Is it possible in C# to create a generic function where the first parameter is an enum (one of several so it has to be generic I guess) and the second parameter is forced to be a value from the enum selected as the first parameter? I understand generics has to be used but I can't think of how to write such an expression, or if it's even possible.
Edit: Added code example
I know that this code example doesn't work but it illustrates a little in the direction I was thinking.
public List<int> Call<EnumValue>(Type enumType, EnumValue enumValue) where EnumValue : Enum.GetValues(typeof(enumType))
{
// Something
}
I don't think it is possible to have a compile-time constraint like that. The best you could do is a run-time check:
public List<int> Call<TEnum>(Type enumType, TEnum enumValue)
{
if(!enumType.IsAssignableFrom(typeof(TEnum)))
throw new ArgumentException();
// Something
}
UPDATE: Although I'm not sure why you need to pass the Type, if it has to be same type as the other parameter anyway. Couldn't you get rid of the first parameter?
public List<int> Call<TEnum>(TEnum enumValue)
{
Type enumType = typeof(TEnum);
// Something
}
The only way I'd think of doing something similar is by adding a condition inside the function and either return something or throw an Exception if the parameter is wrong (with a clear explanation).
Related
I'd like the compiler to infer a type for me, but am unsure if it is possible, or what the best alternative might be.
I'd like to do:
public static TValue Get<TValue>(TKey key) where TValue : Mapped<TKey> { ... }
public class MyObject : Mapped<int> { ... }
And have C# infer that TKey is an int. Is there any way to do something like this? If not, what would be the best alternative?
I'd like to avoid doing something like Get<MyObject, int>(1);
Edit:
For anyone who sees this in the future, similar questions have been asked here and here
No, there is no way to do this in C#. What you're essentially asking for is the ability to specify some of the generic arguments explicitly and have the remainder be inferred. That's not supported in C#; generic type inference needs to be done for all or none of the generic arguments.
#Servy is correct but as it has been pointed out on the other threads, sometimes you can split types up to make things inferrable.
In this example, we specify the non-inferrable type in a class declaration and the inferrable type in the method declaration.
public static class InferHelper<TValue>
where TValue : class
{
public static TValue Get<TKey>(TKey key)
{
// do your magic here and return a value based on your key
return default(TValue);
}
}
and you call it like this:
var result = InferHelper<MyObject>.Get(2);
I know that I can convert an int to an enum using a cast
MyEnumType myEnum = (MyEnumType) myInteger;
The problem here is that the runtime cast won't stop me at build time if myInteger is not of type int
void MyMethod(MyObject myObject)
{
MyEnumType myEnum = (MyEnumType) myObject.someProperty;
....
}
The above is not an uncommon code pattern but it won't protect me at build-time if the object's property type has been changed.
Is there a built-in method to do this conversion that will give me a build-time error? I could, of course, write a generic method rather easily but I'm wondering if one is built in.
You can use
Enum.TryParse(myInteger.ToString(), out result)
to get the Enum value of an int.
Hope this helps,
You can create a method to do this for your one enum easily enough:
public static MyEnumType CastFromInt<T>(int n)
{
return (MyEnumType)n;
}
Sadly, because there is no way to apply a generic constraint such that a generic type argument is an enumeration, there's no good way to genericize this. You could write something like this:
public static T CastFromInt<T>(int n)
{
return (T)(object)n;
}
but that assumes the caller uses an enum as the type of T. If they don't, it has problems. This also needlessly boxes the integer.
