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Typedef-equivalent in C#?

    Question

  • I pass integer IDs to a variety of functions. I want to create a type for each ID so the compiler can check I am passing the right type. In C I would do this with typedefs. E.g.:

    typedef int personID;
    typedef int addressID;

    void DoSomething(personID p, addressID a)
    { ... }

    How can I do this in C#? Creating an entire class seems like a lot of overhead.
    Wednesday, April 05, 2006 1:27 AM

Answers

  • Truthfully, wrapping primitive types just to reimplement the same functionality IMHO would be a waste of time.

    However, if you still want to go ahead with this, then you can overload operators to enable this. Although it is valid to overload operators on classes, it's not something that is commonly done (nor would users expect it), instead, make these wrappers structs:



     struct LongWrapper
        {
            private long _Value;
            public LongWrapper(long value)
            {
                _Value = value;
            }
            public static bool operator ==(LongWrapper value1, LongWrapper value2)
            {
                return value1._Value == value2._Value;
            }
            public static bool operator !=(LongWrapper value1, LongWrapper value2)
            {
                return value1._Value != value2._Value;
            }
        }

     

    You should also override Object.Equals, Object.GetHashCode, implement IEquatable<T> interface on .NET 2.0 and consider implementing IComparable (IComparable<T> as well on .NET 2.0) (in which case you should also overload <, >, <=, >=.

    Implementing a primitive type correctly is not easy, and I don't thing its worth it.

     

    Thursday, April 06, 2006 6:45 PM
  • No, none of those would be allowed regardless of whether PersonID/AddressID are implemented as classes or structs/value types.

    If you need nullability in the C# 1.1 world, you're pretty much going to have to use classes. You could fake it using structs by giving each one a HasValue field or some such and then checking that anywhere you would do a null check, but if class types already provide this functionality, why bother?

    While I agree that defining operators on classes is not generally a good idea, I think it's perfectly appropriate when creating wrapper classes like this. Really, you're just exposing operations that are perfectly valid on the underlying types.

    -Tom Meschter
    Software Dev, Visual C# IDE

    Friday, April 07, 2006 4:05 PM

All replies

  • Apparently, it's the using statement./

    using personID = int;

    I didn't know that, so thanks :-) I just found it online.

     

    • Unmarked as answer by Dour High Arch Friday, April 10, 2009 10:26 PM
    • Proposed as answer by EdgarTaor Wednesday, September 05, 2012 3:21 AM
    Wednesday, April 05, 2006 1:31 AM
  • With the drawback that it is only file scope.  I searched further, but didn't find anything new either.
    Wednesday, April 05, 2006 1:59 AM
  • using personID = int;

    This gives me a syntax error: Invalid token 'using' in class, struct, or interface member declaration.
    Wednesday, April 05, 2006 7:29 PM
  • Short of defining a struct, … why not just name the parameters appropriately (e.g., ‘int personID’) and let the IDE intellisense be your guide?

    Wednesday, April 05, 2006 8:52 PM
  • It's not enough to "guide" the programmer to pass the right type, I want the compiler to enforce it.

    I just discovered that we need to support null IDs as well. I guess a class is the only way to go.

    Is there sample code somewhere for implementing wrappers to CLR types without having to implement every method/property of the base type?
    Wednesday, April 05, 2006 9:23 PM
  • You don't need a class to do this, use nullable value types:



    int? value = null;
    if (value == null)
    {
       value = 10;
    }

     

    Wednesday, April 05, 2006 10:30 PM
  • nullable wha? (google google...)

    That is cool, but this has to work in .Net 1.1.

    Also, can the compiler enforce these types? The reason for this exercise is so that I can define things like:

        AddressID GetAddress( PersonID id ) ...
        string GetCity( AddressID id ) ...

    And call:

        string c = GetCity( GetAddress( new PersonID(...) ) );

    Whereas the following will be compiler errors:

        string GetCity( new PersonID( ... ) );
        int aID = GetAddress( 1024 );
    Wednesday, April 05, 2006 11:48 PM
  • “Forcing” + nullability will require classes as far as I know. If you have “a lot” of these, a small program to generate them is probably in order.

