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在不使用PredicateBuilder的情况下向表达式动态添加谓词

linq-to-entities entity-framework-6 linq entity-framework c#

StepUp · 技术社区 · 6 年前

我用 linqToEntities OR 动态条件。

there is a great library PredicateBuilder by brother Albahari 它可以解决我的任务,但我不能使用它。

我的情况如下:

IEnumerable<Condition> conditions = new List<Condition>()
{
    new Condition(){IdReference = 1, TableName = "Table1" },
    new Condition(){IdReference = 2, TableName = "Table2" },
    new Condition(){IdReference = 3, TableName = "Table3" }, 
    // and so on
};

我得到的是:

var histories = db.History as IQueryable<History>;
foreach (var cond in conditions)
{
     //What code should be here to be translated into:
     /*
     histories = histories
        .Where(h => h.IdReference == 1 && h.TableName =="Table1" 
            || h.IdReference == 2 && h.TableName == "Table2"
            || h.IdReference == 3 && h.TableName == "Table3");
            // and so on
     */
}

不过,我不知道事先有多少 conditions 是的。如何添加条件从 IEnumerable<Condition>

2 回复 | 直到 6 年前

StepUp 6 年前

不确定使用谓词生成器有什么问题-它不一定是LINQ Kit包,所谓的谓词生成器通常是一个带有两个扩展方法的单一静态类,如 Universal Predicate Builder PredicateUtils 从…起 Establish a link between two lists in linq to entities where clause

不管怎样,一旦你想要它,当然可以用普通的 Expression

Expression. 每次通话前:

using static System.Linq.Expressions.Expression;

然后用这样的方法:

if (conditions.Any())
{
    var parameter = Parameter(typeof(History));
    var body = conditions
        .Select(condition => Expression.Constant(condition))
        .Select(condition => Expression.AndAlso(
        Expression.Equal(
            Expression.PropertyOrField(parameter, nameof(History.IdReference)),
            Expression.Convert(
                  Expression.PropertyOrField(condition, nameof(Condition.IdReference))
                , Expression.PropertyOrField(parameter, nameof(History.IdReference)).Type)
        ),
        Expression.Equal(
            Expression.PropertyOrField(parameter, nameof(History.TableName)),
            Expression.Convert(
                  Expression.PropertyOrField(condition, nameof(Condition.TableName))
                , Expression.PropertyOrField(parameter, nameof(History.TableName)).Type)
         )
        ))
        .Aggregate(Expression.OrElse);                      
    var predicate = Lambda<Func<History, bool>>(body, parameter);
    histories = histories.Where(predicate);
}

Hitham Yosri 6 年前

var histories = db.History as IQueryable<History>;
foreach (var cond in conditions)
{
     //What code should be here to be translated into:

     histories = histories
        .Where(h => h.IdReference == cond.IdReference && 
      h.TableName ==cond.TableName );

}