在我自己的系统中解决了这个问题,下面是一个在运行时正确确定任意类型的默认值的方法,该方法已经针对数千种类型进行了测试:
/// <summary>
/// [ <c>public static object GetDefault(this Type type)</c> ]
/// <para></para>
/// Retrieves the default value for a given Type
/// </summary>
/// <param name="type">The Type for which to get the default value</param>
/// <returns>The default value for <paramref name="type"/></returns>
/// <remarks>
/// If a null Type, a reference Type, or a System.Void Type is supplied, this method always returns null. If a value type
/// is supplied which is not publicly visible or which contains generic parameters, this method will fail with an
/// exception.
/// </remarks>
/// <example>
/// To use this method in its native, non-extension form, make a call like:
/// <code>
/// object Default = DefaultValue.GetDefault(someType);
/// </code>
/// To use this method in its Type-extension form, make a call like:
/// <code>
/// object Default = someType.GetDefault();
/// </code>
/// </example>
/// <seealso cref="GetDefault<T>"/>
public static object GetDefault(this Type type)
{
// If no Type was supplied, if the Type was a reference type, or if the Type was a System.Void, return null
if (type == null || !type.IsValueType || type == typeof(void))
return null;
// If the supplied Type has generic parameters, its default value cannot be determined
if (type.ContainsGenericParameters)
throw new ArgumentException(
"{" + MethodInfo.GetCurrentMethod() + "} Error:\n\nThe supplied value type <" + type +
"> contains generic parameters, so the default value cannot be retrieved");
// If the Type is a primitive type, or if it is another publicly-visible value type (i.e. struct/enum), return a
// default instance of the value type
if (type.IsPrimitive || !type.IsNotPublic)
{
try
{
return Activator.CreateInstance(type);
}
catch (Exception e)
{
throw new ArgumentException(
"{" + MethodInfo.GetCurrentMethod() + "} Error:\n\nThe Activator.CreateInstance method could not " +
"create a default instance of the supplied value type <" + type +
"> (Inner Exception message: \"" + e.Message + "\")", e);
}
}
// Fail with exception
throw new ArgumentException("{" + MethodInfo.GetCurrentMethod() + "} Error:\n\nThe supplied value type <" + type +
"> is not a publicly-visible type, so the default value cannot be retrieved");
}
在这些示例中,getdefault方法在静态类defaultvalue中实现。使用如下语句调用此方法:
object Default = DefaultValue.GetDefault(someType);
要将getDefault方法用作类型的扩展方法,请按如下方式调用它:
object Default = someType.GetDefault();
第二,类型扩展方法是一种更简单的客户机代码语法,因为它消除了在调用中引用包含defaultvalue类限定符的需要。
上述getdefault的运行时形式与原语c“default”关键字具有相同的语义,并产生相同的结果。
要使用getdefault的通用形式,可以访问以下函数:
/// <summary>
/// [ <c>public static T GetDefault< T >()</c> ]
/// <para></para>
/// Retrieves the default value for a given Type
/// </summary>
/// <typeparam name="T">The Type for which to get the default value</typeparam>
/// <returns>The default value for Type T</returns>
/// <remarks>
/// If a reference Type or a System.Void Type is supplied, this method always returns null. If a value type
/// is supplied which is not publicly visible or which contains generic parameters, this method will fail with an
/// exception.
/// </remarks>
/// <seealso cref="GetDefault(Type)"/>
public static T GetDefault<T>()
{
return (T) GetDefault(typeof(T));
}
对泛型窗体的调用可以是如下所示:
int? inDefaultVal = DefaultValue.GetDefault<int?>();
当然,上述getdefault的通用形式对于c来说是不必要的,因为它的工作方式与default(t)相同。它只对不支持“default”关键字但支持泛型类型的.NET语言有用。在大多数情况下,一般形式是不必要的。
一个有用的推论方法是确定一个对象是否包含其类型的默认值。为此,我还依赖以下IsObjectSettodeFault方法:
/// <summary>
/// [ <c>public static bool IsObjectSetToDefault(this Type ObjectType, object ObjectValue)</c> ]
/// <para></para>
/// Reports whether a value of type T (or a null reference of type T) contains the default value for that Type
/// </summary>
/// <remarks>
/// Reports whether the object is empty or unitialized for a reference type or nullable value type (i.e. is null) or
/// whether the object contains a default value for a non-nullable value type (i.e. int = 0, bool = false, etc.)
/// <para></para>
/// NOTE: For non-nullable value types, this method introduces a boxing/unboxing performance penalty.
/// </remarks>
/// <param name="ObjectType">Type of the object to test</param>
/// <param name="ObjectValue">The object value to test, or null for a reference Type or nullable value Type</param>
/// <returns>
/// true = The object contains the default value for its Type.
/// <para></para>
/// false = The object has been changed from its default value.
/// </returns>
public static bool IsObjectSetToDefault(this Type ObjectType, object ObjectValue)
{
// If no ObjectType was supplied, attempt to determine from ObjectValue
if (ObjectType == null)
{
// If no ObjectValue was supplied, abort
if (ObjectValue == null)
{
MethodBase currmethod = MethodInfo.GetCurrentMethod();
string ExceptionMsgPrefix = currmethod.DeclaringType + " {" + currmethod + "} Error:\n\n";
throw new ArgumentNullException(ExceptionMsgPrefix + "Cannot determine the ObjectType from a null Value");
}
// Determine ObjectType from ObjectValue
ObjectType = ObjectValue.GetType();
}
// Get the default value of type ObjectType
object Default = ObjectType.GetDefault();
// If a non-null ObjectValue was supplied, compare Value with its default value and return the result
if (ObjectValue != null)
return ObjectValue.Equals(Default);
// Since a null ObjectValue was supplied, report whether its default value is null
return Default == null;
}
以上
IsObjectSetToDefault
方法可以以其本机形式调用,也可以作为类型类扩展访问。