    Thursday, April 06, 2006 2:00 PM
  • First, it's worth pointing out that typedefs in C don't give you type safety. All typedef does is allow you to add a name to an existing type. So in your original example:

    typedef int personID;
    typedef int addressID;

    void DoSomething(personID, addressID a) { ... }

    int, personID, and addressID are all the same type and completely interchangable. Your C compiler won't stop you from passing two personIDs to DoSomething instead of the required personID and addressID, for example. The typedefs give you clarity, but not type safety.

    If you're using C# and want type safety, you're going to have to declare new types for PersonID or AddressID. As for concerns about the "overhead" of using classes/structs in this case, the only way you're going to know for certain if the overhead is too high is to implement your program and then profile it.

    -Tom Meschter
    Software Dev, Visual C# IDE

     

    Thursday, April 06, 2006 5:06 PM
  • Personally, i have never found a need for

    typedef int personID;
    typedef int addressID;

    People should be writing: DoSomething( int personID).  The name of the variable is sufficient; i.e. use the name to describe what it does, and use the type to describe what it is.  If one actually does typedef int personID, they are writing code no better than code that casts things to int.  Both are type-unsafe, and both represent poor C# code.

    I think the demand for typedefs in C# is to hide the complexities of a long type:

    typedef Dictionary<...,...> MyMap;

    MyMap can be interchanged with Dictionary<...,...> in a function call: no type safety is lost by the use of a typedef in this case. 

    Brian

     

    Thursday, April 06, 2006 5:14 PM
  • Good points Tom. You've convinced me that a class is the way to go, but am not sure what needs to be implemented to allow using the new class. For example in a class like this:

        public class LongWrapper
        {
            private long id;

            protected LongWrapper( long id )
            {
                this.id = id;
            }
        }

    The following code:

        LongWrapper w1 = new LongWrapper(1024);
        LongWrapper w2 = new LongWrapper(1024);
        string doesItWork = (w1 == w2) ? "yes" : "no";

    Sets doesItWork to "no". I want two classes with the same internal IDs to be treated as identical. I suppose this is because == compares references, not values, and I need to override ==. But I only found this out after I tried it. If I implement it as a class, what other things do I need to override?
    Thursday, April 06, 2006 5:23 PM
  • I'll kick off the list with:

    • ==
    • <=
    • >=
    • >
    • <
    • !=
    • Equals()
    • GetHashCode()

    I can't think of anything else at the moment.

    Thursday, April 06, 2006 6:38 PM
  • Truthfully, wrapping primitive types just to reimplement the same functionality IMHO would be a waste of time.

    However, if you still want to go ahead with this, then you can overload operators to enable this. Although it is valid to overload operators on classes, it's not something that is commonly done (nor would users expect it), instead, make these wrappers structs:



     struct LongWrapper
        {
            private long _Value;
            public LongWrapper(long value)
            {
                _Value = value;
            }
            public static bool operator ==(LongWrapper value1, LongWrapper value2)
            {
                return value1._Value == value2._Value;
            }
            public static bool operator !=(LongWrapper value1, LongWrapper value2)
            {
                return value1._Value != value2._Value;
            }
        }

     

    You should also override Object.Equals, Object.GetHashCode, implement IEquatable<T> interface on .NET 2.0 and consider implementing IComparable (IComparable<T> as well on .NET 2.0) (in which case you should also overload <, >, <=, >=.

    Implementing a primitive type correctly is not easy, and I don't thing its worth it.

     

    Thursday, April 06, 2006 6:45 PM
  • I agree with making it a value type.
    Thursday, April 06, 2006 6:51 PM
  • Richard - you’re talking about passing an ‘ID’… how about passing the object that contains the ID? Rather than wrapping the fundamental types, can you write your functions like

    void Func ( Person p ) { // … }

     

    ?

    Thursday, April 06, 2006 8:25 PM
  • I understand the suggestions to just use value types, not classes, but that is what got us in the situation we now have.

