As noted in comments, the problem is that there exists no column in the reader for the specified property. The idea is to loop by the column names of reader first, and check to see if matching property exists. But how do one get the list of column names beforehand? One idea is to use … Read more
If you don’t need the title attribute on individual options your code could be simplified to: public static HtmlString SelectFor<TModel, TProperty, TIdProperty, TDisplayProperty, TListItem>( this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<TListItem> enumeratedItems, Expression<Func<TListItem, TIdProperty>> idProperty, Expression<Func<TListItem, TDisplayProperty>> displayProperty, object htmlAttributes ) where TModel : class { var id = (idProperty.Body as MemberExpression).Member.Name; var display = … Read more
While dtb’s answer works for several scenarios, it is suboptimal as such an expression cannot be used in Entity Framework, as it cannot handle Invoke calls. Unfortunately, to avoid those calls one needs a lot more code, including a new ExpressionVisitor derived class: static Expression<Func<A, C>> Compose<A, B, C>(Expression<Func<B, C>> f, Expression<Func<A, B>> g) { … Read more
I think I know what the problem is. Your expression returns type object. If you change this to Expression<Func<T, R>> the return type should be correctly inferred, and UnaryExpression (which I will assume is some boxing operation) should not occur. Update: The signature for Include should be: public void Include<T, R>(Expression<Func<T, R>> expression)
It’s hard to mix compiler-generated expression trees and hand-made ones, precisely because of this sort of thing – extracting out the ParameterExpressions is tricky. So let’s start from scratch: ParameterExpression argParam = Expression.Parameter(typeof(Service), “s”); Expression nameProperty = Expression.Property(argParam, “Name”); Expression namespaceProperty = Expression.Property(argParam, “Namespace”); var val1 = Expression.Constant(“Modules”); var val2 = Expression.Constant(“Namespace”); Expression e1 = … Read more
What you’re looking for is the ability to compose expressions, just as you can compose functions: public static Expression<Func<T, TResult>> Compose<T, TIntermediate, TResult>( this Expression<Func<T, TIntermediate>> first, Expression<Func<TIntermediate, TResult>> second) { return Expression.Lambda<Func<T, TResult>>( second.Body.Replace(second.Parameters[0], first.Body), first.Parameters[0]); } This relies on the following method to replace all instances of one expression with another: public class … Read more