Is it possible to have a delegate as attribute parameter?
Is it possible to have a delegate as the parameter of an attribute?
Like this:
public delegate IPropertySet ConnectionPropertiesDelegate();
public static class TestDelegate
{
public static IPropertySet GetConnection()
{
return new PropertySetClass();
}
}
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface,AllowMultiple=false,Inherited=true)]
public class WorkspaceAttribute : Attribute
{
public ConnectionPropertiesDelegate ConnectionDelegate { get; set; }
public WorkspaceAttribute(ConnectionPropertiesDelegate connectionDelegate)
{
ConnectionDelegate = connectionDelegate;
}
}
[Workspace(TestDelegate.GetConnection)]
public class Test
{
}
And if its not possible, what are the sensible alternatives?
No, you cannot have a delegate as an attribute constructor parameter. See available types: Attribute parameter types
As a workaround (although it's hacky and error prone) you can create a delegate with reflection:
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple = false, Inherited = true)]
public class WorkspaceAttribute : Attribute
{
public ConnectionPropertiesDelegate ConnectionDelegate { get; set; }
public WorkspaceAttribute(Type delegateType, string delegateName)
{
ConnectionDelegate = (ConnectionPropertiesDelegate)Delegate.CreateDelegate(typeof(ConnectionPropertiesDelegate), delegateType, delegateName);
}
}
[Workspace(typeof(TestDelegate), "GetConnection")]
public class Test
{
}
Other possible workaround is creating abstract base Attribute type with abstract method matching your delegate definition, and then implementing the method in concrete Attribute class.
It has following benefits:
- Annotation is more concise and clean (DSL like)
- No reflection
- Easy to reuse
Example:
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface, AllowMultiple=false, Inherited=true)]
public abstract class GetConnectionAttribute : Attribute
{
public abstract IPropertySet GetConnection();
}
public class GetConnectionFromPropertySetAttribute : GetConnectionAttribute
{
public override IPropertySet GetConnection()
{
return new PropertySetClass();
}
}
[GetConnectionFromPropertySet]
public class Test
{
}