How can I get the NuGet package version programmatically from a NuGet feed?

There exist a quite nice NuGet API to accomplish both.

a) Retrieving the NuGet package versions (for the Newtonsoft.Json package in following example):

GET https://api.nuget.org/v3-flatcontainer/Newtonsoft.Json/index.json

b) Downloading a certain version of the NuGet package - e.g.

GET https://api.nuget.org/v3-flatcontainer/utf8json/1.3.7/utf8json.1.3.7.nupkg

Please try to copy the URL to the browser and you could see the results of the examples...

To get more information about API, please visit the Package Content

The following code example could read the versions of a package (using the Microsoft.AspNet.WebApi.Client NuGet package which provides HttpContent's ReadAsAsync<T> extension for parsing the JSON result to an appropriate class - the VersionsResponse in this example)

using System;
using System.Net.Http;
using System.Threading.Tasks;
namespace GetNuGetVer
{
    class VersionsResponse
    {
        public string[] Versions { get; set; }
    }

    class Program
    {
        static async Task Main(string[] args)
        {
            var packageName = "Enums.NET";
            var url = $"https://api.nuget.org/v3-flatcontainer/{packageName}/index.json";
            var httpClient = new HttpClient();
            var response = await httpClient.GetAsync(url);
            var versionsResponse = await response.Content.ReadAsAsync<VersionsResponse>();
            var lastVersion = versionsResponse.Versions[^1]; //(length-1)
            // And so on ..
        }
    }
}

To get rid of Microsoft.AspNet.WebApi.Client (with dependency to Newtonsoft.Json), System.Text.Json can be used out of the box (since .NET Core 3.0) with code changes as follows:

        using System.Text.Json;
        ...
        var response = await httpClient.GetAsync(url);
        var versionsResponseBytes = await response.Content.ReadAsByteArrayAsync();
        var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
        var versionsResponse = JsonSerializer.Deserialize<VersionsResponse>(versionsResponseBytes, options);
        var lastVersion = versionsResponse.Versions[^1]; //(length-1)

or just a different JSON parser based on your own preferences.

As described on NuGet2, Nuget.Core is for version 2 of NuGet.

Version 3 of the NuGet client library has moved to Nuget.Client. See the NuGet API v3 documentation for more information.

NuGet Client SDK

Use the NuGet core package:

string packageID = "ILMerge";

// Connect to the official package repository
IPackageRepository repo = PackageRepositoryFactory.Default.CreateRepository("https://packages.nuget.org/api/v2");
var version =repo.FindPackagesById(packageID).Max(p=>p.Version);

Reference: Play with Packages, programmatically!