> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C# Obsolete Attribute The `System.ObsoleteAttribute` is a predefined metadata attribute used to mark program elements that are deprecated. When the C# compiler encounters code that references an element decorated with this attribute, it evaluates the attribute's parameters to emit either a compiler warning or a hard compiler error, effectively altering the compilation state of the referencing code (the caller). ## Syntax and Constructors The attribute can be applied using three primary constructor overloads, which dictate the severity and messaging of the compiler output. ```csharp theme={"dark"} // 1. Parameterless: Emits a default compiler warning (CS0612) [Obsolete] public class LegacyClass { } // 2. Message parameter: Emits a warning with a custom message (CS0618) [Obsolete("This method is deprecated. Call ModernMethod() instead.")] public void DeprecatedMethod() { } // 3. Message and Error parameters: Emits a compiler error (CS0619) [Obsolete("This property is no longer supported.", error: true)] public string UnsupportedProperty { get; set; } ``` ## Parameter Breakdown * **`message` (`string`)**: A text string injected into the compiler output. It is technically optional but highly recommended to provide redirection to alternative APIs. * **`error` (`bool`)**: A boolean flag determining the compilation behavior. * If `false` (default), the compiler emits a warning (CS0618) and compilation continues. * If `true`, the compiler emits an error (CS0619) and halts the build process. * **`DiagnosticId` (`string`)**: Introduced in .NET 5 (C# 9), this named parameter allows you to assign a custom diagnostic ID to the obsolescence warning. This enables consumers to suppress specific deprecation warnings using `#pragma warning disable ` without suppressing all `CS0618` warnings globally. * **`UrlFormat` (`string`)**: Introduced in .NET 5 (C# 9), this named parameter specifies a URL format string that the compiler uses to generate a clickable link in the IDE pointing to external documentation regarding the deprecation. The format string can optionally include a `{0}` placeholder. If the placeholder is omitted, the compiler treats the provided string as a static URL. If the `{0}` placeholder is included, the compiler replaces it with the value of the `DiagnosticId` property. If `DiagnosticId` is *not* explicitly set, the compiler replaces `{0}` with the default warning code (e.g., `CS0618`). **Modern Syntax Example (.NET 5+):** ```csharp theme={"dark"} // Using a format string with a placeholder and a custom DiagnosticId [Obsolete("Use NewMethod instead.", DiagnosticId = "CONTOSO001", UrlFormat = "https://contoso.com/obsoletions/{0}")] public void OldMethod() { } // Using a static URL without a placeholder [Obsolete("Use AnotherMethod instead.", UrlFormat = "https://contoso.com/obsoletions/another-method")] public void AnotherOldMethod() { } ``` ## Attribute Targets The `[Obsolete]` attribute is defined using a specific bitwise combination of `AttributeTargets`, specifically: `Class | Struct | Enum | Constructor | Method | Property | Field | Event | Interface | Delegate`. Because it is restricted to these explicit targets, it cannot be applied to assemblies, modules, parameters, return values, or generic parameters. ## Compiler Resolution Behavior When the compiler resolves a symbol marked with `[Obsolete]`, it checks the context of the caller. If the calling member is *also* marked as `[Obsolete]`, the compiler suppresses the warning or error. This cascading behavior allows deprecated members to safely reference other deprecated members internally without generating redundant compiler noise.

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