Framework 'Contains' method is declared as Impure

Question

I would think that 'System.Collections.Generic.Stack`1<System.String>.Contains(System.String)' would be free of side-effects and thus eligible to be declared Pure. The error below warns me about this. Is it appropriate to add the [Pure] attribute to the 'Contains' method of Generic.Stack?

using System.Collections.Generic;
using System.Diagnostics.Contracts;

namespace SampleContracts.Test
{
    public class Foo
    {
        public Stack<string> Deletables = new Stack<string>();
        public void Delete(string item)
        {
            Contract.Requires(item != null);
            /* Code Contracts compiler issues this warning
                 * Warning	4	
                 * Detected call to impure method 
                 * 'System.Collections.Generic.Stack`1<System.String>.Contains(System.String)' 
                 * in a pure region in method 
                 * 'SampleContracts.Test.Foo.Delete(System.String)'.	
                 * ...
                 */
            Contract.Requires(Deletables.Contains(item));
        }
    }
}

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Best Regards, David K Allen Minneapolis, Minnesota, USA

Answer 1

Thanks, I added the purity annotation and also on Peek.

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Cheers, -MaF (Manuel Fahndrich)