expression-trees

I'm having issues trying to get the value of an object out of the Expression Tree without using .Compile() The object is quite simple. var userModel = new UserModel { Email = "John@Doe.com"}; The method giving me issues looks like this. private void VisitMemberAccess(MemberExpression expression, MemberExpression left) { var key = left != null ? left.Member.Name : expression.Member.Name; if (expression.Expression.NodeType.ToString() == "Parameter") { // add the string key _strings.Add(string.Format("[{0}]", key)); } else { // add the string parameter _strings.Add(string.Format("@{0}", key)); //

Using anonymous methods you can create empty delegates since C# 2.0. public event EventHandler SomeEvent = delegate {}; public event Action OtherEvent = delegate {}; This is e.g. useful to prevent having to do the null check when invoking events . How can I create the same behavior using Expression Trees ? The only possible option I see now is to use Expression.Lambda() , but as far as I can tell this would require a lot of extra work. An expression tree, by nature of its purpose, always has an expression for a body rather than a statement in the original design. In C# 3 there was no way at

I have a Linq provider that sucessfully goes and gets data from my chosen datasource, but what I would like to do now that I have my filtered resultset, is allow Linq to Objects to process the rest of the Expression tree (for things like Joins, projection etc) My thought was that I could just replace the expression constant that contains my IQueryProvider with the result-sets IEnumerable via an ExpressionVisitor and then return that new expression. Also return the IEnumerable's provider from my IQueryable...but this does not seem to work :-( Any idea's? Edit: Some good answers here, but given

I am working on translating an expression tree to a format that resembles infix notation; I am not evaluating the tree or executing its operations. The tree contains both logical and relational operations, and I would like to emit parentheses in an intelligent manner during the translation. To illustrate, consider the following contrived expression: a < x & (a < y | a == c) & a != d If I walk the expression tree produced by this expression in-order, then I will print out the following expression, which is incorrect. a < x & a < y | a == c & a != d // equivalent to (a < x & a < y) | (a == c & a

I am currently porting a .NET codebase in MonoTouch and I'm currently working on a method that receives an Expression<T> . I'm trying to compile it, and then dynamically invoke it. Here's what I did: // Here's an example of what I could receive Expression<Action<int>> expression = (a => Console.WriteLine (a * 2)); // And here's what I'm trying to do to invoke it expression.Compile().DynamicInvoke(6); This works fine in the iOS Simulator, the result "12" is printed in my console. But then I tried it on an iPad, and I received the following exception. Object reference not set to an instance of

Updated Question Further Down I've been experimenting with expression trees in .NET 4 to generate code at runtime and I've been trying to implement the foreach statement by building an expression tree. In the end, the expression should be able to generate a delegate that does this: Action<IEnumerable<int>> action = source => { var enumerator = source.GetEnumerator(); while(enumerator.MoveNext()) { var i = enumerator.Current; // the body of the foreach that I don't currently have yet } } I've come up with the following helper method that generates a BlockExpression from an IEnumerable: public

Looks like ExpressionTrees compiler should be near with the C# spec in many behaviors, but unlike C# there is no support for conversion from decimal to any enum-type : using System; using System.Linq.Expressions; class Program { static void Main() { Func<decimal, ConsoleColor> converter1 = x => (ConsoleColor) x; ConsoleColor c1 = converter1(7m); // fine Expression<Func<decimal, ConsoleColor>> expr = x => (ConsoleColor) x; // System.InvalidOperationException was unhandled // No coercion operator is defined between types // 'System.Decimal' and 'System.ConsoleColor'. Func<decimal, ConsoleColor>