How can I call default(T) with a type?

Question

In c# I can use default(T) to get the default value of a type. I need to get the default type at run time from a System.Type. How can I do this?

E.g. Something along the lines of this (which doesn't work)

var type = typeof(int);
var defaultValue = default(type);

Answer 1

If you're trying to build an expression tree, use Expression.Default:

Expression expression = Expression.Default(type);

One other way you could do this fairly easily would be:

object defaultValue = Array.CreateInstance(type, 1).GetValue(0);

Ugly, but it'll work :) Note that it's hard to get the default value of a nullable type, as it will always be boxed to a null reference.

Answer 2

That's pretty easy, you just have 2 cases to consider:

• reference types: the default value is always null

• value types: you can easily create an instance using Activator.CreateInstance

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

You could also consider using FormatterServices.GetUninitializedObject instead of Activator.CreateInstance, it's probably faster.

Answer 3

For a reference type return null, for a value type, you could try using Activator.CreateInstance or calling the default constructor of the type.

public static object Default(Type type)
{
   if(type.IsValueType)
   {
      return Activator.CreateInstance(type);
   }

   return null;
}