EnumerableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MyExtensions
{
    public static class EnumerableExtensions
    {
        public static string ToJoinedString<T>(this IEnumerable<T> source, string separator = "")
        {
            return string.Join(separator, source);
        }

        public static IEnumerable<TSource> Concat<TSource>(this IEnumerable<TSource> source, params TSource[] values)
        {
            if (source == null) throw new ArgumentNullException("source");

            return source.ConcatCore(values);
        }

        private static IEnumerable<TSource> ConcatCore<TSource>(this IEnumerable<TSource> source, params TSource[] values)
        {
            foreach (var item in source) yield return item;
            foreach (var x in values) yield return x;
        }
    }
}

MyExtensions.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{B8ECF135-34E9-4582-B0B5-098EC3F6DB01}</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>MyExtensions</RootNamespace>
    <AssemblyName>MyExtensions</AssemblyName>
    <TargetFrameworkVersion>v4.5.2</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\</SolutionDir>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="System.Xml.Linq" />
    <Reference Include="System.Data.DataSetExtensions" />
    <Reference Include="Microsoft.CSharp" />
    <Reference Include="System.Data" />
    <Reference Include="System.Net.Http" />
    <Reference Include="System.Xml" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="EnumerableExtensions.cs" />
    <Compile Include="Properties\AssemblyInfo.cs" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  <Target Name="CopyDll">
    <Copy SourceFiles="$(SolutionDir)MyExtensions\bin\$(ConfigurationName)\MyExtensions.dll" DestinationFolder="$(SolutionDir)CSharpCompilerSlackOuthookCSharp\bin" ContinueOnError="true" />
    <Copy SourceFiles="$(SolutionDir)MyExtensions\bin\$(ConfigurationName)\MyExtensions.dll" DestinationFolder="$(SolutionDir)CSharpCompilerWebhookCSharp\bin" ContinueOnError="true" />
  </Target>
  <Target Name="AfterBuild">
    <CallTarget Condition="'$(Configuration)' == 'Debug'" Targets="CopyDll" />
    <CallTarget Condition="'$(Configuration)' == 'Release'" Targets="CopyDll" />
  </Target>
  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
       Other similar extension points exist, see Microsoft.Common.targets.
  <Target Name="BeforeBuild">
  </Target>
  <Target Name="AfterBuild">
  </Target>
  -->
</Project>