Expression of type 'System.Int32' cannot be used for return type 'System.Object'

Question

I am trying to produce a simple scripting system that will be used to print labels. I have done this in the past with reflection with no problem, but I am now trying to do it with Lambda functions so that I can cache the 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;      }  } 

This seems to work in some cases, but in others it fails. The problem seems to be in my trying to return the values as objects - irrespective of the actual data types. I am trying to do this because I do not know at compile time what the data type will be but in the long run, I only need a string.

I am getting the exception shown in the title of this message whenever I try to access a property of type Int32 - but I am also getting it for Nullable types and others. The exception is thrown when I try to compile the expression into the function.

Can anybody suggest how I might go about this differently whilst maintaining the Lambda functionality so that I can cache the accessors?

Answer 1

Have you tried using Expression.Convert? That will add the boxing/lifting/etc conversion.

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

Answer 2

I hope this code will help 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; }         }     } }