Expand environment variable for My Documents
Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);

Environment.ExpandEnvironmentVariables(@"%MyDocuments%Foo");

Environment.ExpandEnvironmentVariables(@"%USERPROFILE%My DocumentsFoo");

string fullPath = SpecialFolder.ExpandVariables(@"%MyDocuments%Foo");

// this environment variable is created for the current process only
string documents = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
Environment.SetEnvironmentVariable("MYDOCUMENTS", documents);

string path = "%MYDOCUMENTS%\Foo"; // read from config

// expand real env. vars
string expandedPath1 = Environment.ExpandEnvironmentVariables(path);

// expand our "fake" env. var
string documents = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
string expandedPath2 = path.Replace("%MYDOCUMENTS%", documents);

string docs = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
string foo = Path.Combine(docs, "Foo");

string variablePath = "%appdata%".Trim('%'); //read from some config setting
string appdataPath = Environment.GetEnvironmentVariable(variablePath);
string subdir = "foo";  //some other config setting
string myDir = Path.Combine(appdataPath, subdir);

Start-> run -> cmd
set
set |findstr /i documents

Path.Combine(
  Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),
  "Foo");