.NET - 获取反映的PropertyInfo的默认值

Question

今天这真的让我很难过.我确定它不是那么难,但我有一个System.Reflection.PropertyInfo对象.我想根据数据库查找的结果设置其值(想想ORM,将列映射回属性).

我的问题是如果DB返回的值是DBNull,我只想将属性值设置为默认值,与调用相同:

value = default(T);  // where T is the type of the property.

但是,如果你给它一个Type,那么default()方法将无法编译,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);

以上不编译.默认(类型)无效.我还想过:

object myObj = ???;
PropertyInfo myPropInf = ???;
myPropInf.SetValue(myObj, Activator.CreateInstance(myPropInf.PropertyType), null);

但是,如果Type是字符串,那将赋值"new String()",但我真的想要"null",这就是"default(string)"将返回的内容.

那我在这里错过了什么？我想一个非常hacky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);

我宁愿不浪费内存来创建一个全新的实例,只是为了获得属性的默认值.似乎非常浪费.

有任何想法吗？

Answer 1

我相信如果你这样做

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.

Answer 2

object defaultValue = type.IsValueType ? Activator.CreateInstance(type) : null;

Answer 3

"null"技巧会将其设置为类型的零值,该值不一定与属性的默认值相同.首先,如果它是一个新的对象,为什么不单独留下它？或者,使用TypeDescriptor:

PropertyDescriptor prop = TypeDescriptor.GetProperties(foo)["Bar"];
if (prop.CanResetValue(foo)) prop.ResetValue(foo);

这既尊重[DefaultValue]和Reset{name}() 模式(由绑定和序列化使用),使其非常通用和可重复使用.

如果你正在做大量的这个,你也可以用获得的性能提升TypeDescriptor,而不是反射,通过-重新使用的PropertyDescriptorCollection,并使用HyperDescriptor(相同的代码,但很多比任何refletion或原始更快TypeDescriptor).

Answer 4

尝试以下方法,我已经编写并测试了数千种类型:

    /// <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));
    }

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

GetDefault的第一个(通用)版本当然是C#的冗余版本,因为您可能只使用默认(T)关键字.

请享用!

Answer 5

我知道这是一篇旧帖子，但我喜欢达林·季米特洛夫答案的这种转变。它首先检查任何 DefaultValue 属性，然后使用Darin Dimitrov 的答案：

public static object GetDefaultValueForProperty(this PropertyInfo property)
    {
        var defaultAttr = property.GetCustomAttribute(typeof(DefaultValueAttribute));
        if (defaultAttr != null)
            return (defaultAttr as DefaultValueAttribute).Value;

        var propertyType = property.PropertyType;
        propertyType = Nullable.GetUnderlyingType(propertyType) ?? propertyType;
        return propertyType.IsValueType ? Activator.CreateInstance(propertyType) : null;
    }