c# - Lambda expression in attribute constructor

Question

I have created an Attribute class called RelatedPropertyAttribute:

[AttributeUsage(AttributeTargets.Property)]
public class RelatedPropertyAttribute: Attribute
{
    public string RelatedProperty { get; private set; }

    public RelatedPropertyAttribute(string relatedProperty)
    {
        RelatedProperty = relatedProperty;
    }
}

I use this to indicate related properties in a class. Example of how I would use it:

public class MyClass
{
    public int EmployeeID { get; set; }

    [RelatedProperty("EmployeeID")]
    public int EmployeeNumber { get; set; }
}

I would like to use lambda expressions so that I can pass a strong type into my attribute's constructor, and not a "magic string". This way I can exploit compiler type checking. For example:

public class MyClass
{
    public int EmployeeID { get; set; }

    [RelatedProperty(x => x.EmployeeID)]
    public int EmployeeNumber { get; set; }
}

I thought I could do it with the following, but it isn't allowed by the compiler:

public RelatedPropertyAttribute<TProperty>(Expression<Func<MyClass, TProperty>> propertyExpression)
{ ... }

Error:

The non-generic type 'RelatedPropertyAttribute' cannot be used with type arguments

How can I achieve this?

Answer 1

Having a generic attribute is not possible in a conventional way. However C# and VB don't support it but the CLR does. If you want to write some IL code it's possible.

Let's take your code:

[AttributeUsage(AttributeTargets.Property)]
public class RelatedPropertyAttribute: Attribute
{
    public string RelatedProperty { get; private set; }

    public RelatedPropertyAttribute(string relatedProperty)
    {
       RelatedProperty = relatedProperty;
    }
}

Compile the code, open up the assembly with ILSpy or ILDasm and then dump the content to a text file. The IL of you attribute class declaration will look like this:

.class public auto ansi beforefieldinit RelatedPropertyAttribute
extends [mscorlib]System.Attribute

In the text file, you can then make the attribute generic. There are several things that need to be changed.

This can simply be done by changing the IL and the CLR won't complain:

.class public abstract auto ansi beforefieldinit
      RelatedPropertyAttribute`1<class T>
      extends [mscorlib]System.Attribute

and now you can change the type of relatedProperty from string to your generic type.

For Example:

.method public hidebysig specialname rtspecialname 
    instance void .ctor (
        string relatedProperty
    ) cil managed

change it to:

.method public hidebysig specialname rtspecialname 
    instance void .ctor (
        !T relatedProperty
    ) cil managed

There are lot of frameworks to do a "dirty" job like that: Mono.Cecil or CCI.

As I have already said it's not a clean object oriented solution but just wanted to point out another way to break the limit of C# and VB.

There's an interesting reading around this topic, check it out this book.

Hope it helps.