How do you assign "null" to a Delphi.net Nullable

Question

How do you assign null to a Delphi.net Nullable? I have a field which previously contained a value , I need to clear its value back to null.

depth : Nullable<Double>;

Procedure ClearDepth;
begin 
    depth := Nil;
end;

The above creates the error Error: E2010 Incompatible types : 'Nullable<System.Double>' and 'Pointer'. using Null in place of Nil gives the same error with Variant in place of Pointer. I wondered if I should be using a special constructor to generate a null nullable but I couldn't see one in the documentation?

The following works but doesn't seem like a great solution, can anyone suggest a better way?

Procedure ClearDepth;
var
    uninitialisedValue : Nullable<Double>;
begin 
    depth := uninitialisedValue;
end;

@springy76 This looks like Delphi.net rather than Prism because Prism has nullable baked into the language. — David Heffernan, Oct 18 '11 at 14:47

David Heffernan · Answer 1 · 2011-10-18T14:57:41.437

3

You need this syntax:

depth := Default(Nullable<Double>);

edited Oct 18 '11 at 14:57

answered Oct 18 '11 at 14:46

David Heffernan

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perfect, thanks. Should've just tried it to see what happened! – Tom Carver Oct 18 '11 at 14:49
@Thom Out of interest, do both of these versions work? I have to confess to never having used delphi.net! – David Heffernan Oct 18 '11 at 14:51
The first works but not the second (`DEFAULT standard function expects a type identifier`) – Tom Carver Oct 18 '11 at 14:57
@Thom Thanks. Answer updated. Looks pretty verbose when compared to Prism or C#. – David Heffernan Oct 18 '11 at 14:58
Ugh I spoke too soon. That line compiles, but is apparently ignored at runtime. When will I learn, compiling != correct... – Tom Carver Oct 19 '11 at 09:18

svick · Answer 2 · 2011-10-18T15:01:47.103

1

The type Nullable<T> is a struct. That means it has a public parameterless constructor. In the case of this type, that's the value that represents null. In C#, you would use:

new Nullable<Double>()

Another way to get the same value in C# would be

default(Nullable<Double>)

In both cases, I don't know the syntax for Delphi. In the second case, I don't know whether something like this can be even represented there.

EDIT: Apparently, you can use the second version in Delphi, but not the equivalent of the first one. That's quite surprising to me.

edited Oct 18 '11 at 15:01

answered Oct 18 '11 at 14:21

svick

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Unfortunately I can't see how to do either in Delphi. `Nullable.Create(1.0)` (Delphi constructor syntax) works fine, but `Nullable.Create()` gives an error about no overload with that number of parameters, suggesting the default constructor isn't exposed :( – Tom Carver Oct 18 '11 at 14:46
update - sorry I was wrong, even Default(T) doesn't work in Delphi (it compiles, but is completely ignored at runtime, leaving depth with its previous value). – Tom Carver Oct 19 '11 at 09:26

How do you assign "null" to a Delphi.net Nullable

2 Answers2