Skip .NET SDK and runtime install when unneeded

tools/Check-DotNetRuntime.ps1

@@ -0,0 +1,41 @@
+<#
+.SYNOPSIS
+    Checks whether a given .NET Core runtime is installed.
+#>
+[CmdletBinding()]
+Param (
+    [Parameter()]
+    [ValidateSet('Microsoft.AspNetCore.App','Microsoft.NETCore.App')]
+    [string]$Runtime='Microsoft.NETCore.App',
+    [Parameter(Mandatory=$true)]
+    [Version]$Version
+)
+
+$dotnet = Get-Command dotnet -ErrorAction SilentlyContinue
+if (!$dotnet) {
+    # Nothing is installed.
+    Write-Output $false
+    exit 1
+}
+
+Function IsVersionMatch {
+    Param(
+        [Parameter()]
+        $actualVersion
+    )
+    return $actualVersion -and
+           $Version.Major -eq $actualVersion.Major -and
+           $Version.Minor -eq $actualVersion.Minor -and
+           (($Version.Build -eq -1) -or ($Version.Build -eq $actualVersion.Build)) -and
+           (($Version.Revision -eq -1) -or ($Version.Revision -eq $actualVersion.Revision))
+}
+
+$installedRuntimes = dotnet --list-runtimes |? { $_.Split()[0] -ieq $Runtime } |% { $v = $null; [Version]::tryparse($_.Split()[1], [ref] $v); $v }
+$matchingRuntimes = $installedRuntimes |? { IsVersionMatch -actualVersion $_ }
+if (!$matchingRuntimes) {
+    Write-Output $false
+    exit 1
+}
+
+Write-Output $true
+exit 0

tools/Check-DotNetSdk.ps1

@@ -0,0 +1,31 @@
+<#
+.SYNOPSIS
+    Checks whether the .NET Core SDK required by this repo is installed.
+#>
+[CmdletBinding()]
+Param (
+)
+
+$dotnet = Get-Command dotnet -ErrorAction SilentlyContinue
+if (!$dotnet) {
+    # Nothing is installed.
+    Write-Output $false
+    exit 1
+}
+
+# We need to set the current directory so dotnet considers the SDK required by our global.json file.
+Push-Location "$PSScriptRoot\.."
+try {
+    dotnet -h 2>&1 | Out-Null
+    if ($LASTEXITCODE -eq 129) {
+        # This error code indicates no matching SDK exists.
+        Write-Output $false
+        exit 2
+    }
+
+    # The required SDK is already installed!
+    Write-Output $true
+    exit 0
+} finally {
+    Pop-Location
+}

tools/Install-DotNetSdk.ps1

@@ -15,11 +15,14 @@
     When using 'repo', environment variables are set to cause the locally installed dotnet SDK to be used.
     Per-repo can lead to file locking issues when dotnet.exe is left running as a build server and can be mitigated by running `dotnet build-server shutdown`.
     Per-machine requires elevation and will download and install all SDKs and runtimes to machine-wide locations so all applications can find it.
+.PARAMETER Force
+    Installs the preferred .NET SDK and runtime versions even if compatible versions are found to already be installed.
 #>
 [CmdletBinding(SupportsShouldProcess=$true,ConfirmImpact='Medium')]
 Param (
     [ValidateSet('repo','user','machine')]
-    [string]$InstallLocality='user'
+    [string]$InstallLocality='user',
+    [switch]$Force
 )
 
 $DotNetInstallScriptRoot = "$PSScriptRoot/../obj/tools"
@@ -98,15 +101,23 @@ if ($InstallLocality -eq 'machine') {
         $DotNetInstallDir = '/usr/share/dotnet'
     } else {
         $restartRequired = $false
-        if ($PSCmdlet.ShouldProcess(".NET Core SDK $sdkVersion", "Install")) {
-            Install-DotNet -Version $sdkVersion
-            $restartRequired = $restartRequired -or ($LASTEXITCODE -eq 3010)
+        if ($Force -or (-not (& "$PSScriptRoot/Check-DotNetSdk.ps1"))) {
+            if ($PSCmdlet.ShouldProcess(".NET Core SDK $sdkVersion", "Install")) {
+                Install-DotNet -Version $sdkVersion
+                $restartRequired = $restartRequired -or ($LASTEXITCODE -eq 3010)
+            }
+        } else {
+            Write-Verbose "A .NET SDK version compatible with $sdkVersion is already installed."
         }
 
         $runtimeVersions | Get-Unique |% {
-            if ($PSCmdlet.ShouldProcess(".NET Core runtime $_", "Install")) {
-                Install-DotNet -Version $_ -Runtime
-                $restartRequired = $restartRequired -or ($LASTEXITCODE -eq 3010)
+            if ($Force -or (-not (& "$PSScriptRoot/Check-DotNetRuntime.ps1" -Version $_))) {
+                if ($PSCmdlet.ShouldProcess(".NET Core runtime $_", "Install")) {
+                    Install-DotNet -Version $_ -Runtime
+                    $restartRequired = $restartRequired -or ($LASTEXITCODE -eq 3010)
+                }
+            } else {
+                Write-Verbose ".NET runtime $_ is already installed."
             }
         }
 
@@ -150,21 +161,29 @@ if (-not (Test-Path $DotNetInstallScriptPath)) {
 
 $anythingInstalled = $false
 
-if ($PSCmdlet.ShouldProcess(".NET Core SDK $sdkVersion", "Install")) {
-    $anythingInstalled = $true
-    Invoke-Expression -Command "$DotNetInstallScriptPath -Version $sdkVersion $switches"
+if ($Force -or (-not (& "$PSScriptRoot/Check-DotNetSdk.ps1"))) {
+    if ($PSCmdlet.ShouldProcess(".NET Core SDK $sdkVersion", "Install")) {
+        $anythingInstalled = $true
+        Invoke-Expression -Command "$DotNetInstallScriptPath -Version $sdkVersion $switches"
+    } else {
+        Invoke-Expression -Command "$DotNetInstallScriptPath -Version $sdkVersion $switches -DryRun"
+    }
 } else {
-    Invoke-Expression -Command "$DotNetInstallScriptPath -Version $sdkVersion $switches -DryRun"
+    Write-Verbose "A .NET SDK version compatible with $sdkVersion is already installed."
 }
 
 $switches += '-Runtime','dotnet'
 
 $runtimeVersions | Get-Unique |% {
-    if ($PSCmdlet.ShouldProcess(".NET Core runtime $_", "Install")) {
-        $anythingInstalled = $true
-        Invoke-Expression -Command "$DotNetInstallScriptPath -Channel $_ $switches"
+    if ($Force -or (-not (& "$PSScriptRoot/Check-DotNetRuntime.ps1" -Version $_))) {
+        if ($PSCmdlet.ShouldProcess(".NET Core runtime $_", "Install")) {
+            $anythingInstalled = $true
+            Invoke-Expression -Command "$DotNetInstallScriptPath -Channel $_ $switches"
+        } else {
+            Invoke-Expression -Command "$DotNetInstallScriptPath -Channel $_ $switches -DryRun"
+        }
     } else {
-        Invoke-Expression -Command "$DotNetInstallScriptPath -Channel $_ $switches -DryRun"
+        Write-Verbose ".NET runtime $_ is already installed."
     }
 }