    We have a large number of SQL database tables with unique integer primary keys; account, user, staff, provider, supplier, retailer, guest, and on and on. We have an API that wraps and caches database calls on all these tables. You get, for example, account info by passing an account ID, user info by passing a user ID, and so on.

    The problem is that all these IDs are integers, and developers are constantly passing the wrong type of ID to the API. I am working on a project to introduce type-safe wrappers to the API so one can only pass (for example) a UserID to functions that require user IDs, other types must cause compiler errors.
    Thursday, April 06, 2006 9:17 PM
  • There is still no reason why you can't make the wrappers themselves value types instead of classes.
    Thursday, April 06, 2006 9:22 PM
  • Value types in .Net 1.1 can be null, and can't be assigned to each other?
    Thursday, April 06, 2006 9:39 PM
  • Value Types can't be null, not sure what you mean by 'can't be assigned to each other?'.

    Thursday, April 06, 2006 9:55 PM
  • We need to be able to set IDs to null. By "can't be assigned to each other" I mean given:

        PersonID pID = FunctionThatReturnsAPersonID();
        AddressID aID = FunctionThatReturnsAPersonsAddressID( pID );

    The following are all compile-time errors:

        pID = 1024;
        pID = aID;
        int pID2 = FunctionThatReturnsAPersonID();
        AddressID aID2 = FunctionThatReturnsAPersonsAddressID( aID );

    Thursday, April 06, 2006 10:05 PM
  • No, none of those would be allowed regardless of whether PersonID/AddressID are implemented as classes or structs/value types.

    If you need nullability in the C# 1.1 world, you're pretty much going to have to use classes. You could fake it using structs by giving each one a HasValue field or some such and then checking that anywhere you would do a null check, but if class types already provide this functionality, why bother?

    While I agree that defining operators on classes is not generally a good idea, I think it's perfectly appropriate when creating wrapper classes like this. Really, you're just exposing operations that are perfectly valid on the underlying types.

    -Tom Meschter
    Software Dev, Visual C# IDE

    Friday, April 07, 2006 4:05 PM
  • Yes, I agree with that as well.  The only time I bother with typedef in c++ or using newtype=existingtype in c# is to save typing lengthy definitions.  ie, if I'm using a lot of System.Collections.Generic.Dictionary<string, string> and a bunch of System.Collections.Generic.KeyValuePair<string, string>.  It would help if I were not obsessed with always using complete namespaces in declarations, though. 
    Tuesday, July 03, 2007 6:14 PM
  • Personally i dont agree with wrapping a valuetype inside a class/struct to emulate the typedef. 
    Compare this:

    Dictionary<int, int> personAddress;
    Dictionary<personId, addressId> personAddress;

    It is no question that the 2nd declaration is much clearer.
    Wrapping the int as a struct/class would cripple this function:

        personAddress.ConstainsKey(new Person(personId)) ---> doesnt work

    And lots of other operations which is simply not worth the effort.
    Even though the compiler doesn't enforce typedef safety (which i believe is a shortcoming), if we agree that reading code is actually harder than writing, having a clear code helps a lot in the long run.

    Monday, July 28, 2008 3:29 AM
  • I also take exception to the lack of a good typedef facility in C# (pun fully intended).

    Typedefs in C++ help with producing clear code in an efficient manner by providing a functionality-to-name remapping facility. Implementing them as seperate classes / structs carries both a lot of dev time overhead and some potential performance overhead, in that something that was essentially an int suddenly becomes a class with overloaded operators which will no longer map directly to processor instructions.

    But perhaps more importantly, typedefs are a key part of forward-looking architecture in C++. Using well-chosen typedefs gives you the opportunity to later on replace certain types throughout your code with your own classes at no cost, if the need arises. Not only can that insulate you from changing requirements, but also changing implementations in third-party code. This is something that refactoring can't do for you - how would you select 100 different uints out of 500 uses of that type? That is at least as good a reason for using a PersonID typedef as semantic clarity ;)

