将类型转换为PropertyType

Question

I am new to reflection in C# and have something similar like this:

class A
{
    DateTime _time = DateTime.Now;
    public DateTime Time
    {
       set
       {
          _time = value;
       }
       get
       {
          return _time;
       }
    }
}

And this method somewhere in the app:

public Type GetSomeType(int num)
{
    switch (num)
    {
        case 0:
            DateTime time = DateTime.Now;
            return time.GetType();
        case 1:
            int i = 5;
            return i.GetType();
        default:
            return null;
    }
}

What I'm trying to do is set the Time property of class A with the result of the GetSomeType method:

A MyClass = new A();
MyClass.Time = (DateTime)GetSomeType(0); //Clearly, this does not work

I don't know if this is possible at all or am I totally wrong here? In my real application it is more complex since I'm working with PropertyInfo , but for now I would be happy to grasp the concept.

Thanks

Juergen

Answer 1

Naturally this isn't going to work because you're mixing up types with the value (or instances) of types.

What you will likely need to look into is PropertyInfo.GetValue ( PropertyInfo.SetValue might be just as relevant, depending on what you want to do next, too) - but I think you might need to consider exactly what it is you want to do; in your example, for instance, you could just return an object, or perhaps dynamic since you instantiate and handle the value directly. But it sounds like you want to get the value of an existing instance of something.

Say if you have an A and a B , you want to get the value of Ba and set Aa with it, it's not clear from your explanation why you can't just do Ba = Aa , or what the discriminator num is properly for; but if you do have to use reflection and already have the PropertyInfo , then:

public dynamic GetSomeValue(object instance, PropertyInfo property)
{
    return property.GetValue(instance, null);
}

Although, this isn't anywhere near ideal, and mostly flawed if not overkill - it will hopefully be sufficient information to allow you to marry up what you can do with what you need to do.

Answer 2

You don't need reflection to set the type of the property Time. it's defined as a type of DateTime. I'm not sure that this is a problem for reflection.

Also in the GetSomeType method you don't need to instantiate objects to retrieve their type.

public Type GetType(int num)
{
  switch(num)
  {
     case 0:
       return typeof(DateTime);
     case 1:
       return typeof(int)
   }

  return null;
}

Answer 3

There's probably some misunderstanding of what Type represents. It represents only the type object, nothing else. Your code could be simplified to the following and it would behave exactly the same:

public Type GetSomeType(int num)
{
    switch (num)
    {
        case 0:
            return typeof(DateTime);
        case 1:
            return typeof(int);
        default:
            return null;
    }
}

My guess is that you want to return object , or something like that:

public object GetSomeType(int num)
{
    switch (num)
    {
        case 0:
            return DateTime.Now;
        case 1:
            return 5;
        default:
            return null;
    }
}

This should work for you. But I have no idea why would you do that.

Answer 4

First, instead of this:

DateTime time = DateTime.Now;
return time.GetType();

you can do this:

return typeof(DateTime);

also, you don't need to cast if (just guessing) you set a property via PropertyInfo:

propInfo.SetValue(instance, value, null);

where the variable value can be of type object, only the runtime type has to match, which you can also check like this:

if (value == null || propInfo.PropertyType == value.GetType())  

Still, i'm wondering what you are trying to do.

Answer 5

You would need to change your implementation to use object return type instead of Type as return type and also you would have to use nullable datetime type. Please find below the sample code:

class A
{
    DateTime? _time = DateTime.Now;
    public DateTime? Time
    {
       set
       {
          _time = value;
       }
       get
       {
          return _time;
       }
    }
}


public object GetSomeType(int num)
{
    switch (num)
    {
        case 0:
            DateTime time = DateTime.Now;
            return time;
        case 1:
            int i = 5;
            return i;
        default:
            return null;
    }
}

A MyClass = new A();
MyClass.Time = this.GetSomeType(0) as DateTime?; //This should now work