How to add a constraint of Nullable Types to a generics method

[Joe Bloggs]

Hi,

compiling the following code:

public class App
{
static void Main()
{
int? x = 5;
bool? i = null;
Console.WriteLine(GetType(x).FullName);
Console.WriteLine(" x={0}", GetSafeValue(x));
Console.WriteLine(" i={0}", GetSafeValue(i));
Console.ReadLine();
}


public static object GetSafeValue<T, N>(T value) where T:struct
{

if (!IsNullableValueType(GetType(value)))
{
Console.WriteLine(" {0} is not a Nullable type", value.GetType());
return value;
}
object nullableValue = value;

return nullableValue ??
GetDefaultDataTypeValue((GetType(value)).GetGenericArguments()[0]);

}
}

I get the error message
"The type 'int?' must be a non-nullable value type in order to use it
as parameter 'T' in the generic type or method
'App.GetSafeValue<T>(T)'"

Obviously, I have set the constraint to only accept value types.

Now, how do I modify the constraint so that I can accept either value
types or Nullable types?

Thanks in advance.

 

[Nicholas Paldino [.NET/C# MVP]]

Joe,

Why not just get the Value on the nullable to begin with? It will
return a default value if it is "null" (meaning, it is flagged as null, it
isn't actually null).

Hope this helps.

 

[Barry Kelly]

public static object GetSafeValue<T, N>(T value) where T:struct

Yes, struct excludes nullable types.

Obviously, I have set the constraint to only accept value types.

Now, how do I modify the constraint so that I can accept either value
types or Nullable types?

You can't do it statically. You'll have to do it at runtime.

-- Barry