Question

I want to write filtering controls which take object type T and property name and return Expression<Func<T, bool>> that checks value of passed property. I don’t want to use reflection because I’m afraid such expressions can’t be used by EF. I can’t use delegates because C# doesn’t have delegates for properties. What can I do? Maybe I should use different approach for writing these controls?

Here is my first approach using reflection:

public string FilteringField { get; set; }
public Expression<Func<T, bool>> GetFilterExpression()
{
  if (cmbValue.SelectedIndex == 1)
    return (o => (bool)typeof(T).GetProperty(FilteringField).GetValue(o, null));
  if (cmbValue.SelectedIndex == 2)
    return (o => !(bool)typeof(T).GetProperty(FilteringField).GetValue(o, null));
  return null;
}

Answer 1

Reflection isn’t a problem here; EF won’t even be able to notice the difference. The delegate approach is a non-starter, by the way (since you mention EF); ultimately, it is something like:

public static IQueryable<T> Where<T>(this IQueryable<T> query,
    string propertyName, object value)
{
    PropertyInfo prop = typeof(T).GetProperty(propertyName);
    var param = Expression.Parameter(typeof(T), "x");
    var body = Expression.Equal(
        Expression.Property(param, prop),
        Expression.Constant(value, prop.PropertyType)
        );
    var predicate = Expression.Lambda<Func<T, bool>>(body, param);
    return query.Where(predicate);
}

Note that you can make it easier with Expression.PropertyOrField(propertyName); the reason I haven’t used that here is that it is very handy to know the member-type (prop.PropertyType) when creating the constant – otherwise you can get problems with nulls.