PostSharp Contracts Range

Question

Is it possible to achieve something like this with PostSharp and Contracts?

public class Something
{
    int number = 10;

    public void Remove([Range(1,this.number)] int remove)
    {
        number -= remove;
    }

    public void Add(int add)
    {
        number += add;
    }
}

Answer 1

C# compiler will not allow you to apply the [Range] attribute in this way - you will receive a build error stating that "an attribute argument must be a constant expression, typeof expression or array creation expression".

The workaround is to create an aspect that accepts a field name as an argument. Then import that field into the aspect, so you can read the current max value.

[Serializable]
public class MyRangeAttribute : LocationContractAttribute,
                                ILocationValidationAspect<int>,
                                IInstanceScopedAspect,
                                IAdviceProvider
{
    [NonSerialized]
    private object instance;
    [NonSerialized]
    private string maxValueFieldName;
    private int minValue;
    public ILocationBinding maxValueFieldBinding;

    public MyRangeAttribute(int minValue, string maxValueFieldName)
    {
        this.minValue = minValue;
        this.maxValueFieldName = maxValueFieldName;
    }

    public Exception ValidateValue(int value, string locationName, LocationKind locationKind)
    {
        int maxValue = (int) this.maxValueFieldBinding.GetValue(ref this.instance, Arguments.Empty);
        if (value < minValue || value > maxValue)
            return new ArgumentOutOfRangeException(locationName);

        return null;
    }

    public IEnumerable<AdviceInstance> ProvideAdvices(object targetElement)
    {
        FieldInfo maxValueField = ((LocationInfo)targetElement).DeclaringType
            .GetField( this.maxValueFieldName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance );

        yield return new ImportLocationAdviceInstance(
            typeof (MyRangeAttribute).GetField("maxValueFieldBinding"),
            new LocationInfo(maxValueField));
    }

    public object CreateInstance( AdviceArgs adviceArgs )
    {
        MyRangeAttribute clone = (MyRangeAttribute) this.MemberwiseClone();
        clone.instance = adviceArgs.Instance;
        return clone;
    }

    public void RuntimeInitializeInstance()
    {
    }
}

You can apply this aspect like this:

public class Something
{
    private int number = 10;

    public void Remove([MyRange(1, "number")] int remove)
    {
        number -= remove;
    }

    public void Add(int add)
    {
        number += add;
    }
}