How to detect an auto-implemented property in a custom FxCop rule?

Question

I'm trying to write a rule for DTOs, that only allows auto-implemented properties. How do you detect that a property is auto-implemented using the FxCop API?

Answer 1

Actually there is a difference between auto-properties and properties implemented by user in the IL compiled code. Auto-properties setters and getters are tagged with System.Runtime.CompilerServices.CompilerGeneratedAttribute.

Hence with the tool NDepend, where you can write custom code queries and custom code rules through C# LINQ queries, matching auto-properties getters and setters is just a matter of writing the LINQ query:

from m in Application.Methods
where (m.IsPropertyGetter|| m.IsPropertySetter)
    && m.IsGeneratedByCompiler
select m

Notice in the screenshot above that you might match getters and setters in Resources generated classes, to prevent this you can add a clause like && !m.ParentType.IsUsing("System.Resources.ResourceManager"):

Of course, all this can be adapted to FxCop custom ruling system.

Disclamer: I work for NDepend

Answer 2

Thank you @Patrick from NDepend team. Here's the code for the resulting FxCop rule.

using Microsoft.FxCop.Sdk;
using System.Linq;
using System.Runtime.CompilerServices;

namespace CustomFxRules
{
    internal sealed class IsAutoImplementedProperty : BaseIntrospectionRule
    {
        public IsAutoImplementedProperty()
            : base("IsAutoImplementedProperty", "CustomFxRules.CustomFxRules", typeof(IsAutoImplementedProperty).Assembly)
        {
        }

        public override TargetVisibilities TargetVisibility
        {
            get { return TargetVisibilities.ExternallyVisible; }
        }

        private TypeNode CompilerGeneratedAttributeType { get; set; }

        public override void BeforeAnalysis()
        {
            base.BeforeAnalysis();

            this.CompilerGeneratedAttributeType = FrameworkAssemblies.Mscorlib.GetType(
                Identifier.For("System.Reflection"),
                Identifier.For("CompilerGeneratedAttribute"));
        }

        public override ProblemCollection Check(Member member)
        {
            if (member.NodeType != NodeType.Property)
                return base.Check(member);

            var property = (PropertyNode)member;
            VisitProperty(property);
            return this.Problems;
        }

        private const string AddAutoImplementedPropertyResolutionName = "Add auto-implemented property";

        public override void VisitProperty(PropertyNode property)
        {
            if (property.Getter == null)
            {
                Problems.Add(new Problem(new Resolution(AddAutoImplementedPropertyResolutionName, "{0} property must have a getter", property.FullName)));
            }
            else if (property.Getter.Attributes.All(attributeNode => attributeNode.Type.FullName != typeof(CompilerGeneratedAttribute).FullName))
            {
                Problems.Add(new Problem(new Resolution(AddAutoImplementedPropertyResolutionName, "{0} property must have an auto-impelemented getter", property.FullName)));
            }

            if (property.Setter == null)
            {
                Problems.Add(new Problem(new Resolution(AddAutoImplementedPropertyResolutionName, "{0} property must have a setter", property.FullName)));
            }
            else if (property.Setter.Attributes.All(attributeNode => attributeNode.Type.FullName != typeof(CompilerGeneratedAttribute).FullName))
            {
                Problems.Add(new Problem(new Resolution(AddAutoImplementedPropertyResolutionName, "{0} property must have an auto-impelemented setter", property.FullName)));
            }
        }
    }
}