I'm trying to find a way to treat enums generically but I can't find a way to make it work. Say I have several enums declared something like this:
public enum ABC {
One,
Two,
Three
}
public enum DEF {
Four,
Five,
Six
}
and I want to write a method that takes an Enum as a parameter and simply returns name of the enum like this:
public string GetEnumName(Enum anEnum) {
return anEnum.GetType().Name;
}
but if I call it like GetEnumName(DEF); I get the 'is a type being used as a variable' error. Any ideas? thanks
EDIT Sorry judging by the replies I may not have been clear enough. I merely chose GetEnumName as a very simplistic example to illustrate the problem, not how to get the name from a type. Basically I want a method that I can pass ANY enum to and have it act on that enum directly, thanks
Use:
public static string GetTypeName<T>()
{
return typeof(T).Name;
}
Usage:
var result = GetTypeName<DEF>();
Perhaps this will do the trick?
public static class EnumExtensions
{
public static string GetEnumName<T>(this T value) where T : struct
{
var type = typeof(T);
if (!type.IsEnum)
throw new InvalidOperationException(string.Format("{0} is not an enum", type));
return type.GetEnumName(value);
}
}
What you want to write is something like this:
public string GetEnumName<T>() where T : Enum
{
return typeof(T).Name;
}
That is, a generic method with a type parameter constraint.
Unfortunately, there is no way to define such a constraint for enums in C# (nor it is possible to define one for delegates). People usually go for the solution mentioned by #ananthonline.
In fact such constraint is not supported by the C# language but it is supported at the CLR level. Using a tool like Mono.Cecil for example can help you to modify your assembly and apply the constraint on the method after you get it compiled.
Have a look to this article: Constraining generic constraints
You'll find a tool which eases the process of applying non-C#-supported generic type parameter constraints: Cecil constraint patcher
And don't forget there are a lot of useful static methods on the Enum class if you want to work with the names and values of your enum members.
Your problem is that you are passing in the type instead of the System.Type. Change your method to this:
public string GetEnumName(Type enumType) {
return enumType.Name;
}
Just pass the type:
public string GetEnumName(Type enumType)
{
return enumType.Name;
}
And
GetEnumName(typeof(ABC));
At this point if your method does nothing else you could probably just use typeof(ABC).Name instead.
You can't. Generic constraints are not allowed on enum types (including System.Enum). Use Enum.GetName instead.
I'm currently rewriting parts of a custom RPC mechanism (which cannot be replaced by something else, so don't suggest that ;-) ). The arguments of a call are collected in a custom collection that uses a dictionary internally. There is a method T Get<T>(string) to retrieve a named argument. For optional arguments, I wanted to add a TryGet<T>(string) method that returns the argument or null if it doesn't exist, so that the calling code can provide a default value using the null coalescing operator. Of course, for a value type this doesn't work, but I could use T? instead, which is what I want.
So what I have is this:
public class Arguments
{
// lots of other code here
public T TryGet<T>(string argumentName) where T : class
{
// look up and return value or null if not found
}
public T? TryGet<T>(string argumentName) where T : struct
{
// look up and return value or null if not found
}
}
With that, I'd like to be able to do the following:
return new SomeObject(
args.TryGet<string>("Name") ?? "NoName",
args.TryGet<int>("Index") ?? 1
);
Since the constraints are mutually exclusive, the compiler should be able to produce the correct code (it's always possible to infer the call from the generic type given at the call site). The compiler complains that the type already defines a member called "TryGet" with the same parameter types.
Is there any way to make something like this work without giving the two methods different names?
Constraints are not part of the signature. thus the answer to your question is no.
The way classes in the .NET Framework handle this scenario is TryGetValue with an out parameter. The return value is an indicator of whether the get was successful, where the out parameter contains the value requested (on success) or a suitable default value (on failure).
This pattern makes the implementation very simple for reference and value types. You would only need a single method to handle both scenarios.
For an example of this pattern, see Dictionary<TKey,TValue>.TryGetValue.
