.NET - 获取反映的PropertyInfo的默认值

Question

This is really stumping me today. I'm sure its not that hard, but I have a System.Reflection.PropertyInfo object. I want to set its value based on the result of a database lookup (think ORM, mapping a column back to a property).

My problem is if the DB returned value is DBNull, I just want to set the property value to its default, the same as calling:

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

However, the default() method won't compile if you give it a Type, which is what I have:

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);

The above doesn't compile. default(Type) is invalid. I also thought about doing:

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

However, if the Type is string, that would assign the value "new String()", but I really want "null", which is what "default(string)" would return.

So what am I missing here? I suppose a really hacky way would be to create a new instance of myObj's Type and copy the property over, but that just seems stupid...

object myObj = ???;
PropertyInfo  myPropInf = ???;
var blank = Activator.CreateInstance(myObj.GetType());
object defaultValue = myPropInf.GetValue(blank, null);
myPropInf.SetValue(myObj, defaultValue, null);

I'd rather not waste the memory to make a whole new instance, just to get the default for the property though. Seems very wasteful.

Any ideas?

Answer 1

I believe if you just do

prop.SetValue(obj,null,null);

If it's a valuetype, it'll set it to the default value, if it's a reference type, it'll set it to null.

Answer 2

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

Answer 3

The "null" trick will set it to the zero value for the type, which is not necessarily the same as the default for the property. Firstly, if it is a new object, why not just leave it alone? Alternatively, use TypeDescriptor :

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

This respects both [DefaultValue] and the Reset{name}() patterns (as used by binding and serialization), making it very versatile and re-usable.

If you are doing lots of this, you can also get a performance boost using TypeDescriptor instead of reflection, by re-using the PropertyDescriptorCollection and using HyperDescriptor (same code, but much faster than either refletion or raw TypeDescriptor ).

Answer 4

Try the following methods, which I have written and tested against thousands of types:

    /// <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");
    }

The first (generic) version of GetDefault is of course redundant for C#, since you may just use the default(T) keyword.

Enjoy!

Answer 5

This is a more polished version that maintains the .NET Runtime's functionality without adding any unnecessary custom code.

NOTE: This code written for .NET 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);
    }
}

}

USAGE:

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();

Rashad Rivera (OmegusPrime.com)

Answer 6

I know this is an old post, but I like this twist on Darin Dimitrov's answer. It first checks for any DefualtValue attributes then uses 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;
        return propertyType.IsValueType ? Activator.CreateInstance(propertyType) : null;
    }