I want to add a project as a subproject. When I drag the subproject into the main project it appears as a file reference rather than subproject. The picture below explains the problem:

Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
The solution is to ensure that the subproject is not open. Once the project is closed Xcode can add the project as a subproject.