Using code contracts to make a generic to be of type enum

问题

A few days ago I asked a question titled How to constraint a generic to be of type enum?. To summarize the problem is the following code:

class MyClass<T> where T : enum // Not possible in C#
{
}

I was introduced to code contracts and that I could produce a compile time warning for the problem, which is all I want (to be informed at compile time that T should be an enum). I tried the follwing code (Full source).

class MyClass<T>
{
  public MyClass()
  {
    Contract.Requires(typeof(System.Enum).IsAssignableFrom(typeof(T)));
  }
}

It only produces a useless runtime error. I should be able to produce a compile time warning but I can't get it to work. Can anyone tell me what I'm doing wrong?

Here's a picture of project's Code Contracts setting:

回答1:

So, I wrote the following code in a file:

public class SomeClass<T>
{
    [ContractInvariantMethod]
    private void Invariants()
    {
        Contract.Invariant(typeof(System.Enum).IsAssignableFrom(typeof(T)));
    }

    /// <summary>Initializes a new instance of the SomeClass class.</summary>
    /// <param name="dependency"></param>
    public SomeClass()
    {

    }
}

public class SomeOtherClass
{
    public SomeOtherClass()
    {
        var myClass = new SomeClass<int>();

    }
}

From there, I went into the Code Contracts section of the project options and checked all checkboxes under "static checking". I then turned the warning level to "high". When I rebuilt the solution, I received a warning: "Code Contracts: invariant requires unproven: typeof(...)" that corresponded to the class invariant.

From there, I set the warning level back to low and saw that there was no warning, ala what you're reporting. So, I think that setting the warning level to high is what you need.

If that doesn't work, you might try following what I did and defining your contract as a class invariant (which, pedantically, I would suggest doing anyway because this is more of a class level invariant, conceptually, than a result of your constructor's execution).

Edit: I saw your screenshot after I posted, so I'd amend this to suggest using my "if that doesn't work" suggestion with the class level invariant instead of the requires invocation from the xtor.

来源：https://stackoverflow.com/questions/8716804/using-code-contracts-to-make-a-generic-to-be-of-type-enum

标签

inheritance

enums

constraints

code-contracts