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src/DotNetInstallSdk/Acquirables/GlobalJson/GlobalJsonFileLocator.cs

@@ -1,4 +1,5 @@
 using System.IO;
+using System.Linq;
 using System.Text.Json;
 
 namespace DotNet.InstallSdk.Acquirables.GlobalJson
@@ -11,13 +12,24 @@ public GlobalJsonFileLocator(ITextWriter _)
 
         public GlobalJsonParseResult Parse(string file = "global.json")
         {
-            if (!File.Exists(file))
-                return GlobalJsonParseResult.Failure($"A {file} file could not be found in the current directory.");
-
-            var contents = File.ReadAllText(file);
-            var result = JsonSerializer.Deserialize<GlobalJsonFile>(contents);
+            var dir = new DirectoryInfo(Directory.GetCurrentDirectory());
+
+            while (dir != null)
+            {
+                var searchFiles = dir.GetFiles(file);
+                if (!searchFiles.Any())
+                {
+                    dir = dir.Parent;
+                    continue;
+                }
+
+                var contents = File.ReadAllText(Path.Combine(dir.ToString(), file));
+                var result = JsonSerializer.Deserialize<GlobalJsonFile>(contents);
+
+                return GlobalJsonParseResult.Success(result);
+            }
 
-            return GlobalJsonParseResult.Success(result);
+            return GlobalJsonParseResult.Failure($"A {file} file could not be found.");
         }
     }
 }

src/DotNetInstallSdk/DotNetInstallSdk.csproj

@@ -22,6 +22,7 @@
     <ItemGroup>
         <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="2.3.4" />
         <PackageReference Include="System.Text.Json" Version="4.6.0-preview7.19362.9" />
+        <PackageReference Include="SimpleExec" Version="6.0.0" />
     </ItemGroup>
 
     <ItemGroup Condition="'$(Configuration)' == 'Debug'">

src/DotNetInstallSdk/InstallSdkTool.cs

@@ -1,4 +1,3 @@
-using System;
 using System.IO;
 using System.Net.Http;
 using System.Threading.Tasks;
@@ -8,12 +7,12 @@ namespace DotNet.InstallSdk
 {
     public static class InstallSdkTool
     {
-        public static void Run(Acquirable acquirable, ConsoleTextWriter writer)
+        public static async Task RunAsync(Acquirable acquirable, ITextWriter writer)
         {
             try
             {
                 var sdkAcquirer = new SdkAcquirer(new HttpClient(), writer, new InstallerLauncher(), new PlatformIdentifier());
-                sdkAcquirer.Acquire(acquirable).GetAwaiter().GetResult();
+                await sdkAcquirer.Acquire(acquirable);
             }
             catch (FileNotFoundException e)
             {

src/DotNetInstallSdk/Program.cs

@@ -1,4 +1,5 @@
-using DotNet.InstallSdk.Acquirables;
+using System.Threading.Tasks;
+using DotNet.InstallSdk.Acquirables;
 using DotNet.InstallSdk.Acquirables.GlobalJson;
 using McMaster.Extensions.CommandLineUtils;
 
@@ -16,7 +17,7 @@ public static int Main(string[] args)
         [Option("-L|--latest", Description = "Install the latest non-preview version of the .NET Core SDK")]
         public bool Latest { get; } = false;
 */
-        private void OnExecute()
+        private async Task OnExecuteAsync()
         {
             var writer = new ConsoleTextWriter();
 
@@ -29,8 +30,8 @@ private void OnExecute()
             {
                 a = new GlobalJsonVersion(writer);
             }
-            
-            InstallSdkTool.Run(a, writer);
+
+            await InstallSdkTool.RunAsync(a, writer);
         }
     }
 }

src/DotNetInstallSdk/SdkAcquirer.cs

@@ -8,6 +8,7 @@
 using System.Text.Json;
 using System.Threading.Tasks;
 using DotNet.InstallSdk.Acquirables;
+using static SimpleExec.Command;
 
 namespace DotNet.InstallSdk
 {
@@ -32,6 +33,12 @@ public async Task Acquire(Acquirable acquirable)
             if (!result.IsSuccess)
                 return;
 
+            if (await CheckSdkExists(result.Version))
+            {
+                _textWriter.WriteLine($"SDK version {result.Version} is already installed.");
+                return;
+            }
+
             using var channelResponse = await JsonDocument.ParseAsync(await _httpClient.GetStreamAsync(result.ChannelJson));
 
             var file = channelResponse
@@ -113,5 +120,29 @@ void CheckHash(string filePath, string fileHash)
             if (hashString != fileHash)
                 _textWriter.WriteLine("The downloaded file contents did not match expected hash.");
         }
+
+        static async Task<bool> CheckSdkExists(string version)
+        {
+            try
+            {
+                var dotnetInfo =
+                    (await ReadAsync("dotnet", "--info", noEcho: true)).Split(new[] {'\r', '\n'},
+                        StringSplitOptions.RemoveEmptyEntries);
+
+                var existingSdks =
+                    dotnetInfo
+                        .SkipWhile(x => !x.Contains(".NET Core SDKs installed:")) // TODO localize?
+                        .TakeWhile(x => !string.IsNullOrWhiteSpace(x))
+                        .Select(x => x.Split(' ', StringSplitOptions.RemoveEmptyEntries))
+                        .Where(x => x.Length > 0)
+                        .Select(x => x.First());
+
+                return existingSdks.Contains(version);
+            }
+            catch
+            {
+                return false; // If we fail to detect installed sdks, just assume we need to aquire it
+            }
+        }
     }
 }