[英]C# Use Expression to get value from an object
the object cloud be any class, and I don't want to use reflection. object 云是任何 class,我不想使用反射。 this is my code:这是我的代码:
class Dog
{
public int Id { get; set; }
public string Name { get; set; }
}
static void test()
{
object d = new Dog
{
Id = 21,
Name = "adsf"
};
genericeTest(d);
}
static void genericeTest<T>(T t)
{
var type = t.GetType();
var arg = Expression.Parameter(type, "x");
var expr = Expression.Property(arg, "Id");
var compiled = Expression.Lambda<Func<T, int>>(expr, arg).Compile();
var value = compiled.Invoke(t);
}
but there is exception System.ArgumentException: ParameterExpression of type 'ExpressionDemo.Program+Dog' cannot be used for delegate parameter of type 'System.Object'
at var compiled = Expression.Lambda<Func<T, int>>(expr, arg).Compile();
但有异常System.ArgumentException: ParameterExpression of type 'ExpressionDemo.Program+Dog' cannot be used for delegate parameter of type 'System.Object'
var compiled = Expression.Lambda<Func<T, int>>(expr, arg).Compile();
if I write like this如果我这样写
var compiled = Expression.Lambda<Func<Dog, int>>(expr, arg).Compile();
var value = compiled.Invoke(t as Dog);
it works well.它运作良好。 can I fix it, thank you.我可以修一下吗,谢谢。
The problem is that your T
is an object
, because d
has a compile-time type of object
, but t.GetType()
is Dog
, because the runtime type of t
is Dog
.问题是您的T
是object
,因为d
的编译时类型为object
,但t.GetType()
是Dog
,因为t
的运行时类型是Dog
。
So you've got an expression which accepts a Dog
, but you're trying to pass it a variable of type object
-- you're missing the cast which would turn the object
into a Dog
.因此,您有一个接受Dog
的表达式,但您试图将object
类型的变量传递给它——您缺少将object
变成Dog
的演员表。
In other words, you're writing something roughly equivalent to:换句话说,您正在编写的内容大致相当于:
public int GeneratedMethod(object o)
{
Dog d = o; // <-- This line will cause a compile-time error, of course
return d.Id;
}
The question is, what do you want your generated method to look like?问题是,您希望生成的方法是什么样的? Should it look like this?它应该看起来像这样吗?
public int GeneratedMethod(object o)
{
Dog d = (Dog)o;
return d.Id;
}
If so, you'll need to insert the cast from object
to Dog
.如果是这样,您需要将演员表从object
插入到Dog
。 The way to do this is using Expression.Convert
:这样做的方法是使用Expression.Convert
:
var type = t.GetType();
var arg = Expression.Parameter(typeof(T), "x");
var expr = Expression.Property(Expression.Convert(arg, type), "Id");
var compiled = Expression.Lambda<Func<T, int>>(expr, arg).Compile();
Here we're declaring a lambda which accepts a T
, but then casts it to Dog
.在这里,我们声明了一个 lambda ,它接受一个T
,然后将其转换为Dog
。
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