When implementing a microsoft.build.utilities.task how to i get access to the various environmental variables of the build?

When implementing a microsoft.build.utilities.task how to i get access to the various environmental variables of the build?

For example "TargetPath"

I know i can pass it in as part of the task XML

<MyTask TargetPath="$(TargetPath)" />

But i don't want to force the consumer of the task to have to do that if I can access the variable in code.

http://msdn.microsoft.com/en-us/library/microsoft.build.utilities.task.aspx

I worked out how to do this

public static class BuildEngineExtensions
{
    const BindingFlags bindingFlags = BindingFlags.NonPublic | BindingFlags.FlattenHierarchy | BindingFlags.Instance | BindingFlags.Public;

    public static IEnumerable GetEnvironmentVariable(this IBuildEngine buildEngine, string key,bool throwIfNotFound)
    {
        var projectInstance = GetProjectInstance(buildEngine);

        var items = projectInstance.Items
            .Where(x => string.Equals(x.ItemType, key, StringComparison.InvariantCultureIgnoreCase)).ToList();
        if (items.Count > 0)
        {
            return items.Select(x => x.EvaluatedInclude);
        }


        var properties = projectInstance.Properties
            .Where(x => string.Equals(x.Name, key, StringComparison.InvariantCultureIgnoreCase)).ToList();
        if (properties.Count > 0)
        {
            return properties.Select(x => x.EvaluatedValue);
        }

        if (throwIfNotFound)
        {
            throw new Exception(string.Format("Could not extract from '{0}' environmental variables.", key));
        }

        return Enumerable.Empty();
    }

    static ProjectInstance GetProjectInstance(IBuildEngine buildEngine)
    {
        var buildEngineType = buildEngine.GetType();
        var targetBuilderCallbackField = buildEngineType.GetField("targetBuilderCallback", bindingFlags);
        if (targetBuilderCallbackField == null)
        {
            throw new Exception("Could not extract targetBuilderCallback from " + buildEngineType.FullName);
        }
        var targetBuilderCallback = targetBuilderCallbackField.GetValue(buildEngine);
        var targetCallbackType = targetBuilderCallback.GetType();
        var projectInstanceField = targetCallbackType.GetField("projectInstance", bindingFlags);
        if (projectInstanceField == null)
        {
            throw new Exception("Could not extract projectInstance from " + targetCallbackType.FullName);
        }
        return (ProjectInstance)projectInstanceField.GetValue(targetBuilderCallback);
    }
}

And it can be used like this

string targetPath = buildEngine.GetEnvironmentVariable("TargetPath", true).First();
string intermediateAssembly = buildEngine.GetEnvironmentVariable("IntermediateAssembly", true).First();
IEnumerable<string> referencePaths = buildEngine.GetEnvironmentVariable("ReferencePath", true);

Yes it is ugly and black magic but it works.