Built-in attributes

C# course · Advanced · Lesson 53/83

Learn attributes like [Obsolete], [Conditional] and [Serializable].

Built-in attributes

C# ships several common built-in attributes. [Obsolete] marks an element as deprecated and the compiler emits a warning or error. [Conditional("DEBUG")] makes calls to a method compile only when the relevant symbol is defined.

[Serializable] declares that a type can be serialized; it is read by legacy serialization APIs. These are common attributes that steer compiler and runtime behavior without custom code.

[Conditional] details

[Conditional("DEBUG")] marks a method; calls to it compile only when DEBUG is defined. The method itself is always compiled; only the call sites are affected. This is common in logging and diagnostic code.

The Conditional attribute applies only to void-returning methods. Multiple symbols are allowed: [Conditional("DEBUG"), Conditional("TRACE")]. The condition is evaluated at compile time; there is no runtime cost.

Exercise Apply Obsolete and print Message

Apply [Obsolete("Use NewMethod")] to OldMethod in the Tool class below. In Main, read it with Attribute.GetCustomAttribute(typeof(Tool).GetMethod("OldMethod"), typeof(ObsoleteAttribute)) as ObsoleteAttribute and print attr.Message with Console.WriteLine.

Show example solution

var attr = Attribute.GetCustomAttribute(typeof(Tool).GetMethod("OldMethod"), typeof(ObsoleteAttribute)) as ObsoleteAttribute; Console.WriteLine(attr.Message);
Example solution:
using System;

class Tool
{
    [Obsolete("Use NewMethod")]
    public static void OldMethod() { }
}

class Program
{
    static void Main()
    {
        var attr = Attribute.GetCustomAttribute(typeof(Tool).GetMethod("OldMethod"), typeof(ObsoleteAttribute)) as ObsoleteAttribute;
        Console.WriteLine(attr.Message);
    }
}

Test yourself

Complete the Obsolete attribute

[____]
void Old() { }

Show answer

[Obsolete]
void Old() { }

What does [Conditional("DEBUG")] do?
- Always removes the method
- Compiles the method only in DEBUG
- Keeps the call if DEBUG is defined, otherwise removes it
- Makes the method async
Show answer

Keeps the call if DEBUG is defined, otherwise removes it — Correct. Conditional keeps or removes the call based on the symbol; the method itself always compiles.
Match the attribute to its purpose

[Obsolete]

Marks as deprecated
[Conditional]

Keeps call conditional
[Serializable]

Marks as serializable
[AttributeUsage]

Restricts targets

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Built-in attributes