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Compiler Error CS1614

'attribute' is ambiguous; use either '@attribute' or 'attributeAttribute'

The compiler has encountered an ambiguous attribute specification.

For convenience, the C# compiler allows you to specify ExampleAttribute as just [Example]. However, ambiguity arises if an attribute class named Example exists along with ExampleAttribute, because the compiler cannot tell if [Example] refers to the Example attribute or the ExampleAttribute attribute. To clarify, use [@Example] for the Example attribute and [ExampleAttribute] for ExampleAttribute.

The following sample generates CS1614:

// CS1614.cs
using System;

// Both of the following classes are valid attributes with valid
// names (MySpecial and MySpecialAttribute). However, because the lookup
// rules for attributes involves auto-appending the 'Attribute' suffix
// to the identifier, these two attributes become ambiguous; that is,
// if you specify MySpecial, the compiler can't tell if you want
// MySpecial or MySpecialAttribute.

public class MySpecial : Attribute {
   public MySpecial() {}
}

public class MySpecialAttribute : Attribute {
   public MySpecialAttribute() {}
}

class MakeAWarning {
   [MySpecial()] // CS1614
                 // Ambiguous: MySpecial or MySpecialAttribute?
   public static void Main() {
   }

   [@MySpecial()] // This isn't ambiguous, it binds to the first attribute above.
   public static void NoWarning() {
   }

   [MySpecialAttribute()] // This isn't ambiguous, it binds to the second attribute above.
   public static void NoWarning2() {
   }

   [@MySpecialAttribute()] // This is also legal.
   public static void NoWarning3() {
   }
}
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