Add Get-ParameterValue

With support for blending arguments and defaults and even a config file
TODO: Tests

Source/Public/Get-ParameterValue.ps1

@@ -0,0 +1,103 @@
+function Get-ParameterValue {
+    <#
+        .SYNOPSIS
+            Get parameter values from PSBoundParameters + DefaultValues and optionally, a configuration file
+        .DESCRIPTION
+            This function gives command authors an easy way to combine default parameter values and actual arguments.
+            It also supports user-specified default parameter values loaded from a configuration file.
+
+            It returns a hashtable (like PSBoundParameters) which combines these parameter defaults with parameter values passed by the caller.
+    #>
+    [CmdletBinding()]
+    param(
+        # The base name of a configuration file to read defaults from
+        # If specified, the command will read a ".psd1" file with this name
+        # Suggested Value: $MyInvocation.MyCommand.Noun
+        [string]$FromFile,
+
+        # If your configuration file has defaults for multiple commands, pass
+        # the top-level key which contains defaults for this invocation
+        [string]$CommandKey,
+
+        # Allows extending the valid variables which are allowed to be referenced in configuration
+        # BEWARE: This exposes the value of these variables in the calling context to the configuration file
+        # You are reponsible to only allow variables which you know are safe to share
+        [String[]]$AllowedVariables
+    )
+
+    $CallersInvocation = $PSCmdlet.SessionState.PSVariable.GetValue("MyInvocation")
+    $BoundParameters = @{} + $CallersInvocation.BoundParameters
+    $AllParameters = $CallersInvocation.MyCommand.Parameters
+
+    if ($FromFile) {
+        $FromFile = [IO.Path]::ChangeExtension($FromFile, ".psd1")
+    }
+
+    $FileDefaults = if ($FromFile -and (Test-Path $FromFile)) {
+        $MetadataOptions = @{
+            AllowedVariables = $AllowedVariables
+            PSVariable       = $PSCmdlet.SessionState.PSVariable
+            ErrorAction      = "SilentlyContinue"
+        }
+        $FileValues = Import-Metadata $FromFile @MetadataOptions
+        if ($CommandKey) {
+            $FileValues = $FileValues.$CommandKey
+        }
+        $FileValues
+    } else {
+        @{}
+    }
+
+    # Don't support getting common parameters from the config file
+    $CommonParameters = [System.Management.Automation.Cmdlet]::CommonParameters +
+                        [System.Management.Automation.Cmdlet]::OptionalCommonParameters
+
+    # Layer the defaults below config below actual parameter values
+    foreach ($parameter in $AllParameters.GetEnumerator().Where({ $_.Key -notin $CommonParameters })) {
+        Write-Debug "  Parameter: $($parameter.key)"
+        $key = $parameter.Key
+
+        # If the value is not in the file defaults AND was not set by the user ...
+        if (-not $FileDefaults.ContainsKey($key) -and -not $BoundParameters.ContainsKey($key)) {
+            # Support parameter aliases in the config file (by changing their names)
+            Write-Debug "  Aliases: $($parameter.Value.Aliases -join ', ')"
+            foreach ($k in @($parameter.Value.Aliases)) {
+                if ($null -ne $k -and $FileDefaults.ContainsKey($k)) {
+                    Write-Debug "    ... Update FileDefaults[$key] from $k"
+                    $FileDefaults[$key] = $FileDefaults[$k]
+                    $null = $FileDefaults.Remove($k)
+                    break
+                }
+            }
+        }
+
+        if ($CallersInvocation) {
+            # If it's in the defaults AND it was not already set at a higher precedence
+            if ($FileDefaults.ContainsKey($Parameter) -and -not ($BoundParameters.ContainsKey($Parameter))) {
+                Write-Debug "Export $Parameter = $($FileDefaults[$Parameter])"
+                $BoundParameters[$Parameter] = $FileDefaults[$Parameter]
+                # Set the variable in the _callers_ SessionState as well as our return hashtable
+                $PSCmdlet.SessionState.PSVariable.Set($Parameter, $FileDefaults[$Parameter])
+            }
+
+            # Bound parameter values > build.psd1 values > default parameters values
+            if (-not $FileDefaults.ContainsKey($key) -and -not $BoundParameters.ContainsKey($key)) {
+                # Reading the current value of the $key variable returns either the bound parameter or the default
+                if ($null -ne ($value = $PSCmdlet.SessionState.PSVariable.Get($key).Value)) {
+                    if ($value -ne ($null -as $parameter.Value.ParameterType)) {
+                        $BoundParameters[$key] = $value
+                    }
+                }
+                if ($BoundParameters.ContainsKey($key)) {
+                    Write-Debug "    From Parameter: $($BoundParameters[$key] -join ', ')"
+                } elseif ($BoundParameters[$key]) {
+                    Write-Debug "    From Default: $($BoundParameters[$key] -join ', ')"
+                }
+            } elseif ($FileDefaults[$key]) {
+                Write-Debug "    From File: $($FileDefaults[$key] -join ', ')"
+            }
+        }
+    }
+
+    $BoundParameters
+}

Source/Public/Import-ParameterConfiguration.ps1

@@ -101,7 +101,7 @@ function Import-ParameterConfiguration {
             Permissions : { Access = Administrator }
 
         .EXAMPLE
-            Following up on our earlier example, let's look at a way to use imagine that -FileName parameter.
+            Following up on our earlier example, let's look at a way to use that -FileName parameter.
             If you wanted to use a different configuration files than your Noun, you can pass the file name in.
 
             You could even use one of your parameters to generate the file name. If we modify the function like ...