    The using keyword only provides a local mimicking of the C++ keyword because of the file scope, which isn't really enough. Ideally the typedef keyword should be added to C#, but with the compiler treating typedef'ed types as first-class citizens, and at least attempting to enforce type correctness appropriately.
    Friday, August 15, 2008 10:01 PM
  • Being fairly new to some of the Win32 functions, when I came across them I was overwhelmed by some of the types and how to implement them in C#. My solution was to use 'typedef' which of course wasn't initially possible...

    namespace System.Win32.Types 
        public struct HWND 
        { 
            internal uint? _value; 
            public HWND(uint? value) 
            { 
                this._value = value; 
            } 
            public static implicit operator uint?(HWND hWnd) 
            { 
                return hWnd._value; 
            } 
            public static implicit operator HWND(uint? val) 
            { 
                return new HWND(val); 
            } 
            public static implicit operator uint(HWND hWnd) 
            { 
                if (hWnd._value == null) 
                { 
                    return 0; 
                } 
                else 
                { 
                    return (uint)hWnd._value; 
                } 
            } 
            public static implicit operator HWND(uint val) 
            { 
                return new HWND(val); 
            } 
            public override string ToString() 
            { 
                return ((uint?)this).ToString(); 
            } 
        } 

    namespace System.Win32 
        public static class Functions 
        { 
            public static System.Win32.Types.HWND GetForegroundWindow() 
            { 
                return System.Win32.Functions._GetForegroundWindow(); 
            } 
            [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint="GetForegroundWindow")] 
            internal static extern uint _GetForegroundWindow(); 
        } 

    Then, I can use them like this:

    using System; 
    using System.Windows.Forms; 
    using System.Win32; 
    using System.Win32.Types; 
    namespace Win32Test 
        static class Program 
        { 
            [STAThread] 
            static void Main() 
            { 
                HWND hWnd = Functions.GetForegroundWindow(); 
                MessageBox.Show(hWnd.ToString()); 
                hWnd = 5
                MessageBox.Show(hWnd.ToString()); 
                uint i = 6
                hWnd = i
                MessageBox.Show(hWnd.ToString()); 
                hWnd = null
                MessageBox.Show(hWnd.ToString()); 
                uint? j = null
                hWnd = j
                MessageBox.Show(hWnd.ToString()); 
            } 
        } 

    Of course, this is just the tip of the iceberg, and unfortunately you kinda have to implement your own other functions to allow the use of these types... Still though, handy when you're learning and you can't remember what each type is.



    Thursday, September 25, 2008 10:27 AM
  • You can't assign a value type of one type to that of another type, if that's what you mean.

    If you want to support a nullable value type in 1.1, you can always make the value type's default state indicate 'null'.  The value type would have a property, such as "IsNull" to test for null.   You could put a flag in the value type named "_isNotNull".  If this flag is false, which it will be by default, then that means the struct is effectively null.  In fact, that's pretty much how nullable value types work, except for some syntactic sugar added in.  If 0 is not a valid ID, then you could do without the flag and just test whether the value type's internal integer is 0.

    As far as what methods to implement, I would go with ==, !=, Equals, and GetHashCode.  Relational operators don't make sense for most IDs.  If you implement these types as value types, you wont need to implement ==, !=, Equals, and GetHashCode, as these are implemented with value semantics automatically.  Also, if you implement IConvertible, you can use the object as a parameter value in ADO.NET.  ADO.NET will use the IConvertible interface to convert it to the type it needs based upon the database type (probably an int in this case).

    Also, I don't think implementing ID types like this is a waste of time.  I often do similar things in my code.  For one thing, even if the parameters are descriptively named, there are many ways for errors that the compiler otherwise would not catch to be introduced.  For instance, if the method was refactored to reorder its arguments, it's very possible that one or more calls to that method will not be updated and error wont be revealed until the program is run.

    In reality, the IDs aren't really integers anyway, as one can't meaningfully perform arithmetic opreations on them.  These values are IDs that are being represented in the database as integers.

    Another benefit is that you can add additional validation into the type itself, if necessary.  For instance, an SSN can be easily represented as an int or a string.  But, if you make an SSN type that validates its value when it is created, then when you are passed a value of that type you know you've been given a valid value.  You don't have to introduce validation code for every method that accepts an SSN.

