ParameterPreconditions aren't working as of 1.0.0

[jmazouri]

I noticed this while upgrading my project from 1.0.0-rc2 (Nuget Prerelease) to 1.0.0 "Final" (also Nuget). It seems parameter preconditions simply never run.

Here is a simple repro, which I used in conjunction with the Command Service example in the documentation.

public class TestModule : ModuleBase
{
    [Command("test")]
    public async Task DoTest([TestPrecon]string input)
    {
        //this should never happen
        await ReplyAsync("Test Succeeded");
    }
}

public class TestPrecon : ParameterPreconditionAttribute
{
    public override async Task<PreconditionResult> CheckPermissions(ICommandContext context, ParameterInfo parameter, object value, IServiceProvider services)
    {
        //do some async work
        var ownerId = (await services.GetService<DiscordSocketClient>().GetApplicationInfoAsync()).Owner.Id;

        //always return an error
        return PreconditionResult.FromError("Failure.");
    }
}

With this code, "Test Succeeded" is always sent, regardless of the PreconditionResult being an error result. None of the code within CheckPermissions is executed.

[jmazouri]

After looking into it, it appears to be a regression caused by this large PR: #689
032aba9#diff-c2289b02a37b82dc75e06f172eddff2dL204

You can see at this line, during the refactoring that switched it to modern C# 7 pattern matching, ParameterPreconditionAttribute was mistakenly removed. In a local test build, simply adding the following to the top of the switch block fixed the issue:

case ParameterPreconditionAttribute precon:
    builder.AddPrecondition(precon);
    break;