Re: Creating you own Nullable [modified] - C# Discussion Boards

Creating you own Nullable<t></t>

9-Mar-09 1:39

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look here http://www.codeproject.com/KB/web-image/EasyThumbs.aspx[^]

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Paw Jershauge9-Mar-09 0:25

9-Mar-09 0:25

Im not sure if this can be done.

But i want to create my on Nullable<t> struct.
my problem is that i cant assign null to my struct as it is an non-nullable type, how can i overcome this ?

Heres my attemt:

[Serializable, StructLayout(LayoutKind.Sequential)]
public struct Null<T> where T : struct
{
    private bool hasValue;
    private bool allowNull;
    internal T value;
    public Null(T value)
    {
        this.value = value;
        this.hasValue = true;
        this.allowNull = true;
    }

    public bool HasValue
    {
        get
        {
            return this.hasValue;
        }
    }
    public bool AllowNull
    {
        get
        {
            return this.allowNull;
        }
    }
    public T Value
    {
        get
        {
            if (!this.HasValue)
            {
                throw new InvalidOperationException("No Value!");
            }
            return this.value;
        }
    }
    public T GetValueOrDefault()
    {
        return this.value;
    }

    public T GetValueOrDefault(T defaultValue)
    {
        if (!this.HasValue)
        {
            return defaultValue;
        }
        return this.value;
    }

    public override bool Equals(object other)
    {
        if (!this.HasValue)
        {
            return (other == null);
        }
        if (other == null)
        {
            return false;
        }
        return this.value.Equals(other);
    }

    public override int GetHashCode()
    {
        if (!this.HasValue)
        {
            return 0;
        }
        return this.value.GetHashCode();
    }

    public override string ToString()
    {
        if (!this.HasValue)
        {
            return "";
        }
        return this.value.ToString();
    }

    public static implicit operator Null<T>(T value)
    {
        return new Null<T>(value);
    }

    public static explicit operator T(Null<T> value)
    {
        return value.Value;
    }
}

This is all fine but when i then try to use it i get an non-nullable type exception:

Example:
private Null<int> index = null;

the null; assignment returns the following error:
Error 4 Cannot convert null to 'Null<int>' because it is a non-nullable value type

Can this be done,if so HELP please, or am i on an wrong track here ?

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

ABitSmart9-Mar-09 0:39

ABitSmart

9-Mar-09 0:39

You can have a Nullable structure.

struct Dummy{
    public string dummyvalue;
}

Dummy? dummyStrct = null; //This is possible

Paw Jershauge9-Mar-09 1:19

9-Mar-09 1:19

Well this is exactly what im trying to avoid. i know that ? equals the Nullable<t> but what i want to do here is, create another Nullable<t> structure, where i can put my own members into. please look at my samlpe again, and see what im trying to accomplish. Wink | ;)

Thanks for you awnser. Wink | ;)

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

Re: Creating you own Nullable [modified]

ABitSmart9-Mar-09 1:29

ABitSmart

9-Mar-09 1:29

You have been very polite Rose | [Rose]

harold aptroot9-Mar-09 0:51

9-Mar-09 0:51

Maybe if you make an implicit cast operator from object to Null<T>? The object could be null, obviously, so if you tried to assign null to it it should think the null is an object and call your implicit cast operator.

Last modified: 29mins after originally posted --

Paw Jershauge9-Mar-09 1:26

9-Mar-09 1:26

Hmm maybe your on to something here... So far i accomplished the code by making the structure as an class instead, but in reality, i want it to be an struct.

I tried another approach by casting the implicit operator as an null or trypeof(null), but we all know thats gonna happen. Wink | ;)

so im thinking: lets just convert this struct into an class which can be null. Wink | ;)

I hate the fact, that Microsoft can acoomplish this, by making the Nullable<t> structure, but we as developers cannot... grrrrrrrrr D'Oh! | :doh:

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

harold aptroot9-Mar-09 1:32

9-Mar-09 1:32

They cheat by making the compiler support it, which we can't do Frown | :(

Anyhow did an implicit cast from an object work?

Paw Jershauge9-Mar-09 1:36

9-Mar-09 1:36

Yeah your right... they have the advantage here Wink | ;)

Yes the casting on an object works bacause the object in its nature can be null.

