運行時類型的默認值

Question

對於任何給定的類型，我想知道它的默認值。

在C＃中，有一個名爲默認這樣做的像

object obj = default(Decimal); 

關鍵字，但我有一個類型（稱爲的myType）的實例，如果我說這個，

object obj = default(myType); 

它不不工作

有沒有這樣做的好方法？ 我知道一個巨大的開關塊可以工作，但那不是一個好的選擇。

Answer 1

這確實只有兩種可能：null引用類型和new myType()值類型（對應於0代表int，float等）所以你真的只需要考慮兩種情況：

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

    return null; 
} 

（因爲值類型總是有一個默認構造函數，所以對Activator.CreateInstance的調用永遠不會失敗）。

Answer 2

如何像...

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); 
    } 
} 

它產生以下輸出：

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

Answer 3

你是什麼意思的「默認值」？所有引用類型（「類」）都具有空值作爲默認值，而所有值類型都將具有根據this table的默認值。

Answer 4

這裏是一個將返回默認值可空類型的函數（換句話說，這兩個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); 
} 

Answer 5

你也可以將其添加爲擴展方法體系.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

在自己的系統中已經解決了這個問題，這裏是在運行時，已對數以千計類型測試的正確判斷任意類型的默認值的方法：

/// <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"); 
    } 

在這些示例中，GetDefault方法在靜態類DefaultValue中實現。調用此方法與像聲明：

 object Default = DefaultValue.GetDefault(someType); 

要使用GetDefault方法作爲類型的擴展方法，這樣稱呼它：

 object Default = someType.GetDefault(); 

這第二個，類型擴展方法是一個簡單的客戶端代碼語法，因爲它不需要在調用中引用包含DefaultValue類限定符。

GetDefault的上述運行時形式的工作原理與C＃「default」關鍵字的相同語義相同，併產生相同的結果。

要使用GetDefault的通用形式，您可以訪問以下功能：

/// <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)); 
    } 

的一般形式的呼叫可以是這樣的：

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

當然，上述通用GetDefault的形式對於C＃來說是不必要的，因爲它和默認的（T）一樣。它僅適用於不支持「默認」關鍵字但支持泛型類型的.NET語言。在大多數情況下，通用形式是不必要的。

有用的推論方法是確定某個對象是否包含其Type類型的默認值的方法。我還依賴以下IsObjectSetToDefault方法用於該目的：

/// <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; 
    } 

上面IsObjectSetToDefault方法既可以在其天然形式被稱爲或作爲型類擴展訪問。