app.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<appSettings>
<add key="k" value="v"/>
</appSettings>
</configuration>

AppSettingGettingExample.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">x86</Platform>
    <ProductVersion>10.0.0</ProductVersion>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>{89F1B53F-8309-407A-8985-6CF95B638145}</ProjectGuid>
    <OutputType>Exe</OutputType>
    <RootNamespace>AppSettingGettingExample</RootNamespace>
    <AssemblyName>AppSettingGettingExample</AssemblyName>
    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|x86' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug</OutputPath>
    <DefineConstants>DEBUG;</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <Externalconsole>true</Externalconsole>
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|x86' ">
    <DebugType>full</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release</OutputPath>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <Externalconsole>true</Externalconsole>
    <PlatformTarget>x86</PlatformTarget>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System" />
    <Reference Include="System.Configuration.Abstractions">
      <HintPath>..\packages\System.Configuration.Abstractions.1.1.5064.25905\lib\net45\System.Configuration.Abstractions.dll</HintPath>
    </Reference>
    <Reference Include="System.Configuration" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="Program.cs" />
    <Compile Include="Properties\AssemblyInfo.cs" />
  </ItemGroup>
  <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
  <ItemGroup>
    <None Include="app.config" />
    <None Include="packages.config" />
  </ItemGroup>
</Project>

packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="System.Configuration.Abstractions" version="1.1.5064.25905" targetFramework="net45" />
</packages>

Program.cs

using System;
using System.Configuration;


namespace AppSettingGettingExample
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			Console.WriteLine ("Hello World!");
			foreach (string k in ConfigurationManager.AppSettings) {
				var v = ConfigurationManager.AppSettings [k];
				Console.WriteLine ("k:{0} v:{1}", k, v);
			}
		}
	}
}