong>Centralization</strong>: the setup information is stored in a single well-known place and easily retrievable from anywhere in the application code, by invoking a Record.GET. In case of a single-instance codeunit, any reference to it will retrieve the same instance, so the context will be preserved.</li>
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<li><strong>Persistence</strong>: information remains even after the instance goes out of scope, because it continues to live in memory.</li>
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</ul>
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<p>**Limitation: **The generic object-oriented <strong>Singleton</strong> pattern permits instantiation of a limited number n of objects (where usually n=1, but it can have other positive values too). However, in Dynamics NAV, the Singleton patterns are limited: n is always 1.</p>
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