如何为 C# 命令行开关提供默认值

Question

Following is the commandlet and I need it to be operate able by providing the switch 'Uninstall' or 'install' switch. By default 'install' is present.

    [Cmdlet(VerbsLifecycle.Install, "MyPackage")]
    [UsedImplicitly]
    public class InstallMyPackageCommand : PSCmdlet
    {            
        [UsedImplicitly]
        [Parameter(Position = 0, Mandatory = false, ParameterSetName = "Uninstall")]
        public SwitchParameter Uninstall { get; set; } = !SwitchParameter.Present;

        [UsedImplicitly]
        [ValidateNotNullOrEmpty]
        [ValidateNotNull]
        [Parameter(Position = 1, Mandatory = true)]
        public string Path { get; set; }

        protected override void BeginProcessing()
        {
            base.BeginProcessing();
        }
    }

But above code is not recognizing not provided switch as 'Uninstall' and tries to bind the file path with first switch parameter which is entirely not the file path. Even the first parameter is not manadadatory and default value is present.

Following error i receive when I issue the command "Install-MyPackage "C:\Projects\MsiPackages"

 Cannot convert 'System.String' to the type 'System.Management.Automation.SwitchParameter'

required by parameter 'Uninstall'.

Can any body let me know it's solution.

Answer 1

I think it's ok to privide the default value for SwitchParameter as:

  public SwitchParameter Install { get; set; } = SwitchParameter.Present;

Answer 2

While your own answer technically does answer your question relating to how to set a default value for a switch parameter. Using it in the way you describe is not recommended for a few reasons.

Functions with verbs like Install are supposed to be paired with their associated opposite (where it makes sense). Users will expect to run Install-MyPackage <blah> followed by Uninstall-MyPackage <blah> rather than Install-MyPackage <blah> -Uninstall . For reference, here's the list of Approved Verbs for Powershell .

Switch parameters in general are assumed to be "off" by default and "on" only when specified. While it's possible to specify a switch parameter with an explicit value like -MySwitch:$false , a large percentage of users don't know this. It's also not really discoverable without reviewing the full Get-Help output for the function.

In short, please don't do this. Your users will thank you.