Default value of a type

Question

For any given type i want to know its default value.

In C#, there is a keyword called default for doing this like

object obj = default(Decimal);

but I have an instance of Type (called myType) and if I say this,

object obj = default(myType);

it doesn't work

Is there any good way of doing this? I know that a huge switch block will work but thats not a good choice.

Answer 1

How about something like...

class Program
{
  static void Main(string[] args)
  {
    PrintDefault(typeof(object));
    PrintDefault(typeof(string));
    PrintDefault(typeof(int));
    PrintDefault(typeof(int?));
  }

  private static void PrintDefault(Type type)
  {
    Console.WriteLine("default({0}) = {1}", type,
      DefaultGenerator.GetDefaultValue(type));
  }
}

public class DefaultGenerator
{
  public static object GetDefaultValue(Type parameter)
  {
    var defaultGeneratorType =
      typeof(DefaultGenerator<>).MakeGenericType(parameter);

    return defaultGeneratorType.InvokeMember(
      "GetDefault", 
      BindingFlags.Static |
      BindingFlags.Public |
      BindingFlags.InvokeMethod,
      null, null, new object[0]);
  }
}

public class DefaultGenerator<T>
{
  public static T GetDefault()
  {
    return default(T);
  }
}

It produces the following output:

default(System.Object) =
default(System.String) =
default(System.Int32) = 0
default(System.Nullable`1[System.Int32]) =

Answer 2

There's really only two possibilities: null for reference types and new myType() for value types (which corresponds to 0 for int, float, etc) So you really only need to account for two cases:

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

(Because value types always have a default constructor, that call to Activator.CreateInstance will never fail).

Answer 3

What do you mean by "Default Value"? All reference Types ("class") have null as default value, while all value types will have their default values according to this table.

Answer 4

Here's a function that will return the default value for a nullable type (in other words, it returns 0 for both Decimal and Decimal?):

public static object DefaultValue(Type maybeNullable)
{
    Type underlying = Nullable.GetUnderlyingType(maybeNullable);
    if (underlying != null)
        return Activator.CreateInstance(underlying);
    return Activator.CreateInstance(maybeNullable);
}

Answer 5

You could also add it as an extension method to System.Type:

public static class TypeExtensions
{
    public static object GetDefaultValue(this Type t)
    {
        if (t.IsValueType && Nullable.GetUnderlyingType(t) == null)
            return Activator.CreateInstance(t);
        else
            return null;
    }
}