Locating XML file for tests - DataSource DataDirectory value

I have problem with locating XML file used for tests. I changed variables names a little. DataSource and other variables are defined in this way:

[DataSource(TestData.XML_PROVIDER_NAME, TestData.CONNECTION_STRING, NAME_OF_TEST, DataAccessMethod.Sequential)]
TestData.CONNECTION_STRING = "|DataDirectory|\\TestData\\Drawing\\Entities\\FolderName\\TestData.xml"

I have no file added in my Visual Studio project at ProjectPath\TestData\Drawing\Entities\FolderName\TestName.xml path. How can I check the value of DataDirectory? I want to locate the file TestData.xml. I tried using:

AppDomain.CurrentDomain.GetData("DataDirectory");

but it returns null.