.NET-获取反映的PropertyInfo的默认值

我相信如果你这样做

prop.SetValue(obj,null,null);

如果是值类型,则将其设置为默认值;如果是引用类型,则设置为null。

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

“null”技巧会将其设置为 零 类型的值,该值不一定与属性的默认值相同。首先,如果它是一个新对象,为什么不把它放在一边呢?或者,使用 TypeDescriptor :

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

这既尊重 [DefaultValue] 以及 Reset{name}() 模式(如绑定和序列化所使用的),使其非常通用和可重用。

如果你做了很多这样的事情,你也可以通过使用 类型描述符 通过重复使用 PropertyDescriptorCollection 并使用 HyperDescriptor (代码相同,但 很 比反射或原始更快 类型描述符 ).

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)版本对于C#来说当然是多余的,因为您可以只使用default(T)关键字。

享受!

我知道这是一篇老文章,但我喜欢Darin Dimitrov的回答。它首先检查是否有任何DefaultValue属性,然后使用 Darin Dimitrov's answer :

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;
    }

这是一个更精致的版本,可以维护。NET运行时的功能,而无需添加任何不必要的自定义代码。

注意:此代码是为编写的。净3.5 SP1

namespace System {

public static class TypedDefaultExtensions {

    public static object ToDefault(this Type targetType) {

        if (targetType == null)
            throw new NullReferenceException();

        var mi = typeof(TypedDefaultExtensions)
            .GetMethod("_ToDefaultHelper", Reflection.BindingFlags.Static | Reflection.BindingFlags.NonPublic);

        var generic = mi.MakeGenericMethod(targetType);

        var returnValue = generic.Invoke(null, new object[0]);
        return returnValue;
    }

    static T _ToDefaultHelper<T>() {
        return default(T);
    }
}

}

用法:

PropertyInfo pi; // set to some property info
object defaultValue = pi.PropertyType.ToDefault();

public struct MyTypeValue { public int SomeIntProperty { get; set; }
var reflectedType = typeof(MyTypeValue);
object defaultValue2 = reflectedType.ToDefault();

拉沙德·里维拉(OmegusPrime.com)