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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).

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Syntax and Constructors

The attribute can be applied using three primary constructor overloads, which dictate the severity and messaging of the compiler output. 
// 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; }

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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 <DiagnosticId> 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+): 
// 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() { }

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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.

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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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