Using Expression.Dynamic to generate a converter for value types.

Question

Part of the data layer for my application is a converter cache similar to System.Data.DataRowExtensions.UnboxT, used in Linq-to-Data. The premise of the cache for all known types I generate a simple converter and cache it. The converter casts the object as T, or in the case of DBNull.Value, returns default(T) (unlike UnboxT which throws an exception on non-nullable value types.) The reason I can't use UnboxT is that our developers don't like checking for DBNull before assigning a value from the datarow and they just want it done for them. We also have a factory to generate helper delegate that instantiates objects off of a DataRow, and it'd be annoying to add logic in the delegate. i.e. it generates something like this this::

datarow =>
    new MyObject()
    {
        property1 = DBConverterCache<TProperty1>.Converter(datarow[columnName1]),
        property2 = DBConverterCache<TProperty2>.Converter(datarow[columnName2]),
        /*etc...*/

    };

Even more so, I have another annoyance. Objects in the object layer may not match correctly to ones in the database. This is a problem because you can't unbox things to the "wrong" type. I.e. the property in the object layer is an Int32, the column in the DB is an Int64. To fix this I have for the struct IConvertibles the converter basically does this::

value => value == DBNull.Value ? default(T) : (value as IConvertible).To<Type*>(null);

* in the case of Nullable<T> or Enum it casts to underlying type, then casts up to T

This is ugly as we have to use reflection to generate the call to ToType, which relies on the assumption that they will never expand the IConvertible interface to add more objects that are convertible. It's a hack but it avoids boxing the return type. Which a method like IConvertible.ToType does.

of course this works just as well:

value => value == DBNull.Value ? default(T) : (T)(value as dynamic);

Maybe even better, as I don't have to specialize the call based on the type I can just make that my default converter. The only issue I haven't a clue as to how to use Expression.Dynamic, and I can't create an expression that takes dynamic as a parameter. I suppose I could just bind it to a static method or the above lambda expression, but I'd like to do everything as expression trees, if possible.

Answer 1

Well I got this to work, with some good old reflector.

    public static Converter<Object, T> CreateDynamicConverter<T>()
    {
        var param = Expression.Parameter(typeof(object)); 
        var expression = Expression.Lambda<Converter<object, T>>(
            Expression.Condition(
                Expression.Equal(
                    param,
                    Expression.Constant(
                        DBNull.Value
                    )
                ),
                Expression.Default(
                    typeof(T)
                ),
                Expression.Dynamic(
                    Binder.Convert(
                        CSharpBinderFlags.ConvertExplicit, 
                        typeof(T), 
                        typeof(MyApplicationNameHere)
                    ), 
                    typeof(T), 
                    param
                )
            ),
            param
        );
        return expression.Compile();
    }

Seems to be working just fine.