How to get a default instance of built-in type?

  • Thread starter Thread starter Michael Bray
  • Start date Start date
M

Michael Bray

Lets say I want to get the default value for a system type... how would I
implement the following function:

public object DefaultValueOfType(Type t)
{
return ????;
}

I want to be able to call it like this:

DefaultValueOfType(typeof(string));
DefaultValueOfType(typeof(int));
DefaultValueOfType(typeof(double));

I have tried:

return t.TypeInitializer.Invoke(null);

but it says "Type initializer is not callable."

-mdb
 
If you use generics you can use the default keyword...

Otherwise you can use Reflect to get the default c'tor, if it exists (i.e.
perform a check):
return t.GetConstructor(BindingFlags.Instance
| BindingFlags.Default
| BindingFlags.Public
, null
, Type.EmptyTypes
, null).Invoke(null);
 
....forgot an example for generics:
public static void GetDefaultValue<T> ( out T value )
{
value = default(T);
}
// ...
string text;
GetDefaultValue(out text);

Notice you don't have to deal with typeof at all... Keep in mind, that
won't create an instance for reference types--which will be null.
 
Michael Bray said:
Lets say I want to get the default value for a system type... how would I
implement the following function:

public object DefaultValueOfType(Type t)
{
return ????;
}

I want to be able to call it like this:

DefaultValueOfType(typeof(string));
DefaultValueOfType(typeof(int));
DefaultValueOfType(typeof(double));
Click to expand...

For reference types, the default value is always null. For value types,
the default value can be obtained by calling the parameterless
constructor which is guaranteed to exist:

static object DefaultValueOfType (Type t)
{
return t.IsValueType ? Activator.CreateInstance(t)
: null;
}
 
Peter said:
Notice you don't have to deal with typeof at all... Keep in mind, that
won't create an instance for reference types--which will be null.
Click to expand...

Maybe new was better than default.

public static T GetDefault<T>() where T : new()
{
return new T();
}

will work for reference types with a no arg constructor.

Arne
 
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