从互联网上的各种来源中,我总结出了以下函数:
public static Nullable<T> TryParseNullable<T>(this Nullable<T> t, string input) where T : struct
{
if (string.IsNullOrEmpty(input))
return default(T);
Nullable<T> result = new Nullable<T>();
try
{
IConvertible convertibleString = (IConvertible)input;
result = new Nullable<T>((T)convertibleString.ToType(typeof(T), CultureInfo.CurrentCulture));
}
catch (InvalidCastException) { }
catch (FormatException) { }
return result;
}
我已将其制作为扩展方法,如果我直接调用它,它就可以正常工作:
int? input = new int?().TryParseNullable("12345");
我的问题出现在我试图从另一个通用函数的上下文中使用反射调用它时。SO充满了描述如何获取通用方法和静态方法的MethodInfo的答案,但我似乎无法正确地将它们放在一起。
我已经正确确定传递的泛型类型本身是一个泛型类型(
Nullable<>
),现在我想使用反射调用Nullable<>
上的TryParseNullable
扩展方法:public static T GetValue<T>(string name, T defaultValue)
{
string result = getSomeStringValue(name);
if (string.IsNullOrEmpty(result)) return defaultValue;
try
{
if (typeof(T).IsGenericType && typeof(T).GetGenericTypeDefinition() == typeof(Nullable<>))
{
MethodInfo methodInfo;
//using the TryParse() of the underlying type works but isn't exactly the way i want to do it
//-------------------------------------------------------------------------------------------
NullableConverter nc = new NullableConverter(typeof(T));
Type t = nc.UnderlyingType;
methodInfo = t.GetMethod("TryParse", BindingFlags.Public | BindingFlags.Static, Type.DefaultBinder, new[] { typeof(string), t.MakeByRefType() }, null);
if (methodInfo != null)
{
var inputParameters = new object[] { result, null };
methodInfo.Invoke(null, inputParameters);
return (T) inputParameters[1];
}
//start of the problem area
//-------------------------
Type ttype = typeof(T);
//this works but is undesirable (due to reference to class containing the static method):
methodInfo = typeof(ParentExtensionsClass).GetMethod("TryParseNullable", BindingFlags.Public | BindingFlags.Static);
if (methodInfo != null)
Console.WriteLine(methodInfo);
//standard way of getting static method, doesn't work (GetMethod() returns null):
methodInfo = ttype.GetMethod("TryParseNullable", BindingFlags.Public | BindingFlags.Static);
if (methodInfo != null)
Console.WriteLine(methodInfo);
//Jon Skeet's advised method, doesn't work in this case (again GetMethod() returns null):
//(see footnote for link to this answer)
methodInfo = ttype.GetMethod("TryParseNullable");
methodInfo = methodInfo.MakeGenericMethod(ttype);
if (methodInfo != null)
Console.WriteLine(methodInfo);
//another random attempt (also doesn't work):
methodInfo = ttype.GetMethod("TryParseNullable", BindingFlags.Public | BindingFlags.Static, Type.DefaultBinder, new[] { typeof(string) }, null);
if (methodInfo != null)
Console.WriteLine(methodInfo);
}
// if we get this far, then we are not handling the type yet
throw new ArgumentException("The type " + defaultValue.GetType() + " is not yet supported by GetValue<T>.", "T");
}
catch (Exception e)
{
[snip]
}
}
能否让有经验的人帮我解决困境?
typeof(T)
返回正确的类型信息,我想也许我在 GetMethod()
调用中使用它有些不正确,或者我没有用正确的参数调用 GetMethod()
。
input == null
,为什么要将default(T)
返回为T?
?似乎更正确的逻辑应该是返回 null。 - Matthew ScharleyNullable<T>
,其中T
是任何有效类型之一,而不是Nullable<>
。Nullable<>
是它自己的类型,并表示未指定类型参数的类型。通常情况下,它只对反射有用,因为它是一个半成品类,但这就是讨论的主题。您实际上无法实例化Nullable<>
。 - Matthew Scharley