为了在通用类型或方法中将其用作参数“ T”，类型“字符串”必须为非空值类型

Question

I followed the suggestion around this web site to made the following method:

public static T? GetElementValue<T>(this XElement xElement, string s) 
    where T : struct, IComparable
{
     var result = new Nullable<T>();

     try
     {
         if (string.IsNullOrEmpty(s) || s.Trim().Length > 0 || xElement.IsEmpty)
            return result;

         var element = xElement.Element(s);

         if (element == null || element.IsEmpty)
             return result;

         var conv = TypeDescriptor.GetConverter(typeof(T));
         result = (T)conv.ConvertFrom(element.Value);
     }
     catch (Exception ex)
     {
          ex.Message.WriteAsError();
     }

     return result;
}

but when I try to use it with a string I still get the following error: The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method

Someone can tell me why? Thx

Answer 1

You have restricted your T parameter by

where T : struct

The struct restriction means that only value types can be supplied for T . string (more precisely, System.String ) however, is a class, a reference type.

Removing that restriction will not be quite trivial, as you use T as T? , ie a nullable T . Nullable<T> in turn only accepts value types.

You will have to decide on whether you want to allow reference types - if you want to restrict T to reference types, use the class constraint instead of struct (and drop the ? from T? , as reference types are always nullable on their own). If you want to allow both value and reference types, you cannot make a nullable version of T . If you need all of these, please extend your description of the circumstances so we can help you find a better solution.

Answer 2

因为您将T限制为值类型- where T : struct, IComparable ，而string是引用类型。

Answer 3

a string is not a value type, it is an immutable reference type so it violates the generic constraint you have where T must be a struct .

Take a look at the string type documentation for more details about string .