如何解决有没有一种在LINQ DataContext中实现选择条件的方法
|| 我正在MVC3项目中工作，并且正在使用LINQ to SQL。我有一个数据库架构，该数据库架构使用一个字段来指示记录是处于活动状态还是已删除（该字段是名为\“ Active \”的布尔值）。 现在假设有两个表链接，例如State和City，其中City引用State。 假设我有一个返回状态列表的方法：public ActionResult ListStates()
{
return View(_repository.ListStates());
}
public class Repository
{
public IQueryable<State> ListStates()
{
return dataContext.States.Where(p => p.Active == true)
}
}
public partial class DatabaseDataContext
{
partial void InsertState(State instance)
{
instance.Active = true;
this.ExecuteDynamicInsert(instance);
}
}
解决方法
如果我理解正确，则您正在尝试消除在存储库方法中指定.Where（p => p.Active == true）的需要，并且只想定义一次。
我不确定是否可以在不创建数据上下文包装器的情况下实现此目标，因为对于每个查询，您必须组合两个逻辑表达式，即来自存储库的表达式和p => p.Active == true。
最简单的解决方案如下：
/// <summary>
/// A generic class that provides CRUD operations againts a certain database
/// </summary>
/// <typeparam name=\"Context\">The Database context</typeparam>
/// <typeparam name=\"T\">The table object</typeparam>
public class DataContextWrapper<Context> where Context : DataContext,new()
{
Context DataContext;
/// <summary>
/// The name of the connection string variable in web.config
/// </summary>
string ConnectionString
{
get
{
return \"Connection String\";
}
}
/// <summary>
/// Class constructor that instantiates a new DataContext object and associates the connection string
/// </summary>
public DataContextWrapper()
{
DataContext = new Context();
DataContext.Connection.ConnectionString = ConnectionString;
}
protected IEnumerable<T> GetItems<T>([Optional] Expression<Func<T,bool>> query) where T : class,new()
{
//get the entity type
Type entity = typeof(T);
//get all properties
PropertyInfo[] properties = entity.GetProperties();
Expression<Func<T,bool>> isRowActive = null;
//we are interested in entities that have Active property ==> to distinguish active rows
PropertyInfo property = entity.GetProperties().Where(prop => prop.Name == \"Active\").SingleOrDefault();
//if the entity has the property
if (property != null)
{
//Create a ParameterExpression from
//if the query is specified then we need to use a single ParameterExpression for the whole final expression
ParameterExpression para = (query == null) ? Expression.Parameter(entity,property.Name) : query.Parameters[0];
var len = Expression.PropertyOrField(para,property.Name);
var body = Expression.Equal(len,Expression.Constant(true));
isRowActive = Expression.Lambda<Func<T,bool>>(body,para);
}
if (query != null)
{
//combine two expressions
var combined = Expression.AndAlso(isRowActive.Body,query.Body);
var lambda = Expression.Lambda<Func<T,bool>>(combined,query.Parameters[0]);
return DataContext.GetTable<T>().Where(lambda);
}
else if (isRowActive != null)
{
return DataContext.GetTable<T>().Where(isRowActive);
}
else
{
return DataContext.GetTable<T>();
}
}
}
/// <summary>
/// States Repository
/// </summary>
public class StatesRepository : DataContextWrapper<DEMODataContext>
{
/// <summary>
/// Get all active states
/// </summary>
/// <returns>All active states</returns>
public IEnumerable<State> GetStates()
{
return base.GetItems<State>();
}
/// <summary>
/// Get all active states
/// </summary>
/// <param name=\"pattern\">State pattern</param>
/// <returns>All active states tha contain the given pattern</returns>
public IEnumerable<State> GetStates(string pattern)
{
return base.GetItems<State>(s=>s.Description.Contains(pattern));
}
}
StatesRepository repo = new StatesRepository();
var activeStates = repo.GetStates();
var filtered = repo.GetStates(\"Al\");
版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 dio@foxmail.com 举报，一经查实，本站将立刻删除。