So heres the code which works just fine (BUT ITS AN CLASS... grrrrrrrr, i wanted an struct. Wink | ;)

)

[Serializable, StructLayout(LayoutKind.Sequential)]
public class Null<T> where T : struct
{
    private bool hasValue;
    private bool allowNull;
    internal T value;
    public Null(T value)
    {
        this.value = value;
        this.hasValue = true;
        this.allowNull = true;
    }
    public Null(T value,bool allownull)
    {
        this.value = value;
        this.hasValue = true;
        this.allowNull = allownull;
    }
    public Null(bool allownull)
    {
        this.hasValue = false;
        this.allowNull = allownull;
    }
    public Null()
    {
        this.hasValue = false;
        this.allowNull = false;
    }

    public bool HasValue
    {
        get
        {
            return this.hasValue;
        }
    }
    public bool AllowNull
    {
        get
        {
            return this.allowNull;
        }
    }
    public T Value
    {
        get
        {
            if (!this.HasValue)
            {
                throw new InvalidOperationException("No Value!");
            }
            return this.value;
        }
    }
    public T GetValueOrDefault()
    {
        return this.value;
    }

    public T GetValueOrDefault(T defaultValue)
    {
        if (!this.HasValue)
        {
            return defaultValue;
        }
        return this.value;
    }

    public override bool Equals(object other)
    {
        if (!this.HasValue)
        {
            return (other == null);
        }
        if (other == null)
        {
            return false;
        }
        return this.value.Equals(other);
    }

    public override int GetHashCode()
    {
        if (!this.HasValue)
        {
            return 0;
        }
        return this.value.GetHashCode();
    }

    public override string ToString()
    {
        if (!this.HasValue)
        {
            return "";
        }
        return this.value.ToString();
    }

    public static implicit operator Null<T>(T value)
    {
        return new Null<T>(value);
    }

    public static explicit operator T(Null<T> value)
    {
        return value.Value;
    }
}

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

harold aptroot9-Mar-09 1:41

9-Mar-09 1:41

Ok well the idea was that make a

public static implicit operator Null<T>(object value)
{
    if (value is T)
        return new Null<T>(value);
    else if (value == null)
        return new Null<T>(null, true);
    else
        throw new Exception("this is bad, but find a better exception");
}

And then Null<t> can be a struct, I think. Try first..

Paw Jershauge9-Mar-09 1:50

9-Mar-09 1:50

Ahhh sorry didnt see that one, no this is not allowed, as it tries to convert from a base class which is not allowed.

Following error returned:

Null<T>.implicit operator Null<T>(object)': user-defined conversions to or from a base class are not allowed

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

harold aptroot9-Mar-09 2:14

9-Mar-09 2:14

Ok, didn't think of that, than use something else that is not a base type - say string, or some little holder class that does nothing except have the ability to be null.
Does it even say that when Null<T> is a struct?

Paw Jershauge9-Mar-09 2:55

9-Mar-09 2:55

Yes it does tell me that the Null is as struct.
And i think i found a work around.

oddly enought this seems to work:

public static implicit operator Null<T>(T? value)
{
    return new Null<T>(value.Value);
}

public static explicit operator T?(Null<T> value)
{
    return value.Value;
}

but at what cost this works, i havnt seen yet.

I will be trying with it for now. or go back to my struct as an class. Wink | ;)

Thanks alot for tackling this usue with me, Harold.

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

harold aptroot9-Mar-09 2:59

9-Mar-09 2:59

Ok good, you're welcome Smile | :)

Paw Jershauge9-Mar-09 3:38

9-Mar-09 3:38

Hmm Harold, Blush | :O

normaly this would be the time, where i give beer. D'Oh! | :doh:

Mannnnnnn i somehow got so tangled up in this, that i somwehow swaped the
explicit and the implicit... man ohhh man ohhh man.... sooooooo sorry

this works just fint in the struct:

public static explicit operator Null<T>(T value)
{
    return new Null<T>(value);
}

public static implicit operator T(Null<T> value)
{
    return value.Value;
}

as where i before used (which of cause did not work.):

public static implicit operator Null<T>(T value)
{
    return new Null<T>(value);
}

public static explicit operator T(Null<T> value)
{
    return value.Value;
}

With great code, comes great complexity, so keep it simple stupid... $Shucks | :-\$ $Shucks | :-\$

DaveyM699-Mar-09 2:22

DaveyM69

9-Mar-09 2:22

Can't you use
private Null<int?> index = null;
or
private Null<int>? index = null;

Dave
BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

Paw Jershauge9-Mar-09 2:57