尝试调用调用会导致“对象与目标类型不匹配”

Question

I'm trying to do a bit of factoring on my use of a series of Enums defined in .net

    private static String GetSystemTranslatedField(Type enumtype, int? val) {
        if (val == null)
            return "";

        int value = (int)val;
        String tempstr;
        if (Enum.IsDefined(enumtype, value))
        {
            tempstr = (String) enumtype.GetMethod("ToString",Type.EmptyTypes).Invoke(val,null); 
            //equivalent to ((enumtype)val).ToString();
        }
    }

The problem is that when trying to run this, i get the infamous System.Reflection.TargetException: Object does not match target type.

As an extra question, is there a way to change the signature of my method so that only types deriving from Enum are accepted? It's not a big concern but i got curious about type restriction and couldn't find anything.

Answer 1

Enum is a class , so you can set that as your base for method signature and avoid reflection entirely . Try:

    private static String GetSystemTranslatedField(Enum enumtype, int? val) {
        if (!val.HasValue)
            return "";

        String tempstr;
        if (Enum.IsDefined(enumtype, val.Value))
        {
            tempstr = ((enumtype)val.Value).ToString(); 
        }

        ... // Rest of your code here
    }

Note that I cleaned your method up a bit, mainly cosmetic stuff.

Answer 2

You are trying to call ToString coming from an enum type on an Int32 which is causing an exception in the Reflection API.

If you want to get the identifier related to a value, use Enum.GetName which will work on int's as well (although the documentation does not seem to indicate this):

if (Enum.IsDefined(enumtype, value))
{
    tempstr = Enum.GetName(enumType, value);
}

In fact it will return the empty string if the value is not defined, so that if could be removed as well (depending on what is in the rest of the code):

tempstr = Enum.GetName(enumType, value);

C# does not allow generic constraints to Enum derived types. It is doable on a theoretical level (it is not disallowed by the CLR) however it requires some IL weaving, which Jon Skeet demonstrated in his Unconstrained Melody project .