Extension Attribute

Field Extensions and Schema Directives use the [ExtensionAttribute] or IExtensionAttributeHandler interface to apply extensions to IFields and ISchemaTypes.

ExtensionAttribute is useful when building new Attributes (see GraphQLOneOfAttribute) whereas IExtensionAttributeHandler allows you to use existing attributes (see ObsoleteAttributeHandler which marks items with the [ObsoleteAttribute] as @deprecated).

public abstract class ExtensionAttribute : Attribute
{
    public virtual void ApplyExtension(IField field) { }
    public virtual void ApplyExtension(ISchemaType type) { }
}

public abstract class AbstractExtensionAttributeHandler<TAttribute> : IExtensionAttributeHandler where TAttribute : Attribute
{
    public IEnumerable<Type> AttributeTypes { get => new List<Type> { typeof(TAttribute) }; }

    public virtual void ApplyExtension(IField field, Attribute attribute)
    {
        if (attribute is TAttribute tAttribute)
            ApplyExtension(field, tAttribute);
        else
            throw new ArgumentException($"Attribute must be of type {typeof(TAttribute).Name}");
    }
    public virtual void ApplyExtension(ISchemaType type, Attribute attribute)
    {
        if (attribute is TAttribute tAttribute)
            ApplyExtension(type, tAttribute);
        else
            throw new ArgumentException($"Attribute must be of type {typeof(TAttribute).Name}");
    }

    public virtual void ApplyExtension(IField field, TAttribute attribute) { }
    public virtual void ApplyExtension(ISchemaType type, TAttribute attribute) { }
}
public interface IExtensionAttributeHandler
{
    IEnumerable<Type> AttributeTypes { get; }

    public void ApplyExtension(IField field, Attribute attribute);
    public void ApplyExtension(ISchemaType type, Attribute attribute);
}