运行时类型的默认值 [重复]

Question

对于任何给定的类型，我想知道它的默认值。

在 C# 中，有一个叫做 default 的关键字可以这样做

object obj = default(Decimal);

但我有一个 Type 实例（称为 myType），如果我这样说，

object obj = default(myType);

没用

有什么好的方法吗？ 我知道一个巨大的开关块会起作用，但这不是一个好的选择。

Answer 1

诸如...之类的东西怎么样？

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]) =

Answer 2

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 等的 0）所以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默认构造函数，所以对 Activator.CreateInstance 的调用永远不会失败）。

Answer 3

“默认值”是什么意思？所有引用类型（“类”）都将 null 作为默认值，而所有值类型都将根据this table 拥有其默认值。

Answer 4

这是一个返回可空类型的默认值的函数（换句话说，它对Decimal 和Decimal? 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);
}

Answer 5

您也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;
    }
}

Answer 6

在我自己的系统中解决了这个问题，下面是一种在运行时正确确定任意类型默认值的方法，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&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);

要将 GetDefault 方法用作 Type 的扩展方法，请像这样调用它：

        object Default = someType.GetDefault();

第二种类型扩展方法是一种更简单的客户端代码语法，因为它消除了在调用中引用包含 DefaultValue 类限定符的需要。

GetDefault 的上述运行时形式与原始 C# 'default' 关键字使用相同的语义，并产生相同的结果。

要使用通用形式的 GetDefault，您可以访问以下函数：

    /// <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，因为它的工作方式与 default(T) 相同。它仅对不支持“默认”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;
    }

上述IsObjectSetToDefault 方法既可以以其本机形式调用，也可以作为类型类扩展访问。