blocks|key|1660018|text|object+defaultValue+=+type.IsValueType+?+Activator.CreateInstance(type)+:+null;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1660019|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>object defaultValue = type.IsValueType ? Activator.CreateInstance(type) : null;
</code></pre>

blocks|key|1660039|text|我相信只要你这么做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1660040|prop.SetValue(obj,null,null);|code-block|syntax|javascript|1660041|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。|1660042|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

I believe if you just do 

<pre><code>prop.SetValue(obj,null,null);
</code></pre>

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.

blocks|key|1660080|text|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：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1660081|++++///+<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+%7C%7C+!type.IsValueType+%7C%7C+type+==+typeof(void))
++++++++++++return+null;

++++++++//+If+the+supplied+Type+has+generic+parameters,+its+default+value+cannot+be+determined
++++++++if+(type.ContainsGenericParameters)
++++++++++++throw+new+ArgumentException(
++++++++++++++++"{"+%2B+MethodInfo.GetCurrentMethod()+%2B+"}+Error:\n\nThe+supplied+value+type+<"+%2B+type+%2B
++++++++++++++++">+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+%7C%7C+!type.IsNotPublic)
++++++++{
++++++++++++try
++++++++++++{
++++++++++++++++return+Activator.CreateInstance(type);
++++++++++++}
++++++++++++catch+(Exception+e)
++++++++++++{
++++++++++++++++throw+new+ArgumentException(
++++++++++++++++++++"{"+%2B+MethodInfo.GetCurrentMethod()+%2B+"}+Error:\n\nThe+Activator.CreateInstance+method+could+not+"+%2B
++++++++++++++++++++"create+a+default+instance+of+the+supplied+value+type+<"+%2B+type+%2B
++++++++++++++++++++">+(Inner+Exception+message:+\""+%2B+e.Message+%2B+"\")",+e);
++++++++++++}
++++++++}

++++++++//+Fail+with+exception
++++++++throw+new+ArgumentException("{"+%2B+MethodInfo.GetCurrentMethod()+%2B+"}+Error:\n\nThe+supplied+value+type+<"+%2B+type+%2B+
++++++++++++">+is+not+a+publicly-visible+type,+so+the+default+value+cannot+be+retrieved");
++++}|code-block|syntax|javascript|1660082|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。|1660083|享受吧！|1660084|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

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

<pre><code> /// &lt;summary&gt;
 /// [ &lt;c&gt;public static T GetDefault&amp;lt; T &amp;gt;()&lt;/c&gt; ]
 /// &lt;para&gt;&lt;/para&gt;
 /// Retrieves the default value for a given Type
 /// &lt;/summary&gt;
 /// &lt;typeparam name="T"&gt;The Type for which to get the default value&lt;/typeparam&gt;
 /// &lt;returns&gt;The default value for Type T&lt;/returns&gt;
 /// &lt;remarks&gt;
 /// 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.
 /// &lt;/remarks&gt;
 /// &lt;seealso cref="GetDefault(Type)"/&gt;
 public static T GetDefault&lt;T&gt;()
 {
 return (T) GetDefault(typeof(T));
 }

 /// &lt;summary&gt;
 /// [ &lt;c&gt;public static object GetDefault(Type type)&lt;/c&gt; ]
 /// &lt;para&gt;&lt;/para&gt;
 /// Retrieves the default value for a given Type
 /// &lt;/summary&gt;
 /// &lt;param name="type"&gt;The Type for which to get the default value&lt;/param&gt;
 /// &lt;returns&gt;The default value for &lt;paramref name="type"/&gt;&lt;/returns&gt;
 /// &lt;remarks&gt;
 /// 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.
 /// &lt;/remarks&gt;
 /// &lt;seealso cref="GetDefault&amp;lt;T&amp;gt;"/&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 &lt;" + type +
 "&gt; 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 &lt;" + type +
 "&gt; (Inner Exception message: \"" + e.Message + "\")", e);
 }
 }

 // Fail with exception
 throw new ArgumentException("{" + MethodInfo.GetCurrentMethod() + "} Error:\n\nThe supplied value type &lt;" + type + 
 "&gt; is not a publicly-visible type, so the default value cannot be retrieved");
 }
</code></pre>

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

Enjoy!

blocks|key|1445347|text|这是一个更精致的版本，在不添加任何不必要的自定义代码的情况下保持了.NET运行时的功能。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1445348|注意:此代码是为.NET+3.5+SP1编写的|1445349|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+%7C+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);
++++}
}|code-block|syntax|javascript|1445350|}|1445351|用法：|1445352|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();|1445353|拉沙德·里维拉(OmegusPrime.com)|1445354|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|X|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

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 

<pre><code>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&lt;T&gt;() {
 return default(T);
 }
}
</code></pre>

}

USAGE:

<pre><code>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();
</code></pre>

Rashad Rivera (OmegusPrime.com)

blocks|key|1660160|text|我知道这是一个古老的帖子，但我喜欢对Darin+Dimitrov的回答的这个曲解。它首先检查所有DefualtValue属性，然后使用Darin+Dimitrov's+answer|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1660161|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;
++++}|code-block|syntax|javascript|1660162|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/407345/7446265^0|1V|N|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

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 <a href="https://stackoverflow.com/a/407345/7446265" >Darin Dimitrov's answer</a>:

<pre><code>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;
 }
</code></pre>

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:

<pre><code>value = default(T); // where T is the type of the property.
</code></pre>

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

<pre><code>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);
</code></pre>

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

<pre><code>object myObj = ???;
PropertyInfo myPropInf = ???;
myPropInf.SetValue(myObj, Activator.CreateInstance(myPropInf.PropertyType), null);
</code></pre>

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...

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

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?

.NET - Get default value for a reflected PropertyInfo

数据库

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);  // w...

问.NET -获取反射PropertyInfo的默认值
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