GenericStuff.cs

class BindableBase {}

class ValidationRule<T>// where T : BindableBase
{}

class RuleCollection<VM> where VM : BindableBase
{
    public RuleCollection<VM> AddRule<T>(VM _, Expression<Func<VM, object>> expression, params ValidationRule<T>[] rules)
    { return this; }
}

public class ValidationService
{
    private BindableBase bindableBase;
    private RuleCollection<BindableBase> ruleCollection;
	
	internal RuleCollection<VM> For<VM>(VM bindableBase) where VM : BindableBase
    {
        this.bindableBase = bindableBase;
        var a = new RuleCollection<VM>();
		ruleCollection = (RuleCollection<BindableBase>)a;
        return a;
    }
}