Is there anyway to #define CONSTANT on a solution basis?

I have another approach for doing this:

1. Edit global config file. For example in my case it's .netcf2.0 so it's $(MSBuildBinPath)\microsoft.compactframework.csharp.targets.

   Add the following line:

   <Import Project="$(SolutionDir)CommonSettings.targets" Condition="exists('$(SolutionDir)CommonSettings.targets')" />
   

   This is to tell MSBuild import the CommonSettings.targets if it's existed in your solution folder.

2. Create CommonSettings.targets under your solution folder.

   For example to define ABC symbol:

   <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
       <PropertyGroup>
       <DefineConstants>$(DefineConstants);ABC</DefineConstants>
       </PropertyGroup>
   </Project>
   

You can actually use a variation on Ritch's approach with common project settings. Essentially you have to make a single change to the end of each project file in your solution:

  <PropertyGroup Condition="'$(SolutionDir)' == '' or
                     '$(SolutionDir)' == '*undefined*'">
      <SolutionDir>..\..\</SolutionDir>
    </PropertyGroup>
    <Import Project="$(SolutionDir)CommonSettings.targets" />
  </Project>

Then you can define CommonSettings.targets to contain the solution wide settings.

  <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003"
           ToolsVersion="3.5">
      <PropertyGroup>
          <TacoBuild>true</TacoBuild>
      </PropertyGroup>
  </Project>

That's basically it unless you want to override or merge project settings already defined in each project. The link below discusses how to do this in some detail.

http://www.ademiller.com/blogs/tech/2007/12/common-project-settings-for-your-visual-studio-solution/