C# Add custom attribute that will set property value when setter is called

Question

The problem i am trying to solve is that i want all properties for a given class to escape invalid characters depending on whether a given attribute has been applied.

Take following class:

public class MyClass
{
    [EscapeInvalid]
    public string Prop1 {get; set;}

    public string Prop2 {get; set;}
}

Here is my defined attribute class

public class EscapeInvalidAttribute : Attribute 
{
    string escapedValue;
    public string EscapedValue
    {
        get
        {
            return escapedValue;
        }
        set
        {
            escapedValue= Escape(value);
        }
    } 
}

In a nutshell if the property for a given class has the [EscapeInvalid] applied i want to have the setter escape the value.

Is this possible?

I understand that i can just update each setter to apply the escape but was looking for a cleaner way to do this. Thanks in advance for any guidance.

Are you okay with using a class instead of String? For a cleaner way of doing things, I think that's the way I'd go. You can overrdide / extend your class to make it do other things string does if needed. — Nikki9696
– Nikki9696, Commented Dec 11, 2019 at 17:30
@Nikki9696 are you referring to a class inside the attribute definition? — Maxqueue
– Maxqueue, Commented Dec 11, 2019 at 17:32
No, you'd have a custom class, like MyEscapedString, and use that for your property instead of "string" — Nikki9696
– Nikki9696, Commented Dec 11, 2019 at 17:33
Because I don't think you can use attributes for this. I could be wrong of course. =) — Nikki9696
– Nikki9696, Commented Dec 11, 2019 at 17:34
No i can't do that because this class is a request that is sent off to api and needs to not change — Maxqueue
– Maxqueue, Commented Dec 11, 2019 at 17:34

vendettamit · Accepted Answer · 2019-12-11 17:30:15Z

2

Attributes are supported in C# for supplying run time information. It doesn't have pre processor like other Attribute Based programming languages that can change the behavior of data type at runtime.

There is a hard way to do it by creating an IL injector to support this behavior, but that's not very popular in .Net world and requires a lot of plumbing code to achieve it.

answered Dec 11, 2019 at 17:30

vendettamit

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2 Comments

Maxqueue Over a year ago

Thanks for feedback. Ya i wasn't sure if this was possible. It's not the end of the world for me to just update setters to apply the escape where applicable.

vendettamit Over a year ago

Just wondering why a downvote here? would be nice to provide reasoning for this.

timur · Accepted Answer · 2019-12-12 00:31:58Z

If you are open to making your properties virtual, you can use Castle DynamicProxy to intercept calls to setters and change the values as you please. A huge advantage of this method would be - you are relying on a industry proven library that has heaps of support and maintenance. One obvious disadvantage - introducing a layer over all your method invocations will impact performance (must be noted, NHibernate and EF Core seem to not have issues with that)

public class Hook : IProxyGenerationHook
{
    public void MethodsInspected()
    {
        // again, we probably won't use it here
    }

    public void NonProxyableMemberNotification(Type type, MemberInfo memberInfo)
    {
        // we assume everything is correctly marked `virtual` and do nothing here
    }

    public bool ShouldInterceptMethod(Type type, MethodInfo methodInfo)
    {
        // you of course can go way fancier than that. 
        // one thing of note here is that your Property setters would be prefixed with "set_", but nothing stops you from targeting other methods as well
        var prop = type.GetProperties()
        .FirstOrDefault(p => methodInfo.Name == $"set_{p.Name}" && p.GetCustomAttribute(typeof(EscapeInvalidAttribute)) != null);
        return prop != null;
    }
}
public class EscapeInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {        
        invocation.Arguments[0] = EscapeInvalidAttribute.Escape((string)invocation.Arguments[0]);
        invocation.Proceed(); // allow the call to proceed anyway       
    }
}
public class EscapeInvalidAttribute : Attribute
{
    public static string Escape(string input)
    {
        return $"escape|{input}|string";
    }
}
public class MyClass
{
    [EscapeInvalid]
    public virtual string Prop1 { get; set; }

    public virtual string Prop2 { get; set; }

}
void Main()
{
    var generator = new ProxyGenerator();
    var options = new ProxyGenerationOptions
    {
        Hook  = new Hook()
    };
    MyClass myClassProxy = (MyClass)generator.CreateClassProxy(typeof(MyClass), options, new[] {new EscapeInterceptor()});
    myClassProxy.Prop1 = "test";
    myClassProxy.Prop2 = "test";
    Console.WriteLine(myClassProxy.Prop1);
    Console.WriteLine(myClassProxy.Prop2);
}

UPD added more canonical implementation using a IProxyGenerationHook

Ideally, shouldn't you use a ProxyGenerationHook class to filter at proxy creation time the methods to proxy?

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