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

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);

--
Browse http://connect.microsoft.com/VisualStudio/feedback/ and vote.
http://www.peterRitchie.com/blog/
Microsoft MVP, Visual Developer - Visual C#

"Michael Bray" wrote:

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
>

....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.

--
Browse http://connect.microsoft.com/VisualStudio/feedback/ and vote.
http://www.peterRitchie.com/blog/
Microsoft MVP, Visual Developer - Visual C#

"Michael Bray" wrote:

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
>

Michael Bray <mbray@makeDIntoDot_ctiusaDcomwrote:

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));

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;
}

--
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

Peter Ritchie [C# MVP] wrote:

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.

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

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

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