User[] users = new User[] { };
...
Expect.Call(userRepository.GetAll()).Return(users);

dataProviderMock = new DynamicMock(typeof(IDataProvider<User>));
var user = new User {Username = "username", Password = "password"};
var users =new[]{ user };
dataProviderMock.ExpectAndReturn("GetAll",users);

dataProviderMock = new DynamicMock(typeof(IDataProvider<User>));
var user = new User {Username = "username", Password = "password"};
var users =new[]{ user };
dataProviderMock.ExpectAndReturn("GetAll",users.AsQueryable());