An expression of type "System.Int32" cannot be used for the return type of "System.Object"

Question

I am trying to create a simple scripting system that will be used to print labels. I have done this in the past with reflection without problems, but now I am trying to do this using Lambda functions so that I can cache functions for reuse.

The code I have so far is as follows:

public static string GetValue<T>(T source, string propertyPath) { try { Func<T, Object> func; Type type = typeof(T); ParameterExpression parameterExpression = Expression.Parameter(type, @"source"); Expression expression = parameterExpression; foreach (string property in propertyPath.Split('.')) { PropertyInfo propertyInfo = type.GetProperty(property); expression = Expression.Property(expression, propertyInfo); type = propertyInfo.PropertyType; } func = Expression.Lambda<Func<T, Object>>(expression, parameterExpression).Compile(); object value = func.Invoke(source); if (value == null) return string.Empty; return value.ToString(); } catch { return propertyPath; } } 

In some cases this works, but in others it fails. The problem seems to be in my attempt to return values ​​as objects - regardless of the actual data types. I try to do this because at compile time I do not know what data type will be, but in the end, I only need a string.

I get the exception indicated in the header of this post whenever I try to access a property of type Int32, but I also get it for types Nullable and others. An exception occurs when I try to compile an expression in a function.

Can anyone suggest how I can do this differently while supporting Lambda functionality so that I can cache accessors?

Answer 1

Have you tried using Expression.Convert ? This will add a box / boost conversion / etc.

 Expression conversion = Expression.Convert(expression, typeof(object)); func = Expression.Lambda<Func<T, Object>>(conversion, parameterExpression).Compile(); 

Answer 2

I hope this code helps you

 using System; using System.Linq; using System.Linq.Expressions; using System.Reflection; namespace Student { class Program { static void Main(string[] args) { var a = new Student(); PrintProperty(a, "Name"); PrintProperty(a, "Age"); Console.ReadKey(); } private static void PrintProperty<T>(T a, string propName) { PrintProperty<T, object>(a, propName); } private static void PrintProperty<T, TProperty>(T a, string propName) { ParameterExpression ep = Expression.Parameter(typeof(T), "x"); MemberExpression em = Expression.Property(ep, typeof(T).GetProperty(propName)); var el = Expression.Lambda<Func<T, TProperty>>(Expression.Convert(em, typeof(object)), ep); Console.WriteLine(GetValue(a, el)); } private static TPorperty GetValue<T, TPorperty>(T v, Expression<Func<T, TPorperty>> expression) { return expression.Compile().Invoke(v); } public class Student { public Student() { Name = "Albert Einstein"; Age = 15; } public string Name { get; set; } public int Age { get; set; } } } }