C＃Linq使用字段名作为变量查询数据库

Question

这是我的代码。 任何人都可以帮我重构这段代码。 我正在尝试使用字段名称作为变量，所以我不必使用switch case。 有点像dr in db.Users where dr.FieldName.Contains(value) 。

public static List<User> GetUser(string value, string fieldName)
{
    var db = new CCPTDataContext();
    var users = new List<User>();

    switch (fieldName)
    {
        case "FirstName":
            users = (from dr in db.Users where dr.FirstName.Contains(value)
                     select new User
                     {
                         Username = dr.Username,
                         FirstName = dr.FirstName,
                         LastName = dr.LastName
                     }).ToList();

            break;
        case "LastName":
            users = (from dr in db.Users where dr.LastName.Contains(value)
                     select new User
                     {
                         Username = dr.Username,
                         FirstName = dr.FirstName,
                         LastName = dr.LastName
                     }).ToList();
            break;
    }
    return users;
}

Answer 1

您必须使用System.Linq.Expression命名空间中的类手动创建表达式树：

public static Expression<Func<TSource, bool>> GetPropertyContainsValueExpression<TSource>(string propertyName, string value)
{
    var param = Expression.Parameter(typeof(TSource), "x");
    var prop = Expression.Property(param, propertyName);
    var valueExp = Expression.Constant(value, typeof(string));
    var contains = Expression.Call(prop, "Contains", null, valueExp);
    return Expression.Lambda<Func<TSource, bool>>(contains, param);
}

然后你就可以这样使用它：

public static List<User> GetUser(string value, string fieldName)
{
    var db = new CCPTDataContext();

    return db.Users.Where(GetPropertyContainsValueExpression<User>(fieldName, value)).ToList();
}

Answer 2

这应该工作：

users = (from dr in db.Users 
where (fieldName.Equals("FirstName") && dr.FirstName.Contains(value))
      || (fieldName.Equals("LastName") && dr.LastName.Contains(value))
                 select new User
                 {
                     Username = dr.Username,
                     FirstName = dr.FirstName,
                     LastName = dr.LastName
                 }).ToList();