Converting Delphi variant record to C# struct

Question

As I am trying to code a C# application from an existing application but developed in Delphi, Very tough but managed some how till, but now I have come across a problem...

Delphi code contains following code:

type  
  TFruit = record
    name : string[20];
    case isRound : Boolean of // Choose how to map the next section
      True  :
        (diameter : Single);  // Maps to same storage as length
      False :
        (length   : Single;   // Maps to same storage as diameter
         width    : Single);
  end;

i.e. a variant record (with case statement inside) and accordingly record is constructed and its size too. On the other hand I am trying to do the same in C# struct, and haven't succeeded yet, I hope somemone can help me here. So just let me know if there's any way I can implement this in C#. Thanks in advance....

David Heffernan · Accepted Answer · 2012-02-17 16:53:51Z

5

You could use an explicit struct layout to replicate this Delphi variant record. However, I would not bother since it seems pretty unlikely that you really want assignment to diameter to assign also to length, and vice versa. That Delphi record declaration looks like it dates from mid-1990s style of Delphi coding. Modern Delphi code would seldom be written that way.

I would just do it like this:

struct Fruit
{
    string name;
    bool isRound;
    float diameter; // only valid when isRound is true
    float length;   // only valid when isRound is false
    float width;    // only valid when isRound is false
}

A more elegant option would be a class with properties for each struct field. And you would arrange that the property getters and setters for the 3 floats raised exceptions if they were accessed for an invalid value of isRound.

edited Feb 17, 2012 at 16:53

answered Feb 17, 2012 at 16:20

David Heffernan

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2 Comments

Ani Over a year ago

If he is using this struct to call into unmanaged code, that logic wouldn't work.

David Heffernan Over a year ago

No, but that seems unlikely to me and the question did not state that. The question states that this is a Delphi -> C# port.

Ani · Accepted Answer · 2012-02-17 17:39:06Z

1

Perhaps this will do the trick?

This is NOT a copy-and-paste solution, note that the offsets and data sizes may need to be changed depending on how the Delphi structure is declared and/or aligned.

[StructLayout(LayoutKind.Explicit)]
unsafe struct Fruit
{
    [FieldOffset(0)] public fixed char name[20];
    [FieldOffset(20)] public bool IsRound;
    [FieldOffset(21)] public float Diameter;
    [FieldOffset(21)] public float Length;
    [FieldOffset(25)] public float Width;
}

edited Feb 17, 2012 at 17:39

answered Feb 17, 2012 at 16:24

Ani

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4 Comments

David Heffernan Over a year ago

Two things wrong with this. C# char is two bytes but Delphi shortstrings are one byte per char. And you did not account for alignment. That Delphi record will be aligned, although of course the compiler settings could influence that.

Ani Over a year ago

This wasn't meant to be a copy-and-paste solution, but purely indicative. I will update the code to reflect than, thanks.

user743382 Over a year ago

Having unaligned floats is pretty strange. Rather than saying they should be properly aligned if you need it to be laid out exactly as in Delphi, do align them, and only if they aren't aligned in Delphi and they need to be laid out the same in C#, change it back.

Loren Pechtel Over a year ago

@hvd: Yup--the Delphi record isn't declared as packed, it's got padding bytes. Diameter is at 24.

Loren Pechtel · Accepted Answer · 2012-02-17 17:57:58Z

It depends on what you are trying to do.

If you're simply trying to make a corresponding structure then look at David Heffernan's answer. These days there is little justification for mapping two fields on top of each other unless they truly represent the same thing. (Say, individual items or the same items in an array.)

If you're actually trying to share files you need to look along the lines of ananthonline's answer but there's a problem with it that's big enough I couldn't put it in a comment:

Not only is there the Unicode issue but a Delphi shortstring has no corresponding structure in C# and thus it's impossible to simply map a field on top of it.

That string[20] actually comprises 21 bytes, a one-byte length code and then 20 characters worth of data. You have to honor the length code as there is no guarantee of what lies beyond the specified length--you're likely to find garbage there. (Hint: If the record is going to be written to disk always zap the field before putting new data in it. It makes it much easier to examine the file on disk when debugging.)

Thus you need to declare two fields and write code to process it on both ends.

Since you have to do that anyway I would go further and write code to handle the rest of it so as to eliminate the need for unsafe code at all.

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