对于任何给定的类型,我想知道它的默认值。
在C#中,有一个名为default的关键字,用于执行此操作,如
object obj = default(Decimal);
但我有一个Type的实例(称为myType),如果我这样说,
object obj = default(myType);
它不起作用
这样做有什么好办法吗? 我知道一个巨大的开关块可以工作,但这不是一个好的选择。
答案 0 :(得分:238)
实际上只有两种可能:null
用于引用类型,new myType()
用于值类型(对应于0,对于int,float等)所以你真的只需要考虑两种情况:< / p>
object GetDefaultValue(Type t)
{
if (t.IsValueType)
return Activator.CreateInstance(t);
return null;
}
(因为值类型总是有一个默认构造函数,所以对Activator.CreateInstance的调用永远不会失败)。
答案 1 :(得分:30)
您还可以将其作为扩展方法添加到System.Type:
public static class TypeExtensions
{
public static object GetDefaultValue(this Type t)
{
if (t.IsValueType && Nullable.GetUnderlyingType(t) == null)
return Activator.CreateInstance(t);
else
return null;
}
}
答案 2 :(得分:18)
在我自己的系统中解决了这个问题之后,这里有一个方法可以在运行时正确地确定任意类型的默认值,该方法已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Type的扩展方法,请将其命名为:
object Default = someType.GetDefault();
第二种类型扩展方法是一种更简单的客户端代码语法,因为它不需要在调用时引用包含的DefaultValue类限定符。
以上运行时形式的GetDefault使用与原始C#'default'关键字相同的语义,并产生相同的结果。
要使用GetDefault的通用形式,您可以访问以下功能:
/// <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));
}
对通用表单的调用可能类似于:
int? inDefaultVal = DefaultValue.GetDefault<int?>();
当然,上述通用形式的GetDefault对于C#来说是不必要的,因为它的工作原理与默认(T)相同。它仅对不支持'default'关键字但支持泛型类型的.NET语言有用。在大多数情况下,通用表格是不必要的。
一个有用的推论方法是确定一个对象是否包含其Type的默认值。我还为此目的依赖以下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;
}
上述IsObjectSetToDefault
方法既可以以其本机形式调用,也可以作为Type-class扩展进行访问。
答案 3 :(得分:10)
怎么样......
class Program
{
static void Main(string[] args)
{
PrintDefault(typeof(object));
PrintDefault(typeof(string));
PrintDefault(typeof(int));
PrintDefault(typeof(int?));
}
private static void PrintDefault(Type type)
{
Console.WriteLine("default({0}) = {1}", type,
DefaultGenerator.GetDefaultValue(type));
}
}
public class DefaultGenerator
{
public static object GetDefaultValue(Type parameter)
{
var defaultGeneratorType =
typeof(DefaultGenerator<>).MakeGenericType(parameter);
return defaultGeneratorType.InvokeMember(
"GetDefault",
BindingFlags.Static |
BindingFlags.Public |
BindingFlags.InvokeMethod,
null, null, new object[0]);
}
}
public class DefaultGenerator<T>
{
public static T GetDefault()
{
return default(T);
}
}
它产生以下输出:
default(System.Object) =
default(System.String) =
default(System.Int32) = 0
default(System.Nullable`1[System.Int32]) =
答案 4 :(得分:2)
“默认值”是什么意思?所有引用类型(“类”)都将null作为默认值,而所有值类型将根据this table具有默认值。
答案 5 :(得分:1)
这是一个函数,它将返回可空类型的默认值(换句话说,它为Decimal
和Decimal?
返回0):
public static object DefaultValue(Type maybeNullable)
{
Type underlying = Nullable.GetUnderlyingType(maybeNullable);
if (underlying != null)
return Activator.CreateInstance(underlying);
return Activator.CreateInstance(maybeNullable);
}