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I'm teaching myself C#, so forgive me if this seems slightly obvious.

I'm trying to write a generic function that I can pass an array of structs into and then use one of the attributes of the struct. I have no idea how to declare a generic datatype in a function in a way that I can refer to attributes in the way needed.

Maybe what I'm asking can be better communicated in code - this is a non-working function to illustrate what I'm trying to do, how it strikes me as logical that it should work without actually knowing how to write it:

public static int AFunctionIsThis<DataType, int DataType.Value>(DataType passedrecord) { temp = passedrecord.Value * 2 + 1; return temp; }

And I want to be able to call it normally while specifying the attribute of the struct to be passed.

int NewVariable = AFunctionIsThis<ThisIsAStruct, ThisIsAStruct.AnIntAttribute>(ThisIsADeclaredStruct);

Thankyou very much,

Hanii Puppy.

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You can't specify members that a generic type should contain, you can only specify the generic data type.

You would use an interface where the property is defined:

public interface IHaveValue { int Value { get; } }

Your struct would then implement the interface, and you can specify the interface as the generic data type:

public static int AFunctionIsThis<T>(T passedrecord) where T : IHaveValue { return passedrecord.Value * 2 + 1; }

However, with what you are using it for, you don't need to use generics at all, you can just use the interface:

public static int AFunctionIsThis(IHaveValue passedrecord) { return passedrecord.Value * 2 + 1; }

Note that you should most likely not use a struct at all, but a class. A struct is more complicated to implement correctly, so you should stick to classes until you have a really good reason to use a struct.

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And people still ask how Scala's type system is more rich than C#/F#'s :)
I've been fiddling around with interfaces for the last hour or so. Calling the function normally, I get an error about not having a boxing conversion. I know I'm supposed to specify which fields in the interface represent the fields in the struct, but I don't know how to do that. Also, when I attempt to build (I was hoping to copy + paste the error), that error disappears in leu of an error that appears under each variable declared in the interface like you have reading "Interfaces Cannot contain fields".
Right, an interface can't have fields, so you need to use properties. I corrected the code above. The identification between the interface and the implementing type is by name, i.e. you just make a property with the same name as in the interface.
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(To start with, note that the word "attribute" has a different meaning in .NET from the OOP sense.)

Use a Converter<T,int> and Action<T,int> delegate to get and set the member, respectively.

e.g.

public static int AFunctionIsThis<DataType>(DataType passedrecord, Converter<DataType,int> getter) { temp = getter(passedrecord) * 2 + 1; return temp; }

and then call it

AFunctionIsThis(ThisIsADeclaredStruct, x => x.AnIntProperty);

If you also need to set the value, you can use

AFunctionIsThis(ThisIsADeclaredStruct, x => x.AnIntProperty, (x, v) => { x.AnIntProperty = v; });

or do some magic with Expression<Converter<T>> to yank out the member reference and create a matching setter.

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This is what I've gone with. However, when I attempt to call the function in the way you've shown, this is what I get: img41.imageshack.us/img41/6566/screenshot20110804at084.png
EDIT: Ok, I've got it down to two errors xD using this code to declare the function with: static public void Hierarchical<DataType>(ref DataType[] Table, Converter<DataType, int> ID, Converter<DataType, int> SubordinateTo) And this to call the function with: Sort.Hierarchical(ref Categories.Category, ID => Categories.Category[].ID, SubordinateTo => Categories.Category[].SubordinateTo); I know that I have to declare the items going into the convert as arrays - I'm just not sure how ^_^;; Thankyou very much!
@Hanii: Actually, thinking you need to pass arrays to the converter is your problem. Your sort function will pull an individual element of the array and pass it to the converter, to get an individual ID. Try Sort.Hierarchical(Categories.Category, c => c.ID, c => c.SubordinateTo); The array parameter doesn't need to be passed byref either, since an array is a reference type.
Working like a charm :D! Although I tried not passing by reference and the function ended up doing nothing - it only works being passed by reference. Thankyou very much :D As I said, I'm just learning C# and I'm building up a personal library for myself, this'll be invaluable to me for that. Now all I've got to figure out is how to edit those fields of the generic struct in the function. Couldn't give me a slight pointer, could you? ^_^;
@Hanii: For reference types, you can use Action<T, int> to provide a delegate that can change the value (shown in my answer). For value types, you'll need to define your own delegate type that takes a ref parameter.
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Hanii Puppy:

In short, yes, you should be able to do what you are doing, but here is a syntax that works:

public static int AFunctionIsThis<T>(T passedRecord) where T : DataType { var temp = passedRecord.Value; return temp; } public class DataType { public int Value { get; set; } }

Hope that helps. Dave

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That seems like the simplest method of achieving what I want - but how would I call that function and specify the attribute of the struct that I want to be represented by Value? Or would the struct passed in have to have an attribute named "Value"?
Hanii Puppy: Honestly if you want to guarantee that you can access an property of the struct, make the struct implement an interface like Guffa recommended... that way whatever you pass in be it struct or class will have that property.
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What you want to do, is define a generic method that accepts only T that implements a certain interface or is derived from certain base class that has an int member called Value. e.g:

public interface IClass { int Value{get;set;} } public class ExampleImpl : IClass { int Value{get;set;} /* Additional Members\methods here */ } public class HelperClass { public static int GenMethod<T>(T item) where T:IClass { return item.Value * 2 + 1; } }
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