类型'System.Int32'的表达式不能用于返回类型'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; }
        }
    }
}