Question

I have a scenario where I have to get an array of strings that represent each of the property names used within a Func parameter. Here is an example implementation:

public class CustomClass<TSource>
{
  public string[] GetPropertiesUsed
  {
    get
    {
      // do magical parsing based upon parameter passed into CustomMethod
    }
  }

  public void CustomMethod(Func<TSource, object> method)
  {
    // do stuff
  }
}

Here would be an example usage:

var customClass = new CustomClass<Person>();
customClass.CustomMethod(src => "(" + src.AreaCode + ") " + src.Phone);

...

var propertiesUsed = customClass.GetPropertiesUsed;
// propertiesUsed should contain ["AreaCode", "Phone"]

The part I’m stuck on in the above is the “do magical parsing based upon parameter passed into CustomMethod.”

Answer 1

You’ll need to change your CustomMethod to take an Expression<Func<TSource, object>>, and probably subclass the ExpressionVisitor, overriding VisitMember:

public void CustomMethod(Expression<Func<TSource, object>> method)
{
     PropertyFinder lister = new PropertyFinder();
     properties = lister.Parse((Expression) expr);
}

// this will be what you want to return from GetPropertiesUsed
List<string> properties;

public class PropertyFinder : ExpressionVisitor
{
    public List<string> Parse(Expression expression)
    {
        properties.Clear();
        Visit(expression);
        return properties;
    }

    List<string> properties = new List<string>();

    protected override Expression VisitMember(MemberExpression m)
    {
        // look at m to see what the property name is and add it to properties
        ... code here ...
        // then return the result of ExpressionVisitor.VisitMember
        return base.VisitMember(m);
    }
}

This should get you started in the right direction. Let me know if you need some help figuring out the “… code here …” part.

Useful links:

• Expression Trees
• How to Modify Expression Trees
• ExpressionVisitor
• VisitMember
• MemberExpression