.NET -获取反射PropertyInfo的默认值
.NET -获取反射PropertyInfo的默认值
提问于 2009-01-02 16:32:01
今天真的把我难倒了。我确信这并不难，但我有一个System.Reflection.PropertyInfo对象。我想根据数据库查找的结果来设置它的值(比如ORM，将一个列映射回一个属性)。
我的问题是，如果DB返回值是DBNull，我只想将属性值设置为其默认值，这与调用：
value = default(T); // where T is the type of the property.然而，如果你给default()方法一个Type，它将不会被编译，这就是我所拥有的：
object myObj = ???; // doesn't really matter. some arbitrary class.
PropertyInfo myPropInf = ???; // the reflection data for a property on the myObj object.
myPropInf.SetValue(myObj, default(myPropInf.PropertyType), null);上面的代码不能编译。默认(类型)无效。我也考虑过这样做：
object myObj = ???;
PropertyInfo myPropInf = ???;
myPropInf.SetValue(myObj, Activator.CreateInstance(myPropInf.PropertyType), null);但是，如果Type是string，则会赋值为"new String()"，但我确实想要"null"，这是"default(string)“将返回的值。
那么我在这里错过了什么呢？我认为一种非常老套的方法是创建myObj的Type的新实例并复制属性，但这看起来很愚蠢……
object myObj = ???;
PropertyInfo myPropInf = ???;
var blank = Activator.CreateInstance(myObj.GetType());
object defaultValue = myPropInf.GetValue(blank, null);
myPropInf.SetValue(myObj, defaultValue, null);我不想浪费内存来创建一个全新的实例，只是为了获取属性的默认值。看起来很浪费。
有什么想法吗？
回答 5
Stack Overflow用户
回答已采纳
发布于 2009-01-02 16:43:13
我相信只要你这么做
prop.SetValue(obj,null,null);如果它是一个值类型，它会将它设置为默认值，如果它是一个引用类型，它会将它设置为null。
Stack Overflow用户
发布于 2009-01-02 16:37:31
object defaultValue = type.IsValueType ? Activator.CreateInstance(type) : null;Stack Overflow用户
发布于 2011-10-24 05:42:37
尝试以下方法，我已经针对数千种类型编写并测试了这些方法：
/// <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));
}
/// <summary>
/// [ <c>public static object GetDefault(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>
/// <seealso cref="GetDefault<T>"/>
public static object GetDefault(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), 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");
}对于C#来说，GetDefault的第一个(通用)版本当然是多余的，因为您可以只使用default(T)关键字。
享受吧！
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原文链接：
https://stackoverflow.com/questions/407337
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