Hashtable Question

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aperrien@gmail.com - 20 Jan 2008 12:59 GMT

Hello all,
I'm learning C# from a C/C++ background, and I came across an odd
problem.

I have a hashtable that holds a string and a struct like so:

private Hashtable MyTable = new Hashtable();

public class MyData
{
    public long int x,y,number;

    public Tile( long int xx, long int yy, long int num )
    {
        x = xx;
        y = yy;
        number = num;

    }
}

MyData DataPoint;

MyData.Add[SomeString,DataPoint];

----- More code ----

foreach(DictionaryEntry DataPoint in MyTable)
{

    MyData D = (MyData)DataPoint.Value;

    D.x = 10;
}

When I try to access the members of D, I get errors. Why?
Can't I use a hashtable to store struct or class elements like a C++
map?

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Jon Skeet [C# MVP] - 20 Jan 2008 13:10 GMT

> I'm learning C# from a C/C++ background, and I came across an odd
> problem.
>
> I have a hashtable that holds a string and a struct like so:

<snip>

Could you post a short but complete program which demonstrates the
problem?

See http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

Signature

Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
World class .NET training in the UK: http://iterativetraining.co.uk

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Bill Butler - 20 Jan 2008 16:02 GMT

> Hello all,
> I'm learning C# from a C/C++ background, and I came across an odd
> problem.
>
> I have a hashtable that holds a string and a struct like so:

From your definition of MyData below, you have a class, not a struct.
class and struct have very different behavior in C#.
You will most likely want to keep this a class.

> private Hashtable MyTable = new Hashtable();
>
[quoted text clipped - 12 lines]
>
> MyData DataPoint;

DataPoint is an unassigned reference (null)
Try
MyData DataPoint = new MyData(0,0,0);

> MyData.Add[SomeString,DataPoint];
>
[quoted text clipped - 4 lines]
>
> MyData D = (MyData)DataPoint.Value;

This is probably where it is bombing.
- DataPoint is null
- MyData has no Value members

> D.x = 10;
> }
>
> When I try to access the members of D, I get errors. Why?
> Can't I use a hashtable to store struct or class elements like a C++
> map?

It is generally a bad Idea to have public fields in your classes.
Try making the field private and adding public properties for set/get
functionality (if needed)

Many new C# developers that come from a C++ background trip all over
structs.
They should not be treated as lightweight classes the way they generally
are in C++.
Structs are very different critters from classes in C# and they have
many subtle gotchas.

I recommend sticking with classes until you understand all of the
differences.

Good luck
Bill

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