how to get the default value of a type if the type is only known as System.Type? [duplicate]

问题

If I want a method that returns the default value of a given type and the method is generic I can return a default value like so:

public static T GetDefaultValue()
{
  return default(T);
}

Can I do something similar in case I have the type only as a System.Type object?

public static object GetDefaultValue(Type type)
{
  //???
}

回答1:

Since you really only have to worry about value types (reference types will just be null), you can use Activator.CreateInstance to call the default constructor on them.

public static object GetDefaultValue(Type type) {
   return type.IsValueType ? Activator.CreateInstance(type) : null;
}

Edit: Jon is (of course) correct. IsClass isn't exhaustive enough - it returns False if type is an interface.

回答2:

Here is how I normally do it. This avoids the whole 'IsValueType' or searching for constructors issues altogether.

public static object MakeDefault(this Type type)
{
    var makeDefault = typeof(ExtReflection).GetMethod("MakeDefaultGeneric");
    var typed = makeDefault.MakeGenericMethod(type);
    return typed.Invoke(null, new object[] { });
}

public static T MakeDefaultGeneric<T>()
{
    return default(T);
}

回答3:

Without a generic, you can't guarantee that the type has a parameterless constructor, but you can search for one using reflection:

public static object GetDefaultValue(Type type)
{
    ConstructorInfo ci = type.GetConstructor( new Type[] {} );
    return ci.Invoke( new object[] {} );
}

I tried this in a console app, and it returns a "default" instance of the class — assuming it's a class. If you need it to work for reference types as well, you'll need an additional technique.

来源：https://stackoverflow.com/questions/1281161/how-to-get-the-default-value-of-a-type-if-the-type-is-only-known-as-system-type

标签

generics

default