Build_In_Memory_CSharp_Project.txt

[Reflection.Assembly]::LoadWithPartialName('Microsoft.Build');
$proj = [System.Xml.XmlReader]::create("https://gist.githubusercontent.com/bohops/a29a69cf127ffb0e37622d25b9f79157/raw/35fa4c5a0d2db037220f224b5c4c269ea243b3bd/test.csproj");
$e=new-object Microsoft.Build.Evaluation.Project($proj);
$e.Build();

test.csproj

<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Target Name="Hello">
   <FragmentExample />
   <ClassExample />
  </Target>
  <UsingTask
    TaskName="FragmentExample"
    TaskFactory="CodeTaskFactory"
    AssemblyFile="C:\Windows\Microsoft.Net\Framework\v4.0.30319\Microsoft.Build.Tasks.v4.0.dll" >
    <ParameterGroup/>
    <Task>
      <Using Namespace="System" />  
      <Code Type="Fragment" Language="cs">
        <![CDATA[		
        ]]>
      </Code>
    </Task>
	</UsingTask>
	<UsingTask
    TaskName="ClassExample"
    TaskFactory="CodeTaskFactory"
    AssemblyFile="C:\Windows\Microsoft.Net\Framework\v4.0.30319\Microsoft.Build.Tasks.v4.0.dll" >
	<Task>
	<!-- <Reference Include="System.IO" /> Example Include -->		
      <Code Type="Class" Language="cs">
        <![CDATA[
			using System;
			using System.Diagnostics;
			using Microsoft.Build.Framework;
			using Microsoft.Build.Utilities;
				
			public class ClassExample :  Task, ITask
			{
				public override bool Execute()
				{
                    System.Diagnostics.Process proc = new System.Diagnostics.Process();
                    proc.StartInfo.FileName = "c:\\windows\\system32\\notepad.exe";
                    proc.Start();						
					return true;
				}
			}
        ]]>
      </Code>
    </Task>
  </UsingTask>
</Project>