    Thursday, September 25, 2008 2:50 PM
  • Nice post...I have used operator overloading since moving from c to c++ and now c#.

    implicit and explicit casts do exactly what the original poster wanted

    Thursday, October 02, 2008 3:44 PM
  • I appreciate the value of typedefs. I think that the conversion of Windows to 32-bit and then to 64-bit is probably a good example, but I am not sure. This is more of a guess of what was needed for the conversions but hopefully they are a good example.

    When Windows was converted from 16-bits to 32-bits, there was obviously substantial changes needed. I think that one technique used was to create many typedefs. I suspect that the typedefs made conversion from 32-bits to 64-bits easier. There are many typedefs in the Windows SDK for use in C++ programs that make it easier (for us) to write code that can be compiled for either. For C# and .Net, there are other solutions for 32-bit and 64-bit but there are many other ways that a C# equivalent to the C++ typedef would likely make programming easier.
    Sam Hobbs; see my SimpleSamples.Info
    Tuesday, November 11, 2008 4:54 PM
  • I fully agree.

    I've got a similar problem using enums.

    We implement a REST based interface and provide a client class library.

    So, there are resources and resource collections, one collection for each resource.

    We've got a

    public enum ResourceType {ARes,BRes,CRes, XAttr, YAttr etc.};

    I'd love to be able to define a

    public enum CollectionType as ResType; 

    public enum ConstrainedType as ResType{ARes-CRes,YAttr};

    so that I can overload methods to react differently, depending on which type the parameter has.

    Ditto for integers:

    public IDType as int {1,3,5,7-12}; 

    Lots of Greetings!

    Volker

    Thursday, January 29, 2009 3:16 PM
  • After the original post until last two posts, this thread is a hallmark of coding badness.

    Your C/C++ compiler can enforce typedefs. But if you were just assuming it would, it probably doesn't. You'd actually need to define a distinct type to make it do that in all cases by default, which would probably mean defining a base class that implements an integer class, and then subclassing it to your specific indexes with a trait.

    template<const char* const SourceTable, const char* const SourceColumn>
    class ColumnIndex {
      unsigned int value;
      ...
    };

    class AddressID : ColumnIndex<"t_address", "address_id"> {
    public:
     AddressID(int i) : ColumnIndex(i) {}
     AddressID(const AddressID& rhs) : ColumnIndex(rhs.value) {}
    private:
     AddressID(const ColumnIndex& rhs) { // Not allowed }
    };

    You can ditch the template part: my implementation has an additional template parameter that defines whether or not a particular column is allowed null values, making it a component of type definition - and I thought that might be a useful hint here.

    Either way, with this methodology, the path to C# becomes very natural: The closest thing I've found to typedef in C# is

    public class MyTypedef : Dictionary<string, Dictionary<int, List<string>>> {}

    Monday, March 15, 2010 8:32 PM
  • To clarify, I believe you're asking about using typedefs as one type that you may need to change later. Say you need "personID" to be a string later but don't want to go through and change your whole program; just edit one little line. It would cause much overhead to make whole new classes and limit your freedom to use them interchangeably while still being able to edit just one line. Also, it's not about being able to null them either. Maybe C# programmers just don't get this. It's a C thing... I've found that you can just use a "using" statement to define it within your namespace but not inside the classes. 

    Example: 

    namespace myname

    {

        using mytype = String; 

        class program

        {

    static void Main(string[] args) 

    {

        mytype a, b; 

        Console.Read(a); Console.Read(b); 

        mytype c = a + b; 

        Console.WriteLine(c); 

    }

        }

    }

     

    Later you may consider changing the mytype to an int instead of a string, or perhaps an abstract data type which you've made. It's good for testing and interchangeability. 

    Wednesday, July 07, 2010 2:32 PM
  • IMO renaming an "int" via a typedef is pretty silly, be it in C++ or C#. Typically typedefs are used to rename complicated data types into something more friendly to read:

     

    // Imagine you need to return this data type:
    //
    // Dictionary<Foo, Dictionary<Baz, IEnumerable<Qux>>
    
    
    
    // You can do a typedef in C# using inheritance:
    
    class Bar : Dictionary<Baz, IEnumerable<Qux>>{}
    
    
    
    // And now have a much friendlier name to work with:
    
    // Dictionary<Foo, Bar>
    

    Tuesday, January 04, 2011 6:32 PM

  • IMO renaming an "int" via a typedef is pretty silly, be it in C++ or C#.



