public static List<TestCase> GetAllTestCasesInTestPlan(ITestManagementTeamProject testManagementTeamProject, ITestPlan testPlan, bool initializeTestCaseStatus = true)
{
    testPlan.Refresh();
    List<TestCase> testCasesList;
    testCasesList = new List<TestCase>();
    string fullQuery =
        String.Format("SELECT [System.Id], [System.Title] FROM WorkItems WHERE [System.WorkItemType] = 'Test Case' AND [Team Project] = '{0}'", testManagementTeamProject.TeamProjectName);
    IEnumerable<ITestCase> allTestCases = testManagementTeamProject.TestCases.Query(fullQuery);
    foreach (var currentTestCase in allTestCases)
    {
        TestCase testCaseToAdd = new TestCase(currentTestCase, currentTestCase.TestSuiteEntry.ParentTestSuite, testPlan, initializeTestCaseStatus);
        if (!testCasesList.Contains(testCaseToAdd))
        {
            testCasesList.Add(testCaseToAdd);
        }
    }

    return testCasesList;
}