.WHERE(user=>user.id=MYPARAM)

Expression<Func<User,bool>> func = null;

ParameterExpression xParam = Expression.Parameter(typeof(User), "u");

BinaryExpression uidTest = Expression.Equal(Expression.Property(xParam, "id"),
                                                               Expression.Constant("test");

func = Expression.Lambda<Func<User, bool>>(uidTest, xParam)