VsoLogger.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Engage.MsBuild.Logger.Vso
{
    using System.Globalization;

    using Microsoft.Build.Framework;
    using Microsoft.Build.Utilities;

    public class VsoLogger : Logger
    {
        public override void Initialize(IEventSource eventSource)
        {
            eventSource.WarningRaised += (sender, e) =>
                                         {
                                             LogIssue(
                                                 type: IssueType.Warning, 
                                                 code: e.Code,
                                                 columnNumber: e.ColumnNumber,
                                                 lineNumber: e.LineNumber,
                                                 message: e.Message,
                                                 sourcePath: e.File);
                                         };
            eventSource.ErrorRaised += (sender, e) =>
                                         {
                                             LogIssue(
                                                 type: IssueType.Error, 
                                                 code: e.Code,
                                                 columnNumber: e.ColumnNumber,
                                                 lineNumber: e.LineNumber,
                                                 message: e.Message,
                                                 sourcePath: e.File);
                                         };
        }

        private static void LogIssue(
            IssueType type,
            string message,
            string sourcePath = null,
            int? lineNumber = null,
            int? columnNumber = null,
            string code = null)
        {
            var attributes = new[]
                             {
                                 new KeyValuePair<string, string>("type", type.ToString().ToLowerInvariant()),
                                 new KeyValuePair<string, string>("sourcePath", sourcePath),
                                 new KeyValuePair<string, string>("lineNumber", lineNumber?.ToString(CultureInfo.InvariantCulture)),
                                 new KeyValuePair<string, string>("columnNumber", columnNumber?.ToString(CultureInfo.InvariantCulture)),
                                 new KeyValuePair<string, string>("code", code),
                             }.Where(kvp => !string.IsNullOrWhiteSpace(kvp.Value))
                             .Select(kvp => $"{kvp.Key}={kvp.Value};");

            Console.WriteLine(
                "##vso[task.logissue {0}]{1}", 
                string.Join(string.Empty, attributes), 
                message);
        }

        private enum IssueType
        {
            Warning = 0,
            Error,
        }
    }
}