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Using LINQ to write constraints in OCL style

  • Artigo

I wanted to investigate using LINQ to write constraints "OCL-style".   I made a "minimal language" and added a new NamedDomainClass called Property, embedded in ExampleElement.  Then I wrote the following validation method to check that all of the Properties attached to an ExampleElement are uniquely named:

[ValidationState(ValidationState.Enabled)]

public partial class ExampleElement

{

[ValidationMethod(ValidationCategories.Menu | ValidationCategories.Save)]

private void TestExampleElement(ValidationContext context)

{

var propnames = from p in this.Properties select p.Name;

var distinctnames = propnames.Distinct<string>();

if (propnames.Count<string>() != distinctnames.Count<string>())

{

context.LogError("Non-unique property names", "Error 1");

}

}

}

A lot tighter than writing it in good old C#, I think. Then I thought about "flattened sets", i.e. navigating across more than one relationship and creating a single collection containing the results. So I created SubProperty embedded in Property, and extended the constraint like this:

[ValidationState(ValidationState.Enabled)]

public partial class ExampleElement

{

[ValidationMethod(ValidationCategories.Menu | ValidationCategories.Save)]

private void TestExampleElement(ValidationContext context)

{

var propnames = from p in this.Properties select p.Name;

var distinctnames = propnames.Distinct<string>();

if (propnames.Count<string>() != distinctnames.Count<string>())

{

context.LogError("Non-unique property names", "Error 1");

}

var subproperties = this.Properties.Aggregate(Enumerable.Empty<SubProperty>(),

(agg, p) => agg.Union<SubProperty>(p.SubProperties));

var subpropnames = from p in subproperties select p.Name;

var distinctsubpropnames = subpropnames.Distinct<string>();

if (subpropnames.Count<string>() != distinctsubpropnames.Count<string>())

{

context.LogError("Non-unique sub property names", "Error 2");

}

}

}

Not bad: but that line to calculate and flatten the subproperties is a bit complicated.  So, inspired by OCL, I defined a new extension method like this:

 

public static class C

{

    public static IEnumerable<U> Collect<T, U>(this IEnumerable<T> source, Func<T, IEnumerable<U>> func)

    {

        return source.Aggregate(Enumerable.Empty<U>(), (agg, p) => agg.Union<U>(func(p)));

  }

}

 

And now the subproperties line looks like this:

 

var subproperties = this.Properties.Collect(p => p.SubProperties);

 

I think I may be using that Collect method again!

Comments

  • Doesn't LINQ's SelectMany do what you want to do (flatten collections)?
  • Out of curiosity, why do you explicitly specify the type parameter to methods like Count (Count<string>() instead of just count())? Thanks!

  • Anonymous
    August 23, 2007
    SelectMany - I will have to try that. The type parameters to Count and Distinct are unnecessary, as you point out. -- Steve

  • Anonymous
    August 23, 2007
    Oh one other style question (I'm very curious about how people view functional programming): You wrote: var propnames = from p in this.Properties select p.Name; Instead of: var propnames = this.Properties.Select(p => p.Name); Any particular reason?

  • Anonymous
    August 23, 2007
    Really no reason at all.  I haven't decided which I prefer yet.  I suppose to be consistent with OCL I would go for the second style. You are correct about SelectMany - it does just the same as my Collect.  So Collect is unnecessary.  I guess the name confused me. Still, writing the extension method was a good learning experience.

  • Anonymous
    August 23, 2007
    Cool! I love seeing functional styles; please keep blogging things like this. Hopefully, more people will see why this is vastly superior to writing it imperatively. In fact, do you have a snippet of what it would look like without the C# 3.0 features, for comparison?

  • Anonymous
    August 23, 2007
    I guess doing it the “old way” would typically look like this: [ValidationMethod(ValidationCategories.Menu | ValidationCategories.Save)] private void TestExampleElementTheOldWay(ValidationContext context) {    List<string> uniquePropertyNames = new List<string>();    foreach (Property p in this.Properties)    {        if (uniquePropertyNames.Contains(p.Name))        {            context.LogError("Non-unique property names (old way)", "Error 3");        }        else        {            uniquePropertyNames.Add(p.Name);        }    }    List<string> uniqueSubPropertyNames = new List<string>();    foreach (Property p in this.Properties)    {        foreach (SubProperty sp in p.SubProperties)        {            if (uniqueSubPropertyNames.Contains(sp.Name))            {                context.LogError("Non-unique sub property names (old way)", "Error 4");            }            else            {                uniqueSubPropertyNames.Add(sp.Name);            }        }    } }