C# generic method with integer constraint refuses to cast from integer to the generic type

2

If I have a generic method that is constrained to be type 'int' then surely I should be able to cast an integer to the generic T type. For example...

    public T ExampleMethod<T>(int unchanged) where T : int
    {
        return (T)unchanged;
    }

...the compiler complains that Cannot convert type 'int' to 'T' but I have a constraint indicating that the target is as integer. So surely it should work?

Update:

The actual scenario is that I want to a helper method that returns an enum value. So my ideal helper method would be more like this....

public T GetAttributeAsEnum<T>(XmlReader reader, string name) where T : enum
{
    string s = reader.GetAttribute(name);
    int i = int.Parse(s);
    return (T)i;
}

...and use it like this...

StateEnum x = GetAttributeAsEnum<StateEnum>(xmlReader, "State");
CategoryEnum y = GetAttributeAsEnum<CategoryEnum>(xmlReader, "Category");
OtherEnum z = GetAttributeAsEnum<OtherEnum>(xmlReader, "Other");

...but you cannot constrain by enum.

Phil Wright

출처 c# generics
  • Why not just specify the parameter type as T? - gilly3
  • My real situation is more complicated. I want a helper method tthat reads a string from a file, the string is actually a number. Then cast the number to the correct target enum type. So... - Phil Wright
  • This will not compile. Error : 'int' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter. - Adriaan Stander
  • If T is always int then why to use generics? - default locale
  • public T Method<T>(XmlReader reader) where T : enum { string s = reader.GetAttribute("whatever"); int i = int.Parse(s); return (T)i; } - Phil Wright

3 답변

7

"Only class or interface could be specified as constraint." (c) ReSharper

int (Int32) is just a struct. You can constrain that T is a struct. but you can't use any struct as constraint.

the whole list of possible constraints you can find here - http://msdn.microsoft.com/en-us/library/d5x73970.aspx

UPD

and for Enum constraint see this question - Is there a workaround for generic type constraint of "special class" Enum in C# 3.0?

  • A struct can also be used as a constraint. The following compiles: public class Generic<T> where T : struct { }. You can then instantiate the class as Generic<int> gen = new Generic<int>();. - Eric J.
  • @EricJ. you don't understand. we can constrain T to be "a struct", but we can't use any struct as constraint. - Dmitry Khryukin
  • I see where you are coming from. As it stands, though, your answer is misleading. You in addition to struct you can also specify new as a constraint. Neither of those are a "class or interface". - Eric J.
  • @EricJ. updated! now it shouldn't be misleading? - Dmitry Khryukin
  • Looks good to me now. - Eric J.
4

int (and all other numeric types, and enums) cannot be used as a generic constraint.

See

Generic C# Code and the Plus Operator

for further details and options.

For a discussion with Anders Hejlsberg, the creator of C#, about generics and type constraints see

http://www.artima.com/intv/generics.html

One can place a type constraint of struct like this:

public class Generic<T> where T : struct { }

Generic<int> gen = new Generic<int>();

  • Downvoter: What is wrong with this answer? Do share, so that we can all learn. - Eric J.
2

Are you sure its compiling?

Here, it gives following error:

error CS0701: 'int' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

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