    Obviously you don't have as much experience as Microsoft.


    Sam Hobbs; see my SimpleSamples.Info
    Tuesday, January 11, 2011 8:38 AM
  • Am 11.01.2011 09:38, schrieb Simple Samples:

    IMO renaming an "int" via a typedef is pretty silly, be it in C++ or C#.

    Obviously you don't have as much experience as Microsoft.

    LOL and true

    //WinDef.h
    typedef int INT;
    typedef float FLOAT;
     Chris

    Tuesday, January 11, 2011 2:11 PM
  • // You can do a typedef in C# using inheritance:
    class Bar : Dictionary<Baz, IEnumerable<Qux>>{} // And now have a much friendlier name to work with: // Dictionary<Foo, Bar>

     

    That's not a typedef, it's a new type, that derives from another.
    Downcasts may fail. The only mean to create an alias name for a type in C#
    is afaics the using directive.

    using Bar = System.Collections.Generic.Dictionary<Baz, System.Collections.Generic.IEnumerable<Qux>>;


    Chris
    Tuesday, January 11, 2011 2:29 PM
  • I'm surprised at how many people seem to be against C# supporting a standard C++-style typedef, they can be incredibly useful.

    For example:

    typedef MyType int
    
    // Define a 2D array of 'MyType'
    MyType[][]
    

    A month down the road I decide I need my arrays to hold __int64s. Well that's easy enough, I just change my typedef, and every place using my array that was referring to the data within it by MyType should continue to work normally. typedefs also make it very clear to the programmer what type of data they're dealing with.

    Having to create my own class to recreate the comparison, equality, etc... operators for an int is a bit much.

    Now, all this being said, I also know that things like macros and typedefs make the compilers job a nightmare, and it's part of the reason why C# is a dream to work with in Visual Studio compared to C++. Still, it is a bit frustrating not having the typedef capability in C# without a lot of extra hassle.

    Thursday, July 07, 2011 5:52 AM
  • C/C++ typedefs are actually too weak, and I don't think C# needs to replicate that particularly nasty; that is:

     

    typedef _uint32 unsigned int ;
    typedef UINT32 _uint32 ;
    
    void takes_uint32(const _uint32& value_)
    {
     /* stuff */
    }
    
    int main(int argc, const char* const argv[])
    {
     int a = 0 ; // Note: not unsigned.
     UINT32 b = 1 ; // Note: "1" is an unsigned int, and b is a UINT32<br/> unsigned int c = 2 ;
    
     takes_uint32(a) ; // Valid :(
     takes_uint32(b) ; // Also valid :(<br/> takes_uint32(c) ; // ALSO valid :(
    }
    


    During parsing, typedefs are always rolled back to their lowest definition, almost like a macro.

    That causes typedef to fail for type safety. But if it was actually strict, it would be really useful for distinguishing between all these other numbers we otherwise just routinely stuff into int, long, etc, fields.

    All the attempts at type safety and strictness in C# are blown out the window without this feature, because nobody can be bothered to create micro numeric classes for every single instance of a numeric field with a discrete intention.

    A strict typedef mechanism shouldn't put that much of a burden on the IDE, infact it could perhaps just be implemented as a short hand for a class that wraps your aliased entity with appropriate access.

    class _uint32 : unsigned int ;
    class UINT32 : _uint32 ;

    unsigned int a ;
    _uint32 b ;
    UINT32 c ;

    a = 1 ; // valid
    b = a ; // invalid.
    b = 1 ; // invalid.
    b = (_uint32)1 ; // valid
    c = 1 ; // invalid.
    c = a ; // invalid.
    c = b ; // invalid.
    c = (UINT32)c ; // valid

     

     

     

    Friday, July 08, 2011 6:53 AM