运行时类型的默认值

小开

比如说..。

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);
}
}

它的产出如下:

default(System.Object) =
default(System.String) =
default(System.Int32) = 0
default(System.Nullable`1[System.Int32]) =

小开

最佳答案

实际上只有两种可能性: null用于引用类型，new myType()用于值类型(对应于 int、 float 等等的0) ，因此您实际上只需要考虑两种情况:

object GetDefaultValue(Type t)
{
if (t.IsValueType)
return Activator.CreateInstance(t);


return null;
}

(因为值类型总是有一个缺省构造函数，所以对 Activator.createInstance 的调用永远不会失败)。

小开

你说的“默认值”是什么意思？所有引用类型(“ class”)都将 null 作为默认值，而所有值类型都将根据这张桌子具有其默认值。

小开

下面的函数将返回可空类型的默认值(换句话说，对于 Decimal和 Decimal?，它都返回0) :

public static object DefaultValue(Type maybeNullable)
{
Type underlying = Nullable.GetUnderlyingType(maybeNullable);
if (underlying != null)
return Activator.CreateInstance(underlying);
return Activator.CreateInstance(maybeNullable);
}

小开

也可以将其作为扩展方法添加到 System。类型:

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;
}
}

小开

在我自己的系统中已经解决了这个问题，下面是一个在运行时正确确定任意 Type 的默认值的方法，该方法已经在数千种 Type 上进行了测试:

    /// <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&lt;T&gt;"/>
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);

要使用 GetDefault 方法作为 Type 的扩展方法，可以这样调用它:

        object Default = someType.GetDefault();

第二种类型扩展方法是一种更简单的客户机代码语法，因为它不需要在调用中引用包含 DefaultValue 类限定符。

以上 GetDefault 的运行时形式与原始 C # ‘ default’关键字具有相同的语义，并且产生相同的结果。

若要使用 GetDefault 的通用形式，可以访问以下函数:

    /// <summary>
/// [ <c>public static T GetDefault&lt; T &gt;()</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));
}

对通用形式的调用可以类似于:

        int? inDefaultVal = DefaultValue.GetDefault<int?>();

当然，上面的 GetDefault 通用形式对 C # 来说是不必要的，因为它的工作原理与 default (T)相同。它只对。NET 语言，不支持“ default”关键字，但支持泛型类型。在大多数情况下，泛型形式是不必要的。

一个有用的推论方法是确定对象是否包含其 Type 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;
}

上述 IsObjectSetToDefault方法可以以其本机形式调用，也可以作为 Type 类扩展访问。