[Client SDK] Add method to check for package existence (loic-sharma#351)

Author: loic-sharma

samples/Sample02_Search.cs

@@ -2,12 +2,31 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using BaGet.Protocol.Models;
+using NuGet.Versioning;
 using Xunit;
 
 namespace BaGet.Protocol.Samples.Tests
 {
     public class Sample02_Search
     {
+        [Fact]
+        public async Task Exists()
+        {
+            // Check if a package exists.
+            NuGetClient client = new NuGetClient("https://api.nuget.org/v3/index.json");
+
+            if (!await client.ExistsAsync("newtonsoft.json"))
+            {
+                Console.WriteLine("Package 'newtonsoft.json' does not exist!");
+            }
+
+            var packageVersion = NuGetVersion.Parse("12.0.1");
+            if (!await client.ExistsAsync("newtonsoft.json", packageVersion))
+            {
+                Console.WriteLine("Package 'newtonsoft.json' version '12.0.1' does not exist!");
+            }
+        }
+
         [Fact]
         public async Task Search()
         {

src/BaGet.Protocol/NuGetClient.cs

@@ -53,13 +53,54 @@ public NuGetClient(NuGetClientFactory clientFactory)
             _clientFactory = clientFactory ?? throw new ArgumentNullException(nameof(clientFactory));
         }
 
+        /// <summary>
+        /// Check if a package exists.
+        /// </summary>
+        /// <param name="packageId">The package ID.</param>
+        /// <param name="cancellationToken">A token to cancel the task.</param>
+        /// <returns>Whether the package exists.</returns>
+        public virtual async Task<bool> ExistsAsync(
+            string packageId,
+            CancellationToken cancellationToken = default)
+        {
+            var client = await _clientFactory.CreatePackageContentClientAsync(cancellationToken);
+            var versions = await client.GetPackageVersionsOrNullAsync(packageId, cancellationToken);
+
+            return (versions != null && versions.Versions.Any());
+        }
+
+        /// <summary>
+        /// Check if a package exists.
+        /// </summary>
+        /// <param name="packageId">The package ID.</param>
+        /// <param name="packageVersion">The package version.</param>
+        /// <param name="cancellationToken">A token to cancel the task.</param>
+        /// <returns>Whether the package exists.</returns>
+        public virtual async Task<bool> ExistsAsync(
+            string packageId,
+            NuGetVersion packageVersion,
+            CancellationToken cancellationToken = default)
+        {
+            var client = await _clientFactory.CreatePackageContentClientAsync(cancellationToken);
+            var versions = await client.GetPackageVersionsOrNullAsync(packageId, cancellationToken);
+
+            if (versions == null)
+            {
+                return false;
+            }
+
+            return versions
+                .ParseVersions()
+                .Any(v => v == packageVersion);
+        }
+
         /// <summary>
         /// Download a package (.nupkg), or throws if the package does not exist.
         /// </summary>
         /// <param name="packageId">The package ID.</param>
         /// <param name="packageVersion">The package version.</param>
         /// <param name="cancellationToken">A token to cancel the task.</param>
-        /// <returns>The package's content stream. The stream may not be seekable.</returns>
+        /// <returns>The package's content stream. The stream may be unseekable and may be unbuffered.</returns>
         /// <exception cref="PackageNotFoundException">
         ///     The package could not be found.
         /// </exception>