Daniel recently added support for .NET 1.1 and .NET 1.0 to SharpDevelop 2.0.  Let us take a low level look at how SharpDevelop 2.0 targets other frameworks.  Targeting another framework involves a custom .targets file and custom MSBuild tasks. 

SharpDevelop Custom MSBuild Tasks and Targets

Component	Description
ICSharpCode.Build.Tasks.dll	Assembly containing custom MSBuild tasks.
SharpDevelop.Build.Common.targets	Specifies targets common to all SharpDevelop targets.
SharpDevelop.Build.CSharp.targets	Specifies targets for C# projects that target a framework other than the default.
SharpDevelop.Build.Mono.Mcs.targets	Defines the Mcs task.
SharpDevelop.Build.Mono.Gmcs.targets	Defines the Gmcs task.
SharpDevelop.Build.MSIL.targets	Specifies targets for ILAsm projects.

What happens when another framework is targeted?

When the project has been configured to target a framework other than the default, the project is modified to use a customised .targets file and has a property added that specifies the framework being targeted.  Below is a project file for a console application that targets Mono 1.1.

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <RootNamespace>HelloMonoWorld</RootNamespace>
    <AssemblyName>HelloMonoWorld</AssemblyName>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{0534CD74-E5FA-4349-866E-4E0C6428E840}</ProjectGuid>
    <TargetFrameworkVersion>Mono v1.1</TargetFrameworkVersion>
    <NoStdLib>False</NoStdLib>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)' == 'Debug' ">
    <OutputPath>bin\Debug\</OutputPath>
    <Optimize>False</Optimize>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>Full</DebugType>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)' == 'Release' ">
    <OutputPath>bin\Release\</OutputPath>
    <Optimize>True</Optimize>
    <DefineConstants>TRACE</DefineConstants>
    <DebugSymbols>False</DebugSymbols>
    <DebugType>None</DebugType>
  </PropertyGroup>
  <ItemGroup>
    <Compile Include="Main.cs" />
    <Compile Include="AssemblyInfo.cs" />
  </ItemGroup>
  <Import Project="$(SharpDevelopBinPath)\SharpDevelop.Build.CSharp.targets" />
</Project>

The .csproj file has a <TargetFrameworkVersion> element and includes a custom .targets file "SharpDevelop.Build.CSharp.targets" instead of the usual "Microsoft.CSharp.Targets".

The "SharpDevelop.Build.CSharp.targets" file looks at the TargetFrameworkVersion information and targets the appropriate runtime.  If we are targeting Mono, the file imports one of the Mono specific target files.

<Import Condition=" '$(TargetFrameworkVersion)' == 'Mono v1.1' " 
        Project="$(SharpDevelopBinPath)\SharpDevelop.Build.Mono.Mcs.targets"/>
<Import Condition=" '$(TargetFrameworkVersion)' == 'Mono v2.0' "
        Project="$(SharpDevelopBinPath)\SharpDevelop.Build.Mono.Gmcs.targets"/>

These Mono specific files reference the custom MSBuild tasks in the ICSharpCode.Build.Tasks assembly.

If one of the Microsoft .NET frameworks is targeted, various properties, such as CscToolPath, are changed so the correct compiler is used, but no custom tasks are involved.