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是否有一种简单的方法来附加lambda并重用lambda名称,以创建我的Linq where条件?

lambda linq-to-sql linq .net c#

TheSoftwareJedi jac · 技术社区 · 17 年前

我有一个用户控件,它接受一个Func,然后将其传递给IQuery的Linq“Where”扩展方法。这个想法是,从调用代码中,我可以传入所需的搜索函数。

Func<Order, bool> func == a => true;
if (txtName.Text.Length > 0) {
  //add it to the function
  func = a => func(a) && a.Name.StartsWith(txtName.Text);
}
if (txtType.Text.Length > 0) {
  //add it to the function
  func = a => func(a) && a.Type == txtType.Text;
}
..... etc .....

5 回复 | 直到 17 年前

Dario 17 年前

只需将当前lambda保存在临时变量中以防止递归。

var tempFunc = func;
func = a => tempFunc(a) && ...

Marc Gravell 17 年前

如果你想做一个“and”组合,首选是使用多个“where”子句:

IQueryable<Order> query = ...
if (!string.IsNullOrEmpty(txtName.Text.Length)) {
  //add it to the function
  query = query.Where(a => a.Name.StartsWith(txtName.Text));
}
if (!string.IsNullOrEmpty(txtType.Text.Length)) {
  //add it to the function
  query = query.Where(a => a.Type == txtType.Text);
}

您可以使用表达式构建(AndAlso、Invoke等)执行更复杂的操作,但对于“and”组合来说,这不是必需的。

static Expression<Func<T, bool>> OrElse<T>(
    this Expression<Func<T, bool>> lhs,
    Expression<Func<T, bool>> rhs)
{
    var row = Expression.Parameter(typeof(T), "row");
    var body = Expression.OrElse(
        Expression.Invoke(lhs, row),
        Expression.Invoke(rhs, row));
    return Expression.Lambda<Func<T, bool>>(body, row);
}
static Expression<Func<T, bool>> AndAlso<T>(
    this Expression<Func<T, bool>> lhs,
    Expression<Func<T, bool>> rhs)
{
    var row = Expression.Parameter(typeof(T), "row");
    var body = Expression.AndAlso(
        Expression.Invoke(lhs, row),
        Expression.Invoke(rhs, row));
    return Expression.Lambda<Func<T, bool>>(body, row);
}

不容易的。因此,为什么通常最好使用 Queryable.Where Queryable.Concat

bytebender 17 年前

我正在做这件事。..我使用表达式是因为Func是编译代码,其中 Expression<Func<YourObect, boo>>

在您的搜索页面上,您将实现如下代码:

SearchCritera<Customer> crit = new SearchCriteria<Customer>();

if (txtName.Text.Length > 0) {
  //add it to the function
  crit.Add(a.Name.StartsWith(txtName.Text));
}

if (txtType.Text.Length > 0) {
  //add it to the function
  crit.Add(a.Type == txtType.Text));
}

SearchCriteria对象看起来像这样。..

public class SearchCritera<TEntity>
    {
        private List<Expression<Func<TEntity, bool>>> _Criteria = new List<Expression<Func<TEntity, bool>>>();

        public void Add(Expression<Func<TEntity, bool>> predicate)
        {
            _Criteria.Add(predicate);
        }

        // This is where your list of Expression get built into a single Expression 
        // to use in your Where clause
        public Expression<Func<TEntity, bool>> BuildWhereExpression()
        {
            Expression<Func<TEntity, bool>> result = default(Expression<Func<TEntity, bool>>);
            ParameterExpression parameter = Expression.Parameter(typeof(TEntity), "entity");
            Expression previous = _Criteria[0];

            for (int i = 1; i < _Criteria.Count; i++)
            {
                previous = Expression.And(previous, _Criteria[i]);
            }

            result = Expression.Lambda<Func<TEntity, bool>>(previous, parameter);

            return result;
        }
    }

public List<Customer> FindAllCustomers(SearchCriteria criteria)
{
   return LinqToSqlDataContext.Customers.Where(SearchCriteria.BuildWhereExpression()).ToList();
}

这是我第一次把它编码出来,你可能需要为你的目的做一些更改,我知道这很复杂,但当我真正去做的时候,我会对它进行单元测试,但这是我一直在脑海中徘徊的想法。..

TheSoftwareJedi jac 17 年前

PredicateBuilder !!!

此扩展方法将防止递归:

    public static Func<T, bool> And<T>(this Func<T, bool> f1, Func<T, bool> f2)
    {
        return a => f1(a) && f2(a);
    }

    public static Func<T, bool> Or<T>(this Func<T, bool> f1, Func<T, bool> f2)
    {
        return a => f1(a) || f2(a);
    }

Func<Order, bool> func == a => true;
if (txtName.Text.Length > 0) {
  //add it to the function
  func.And(a => a.Name.StartsWith(txtName.Text));
}
if (txtType.Text.Length > 0) {
  //add it to the function
  func.And(a => a.Type == txtType.Text);
}

Amy B 17 年前

Dictionary<Func<bool>, Expression<Func<Order, bool>>> filters =
  new Dictionary<Func<bool>, Expression<Func<Order, bool>>>();
// add a name filter
filters.Add(
  () => txtName.Text.Length > 0,
  a => a.Name.StartsWith(txtName.Text)
);
// add a type filter
filters.Add(
  () => txtType.Text.Length > 0,
  a => a.Type == txtType.Text
);

var query = dc.Orders.AsQueryable();

foreach( var filter in filters
  .Where(kvp => kvp.Key())
  .Select(kvp => kvp.Value) )
{
  var inScopeFilter = filter;
  query = query.Where(inScopeFilter);
}