C# “System.Int32”类型的表达式不能用于返回类型“System.Object”
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Expression of type 'System.Int32' cannot be used for return type 'System.Object'
提问by Martin Robins
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.
我正在尝试制作一个简单的脚本系统，用于打印标签。我过去使用反射没有问题地完成了此操作，但我现在尝试使用 Lambda 函数来完成此操作，以便我可以缓存这些函数以供重用。
The code I have so far is as follows...
我到目前为止的代码如下...
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.
这在某些情况下似乎有效，但在其他情况下却失败了。问题似乎在于我试图将值作为对象返回 - 无论实际数据类型如何。我正在尝试这样做，因为我在编译时不知道数据类型是什么，但从长远来看，我只需要一个字符串。
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.
每当我尝试访问 Int32 类型的属性时，我都会收到此消息标题中显示的异常 - 但我也收到了 Nullable 类型和其他类型的异常。当我尝试将表达式编译到函数中时会引发异常。
Can anybody suggest how I might go about this differently whilst maintaining the Lambda functionality so that I can cache the accessors?
任何人都可以建议我如何在保持 Lambda 功能的同时以不同的方式处理这个问题，以便我可以缓存访问器？
采纳答案by Jon Skeet
Have you tried using Expression.Convert? That will add the boxing/lifting/etc conversion.
您是否尝试过使用Expression.Convert？这将添加拳击/举重/等转换。
Expression conversion = Expression.Convert(expression, typeof(object));
func = Expression.Lambda<Func<T, Object>>(conversion, parameterExpression).Compile();
回答by Zekaee Esmaeel
I hope this code will help you ?
我希望这段代码对你有帮助？
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; }
}
}
}