The reason this doesn't work is because you cannot have two methods with the same name and same argument types (the return type is not taken into account for method overloading). Instead you could define a single method without the generic constraint which will work for both value and reference types:
public T TryGet<T>(string argumentName)
{
if (!_internalDictionary.ContainsKey(argumentName))
{
return default(T);
}
return (T)_internalDictionary[argumentName];
}
An alternative solution could be this one:
public class Arguments {
public T Get<T>(string argumentName,T defaultValue) {
// look up and return value or defaultValue if not found
}
}
return new SomeObject(
args.Get<string>("Name","NoName"),
args.Get<int>("Index",1)
);
In that particular case you would not even have to specify the generic type, as it could be inferred by the default parameter:
return new SomeObject(
args.Get("Name","NoName"),
args.Get("Index",1)
);
Although it does not work directly due to identical argument types, You can do that by adding optional defaultValue parameter which defaults to null:
public class Arguments
{
// lots of other code here
public T? TryGet<T>(string argumentName, T? defaultValue = null) where T : class
{
// look up and return value or null if not found
}
public T? TryGet<T>(string argumentName, T? defaultValue = null) where T : struct
{
// look up and return value or null if not found
}
}
The reason this one works is that second argument type is different for both contraints (In the the method with class constraint it is simply T, and in the method with struct constraint it is Nullbale<T>).
Following code works as you would expect:
var args = new Arguments();
var stringValue = args.TryGet<string>("Name") ?? "NoName";
var intValue = args.TryGet<int>("Index") ?? 1;
I have read the documentation that states that "given the type of the enum, the GetValues() method of System.Enum will return an array of the given enum's base type" i.e. int, byte, etc.
However, I have been using the GetValues() method and all I keep getting back is an array of type Enums. Am I missing something?
public enum Response
{
Yes = 1,
No = 2,
Maybe = 3
}
foreach (var value in Enum.GetValues(typeof(Response)))
{
var type = value.GetType(); // type is always of type Enum not of the enum base type
}
You need to cast the result to the actual array type you want
(Response[])Enum.GetValues(typeof(Response))
as GetValues isn't strongly typed
EDIT: just re-read the answer. You need to explicitly cast each enum value to the underlying type, as GetValues returns an array of the actual enum type rather than the base type. Enum.GetUnderlyingType could help with this.
If you're using NET 3.5 (i.e. you have LINQ) you can do:
var responses = Enum.GetValues(typeof(Response)).Cast<Response>();
Personally I've created a separate method in my Utils project, which I include in my other projects. Here's the code I use:
public static class EnumUtil
{
public static IEnumerable<TEnum> GetAllValues<TEnum>()
where TEnum : struct, IConvertible, IComparable, IFormattable
{
return Enum.GetValues(typeof(TEnum)).Cast<TEnum>();
}
}
And I call it like this:
var enumValues = EnumUtil.GetAllValues<Response>();
Can you please refer to the documentation you mention. The documentation on Enum.GetValues does not mention anything like that (quote from that page):
Return Value
Type: System.Array
An Array of the
values of the constants in enumType.
The elements of the array are sorted
by the binary values of the
enumeration constants.
As Roger mentioned in a comment, it would be nice if there was a Enum.GetValues<MyEnum>() generic implementation, but there is not.
This problem annoyed the heck out of me, as well, so I created a library in C++/CLI that has generic implementations of all of the static methods on the Enum class (as well as a bunch of other generic methods for working with enums).
The library is written in C++/CLI because C# does not support constraining a generic type by System.Enum. C++/CLI (and the CLR) do support constraining by System.Enum and C#/VB.NET has no problem understanding calls to a method that have this constraint.
In the case of your example, you'd use Enums.GetValues<MyEnumType>() which will hand you an array of MyEnumType without the need to cast. Though C# and VB.Net do not support defining an enum constraint, they have no problem with consuming a method/class that has such a constraint and intellisense/the compiler handle it perfectly.
Similar to Joel's Answer but done a slight different way:
public static class Enums<T>
where T : struct, IComparable, IFormattable, IConvertible
{
static Enums()
{
if (!typeof(T).IsEnum)
throw new ArgumentException("Type T must be an Enum type");
}
public static IEnumerable<T> GetValues()
{
var result = ((T[])Enum.GetValues(typeof(T)).ToList()
return result;
}
}
Usage:
IEnumerable<System.Drawing.FontStyle> styles = Enums<System.Drawing.FontStyle>.GetValues();
If you are using .NET 5+ you can now use Enum.GetValues<TEnum>().
https://learn.microsoft.com/en-us/dotnet/api/system.enum.getvalues?view=net-6.0#system-enum-getvalues-1