Use lambda return value for expression tree

Question

I'm trying to play a little with expression trees. I have an object with List<string> , and I want to build an expression tree that will add a value to this property, but I want to specify a value to add through Func . I am currently trying this ...

 public static Action<T> CreateMethodAddObjectToList<T, C>(this Type type, string property, Func<C> ctorFunction) { PropertyInfo fieldInfo = type.GetProperty(property); if (fieldInfo == null) { return null; } ParameterExpression targetExp = Expression.Parameter(type, "target"); MemberExpression fieldExp = Expression.Property(targetExp, property); var method = fieldExp.Type.GetMethod("Add", BindingFlags.Public | BindingFlags.Instance); Expression<Func<C>> ctorExpression = () => ctorFunction(); // but this doesnt work because I can't use the ctorExpression in this way var callExp = Expression.Call(fieldExp, method, ctorExpression); var function = Expression.Lambda<Action<T>>(callExp, targetExp).Compile(); return function; } 

the call looks like

 var dummyObject = new DummyObject { IntProperty = 5 }; Action<DummyObject> setter = typeof (DummyObject).CreateMethodAddObjectToList<DummyObject, string>("StringList", () => "Test" ); 

Answer 1

You can change ctorFunction to Expression<Func<C>> and then call it in the generated action:

 public static Action<T> CreateMethodAddObjectToList<T, C>(this Type type, string property, Expression<Func<C>> createExpr) { PropertyInfo fieldInfo = type.GetProperty(property); if (fieldInfo == null) { return null; } ParameterExpression targetExp = Expression.Parameter(type, "target"); MemberExpression fieldExp = Expression.Property(targetExp, property); var method = fieldExp.Type.GetMethod("Add", BindingFlags.Public | BindingFlags.Instance); var valueExpr = Expression.Invoke(createExpr); var callExpr = Expression.Call(fieldExp, method, valueExpr); var function = Expression.Lambda<Action<T>>(callExpr, targetExp).Compile(); return function; }