将 Linq to EF 查询构建到可变列数中的可变关键字列表？

Question

我正在尝试提出一种实用方法来构建 Linq 查询或 Linq 谓词，以将其添加到 Linq to EF 查询中，以在可变列数的术语列表中搜索所有术语。

我正在尝试使用 PredicateBuilder 构建 where 子句。使用一个搜索词和一个固定的列列表就相对容易了。到目前为止，我尝试编写的伪代码如下所示：

private static Predicate<Project> CreateDynamicSearch(IEnumerable<strings> searchableColumns, string[] searchTerms)
{
      var predicate = PredicateBuilder.True<Project>();
      foreach (var columnName in searchableColumns) 
      { 
        foreach (var term in searchTerms)
        {
          predicate = predicate.And(a => a.**columnName**.Contains(term));
        }
       predicate = predicate.Or(predicate);
      }
      return predicate;
}

我遇到的最大问题是处理 columnName 的表达式。以前的建议是使用表达式树，但我不明白在这种情况下它是如何工作的。

** 更新** 更新后，我已经采用了您拥有的代码。它构建但是当我实际调用它时 Extension.Property(param,columnName); 行出错，错误 Instance 属性 'Name' 没有为类型 'System.Func`2[Myclass,System.Boolean]' 消息定义。 columnName = "Name"

** 更新 2 ** 它的调用方式：

var test = CreateDynamicSearch<Func<Project, bool>>(searchCols, searchTerms);

Answer 1

你可以自己构建谓词的表达式，在这种情况下相对容易：

private static Expression<Func<T, bool>> CreateDynamicSearch<T>(IEnumerable<string> searchableColumns, string[] searchTerms) {
    // start with true, since we combine with AND
    // and true AND anything is the same as just anything
    var predicate = PredicateBuilder.True<T>();
    foreach (var columnName in searchableColumns) {                
        // start with false, because we combine with OR
        // and false OR anything is the same as just anything
        var columnFilter = PredicateBuilder.False<T>();
        foreach (var term in searchTerms) {
            // a =>
            var param = Expression.Parameter(typeof(T), "a");
            // a => a.ColumnName
            var prop = Expression.Property(param, columnName);
            // a => a.ColumnName.Contains(term)
            var call = Expression.Call(prop, "Contains", new Type[0], Expression.Constant(term));
            columnFilter = columnFilter.Or(Expression.Lambda<Func<T, bool>>(call, param));
        }
        predicate = predicate.And(columnFilter);
    }
    return predicate;
}

回应评论

I was just curious if there was some way you could combine the expression created by Expression.Property(param, columnName) with the one the compiler generates for (string s) -> s.Contains(term)

您可以（例如）像这样：

// a =>
var param = Expression.Parameter(typeof(T), "a");                    
// a => a.ColumnName
var prop = Expression.Property(param, columnName);                    
// s => s.Contains(term)
Expression<Func<string, bool>> contains = (string s) => s.Contains(term);
// extract body - s.Contains(term)
var containsBody = (MethodCallExpression)contains.Body;                    
// replace "s" parameter with our property - a.ColumnName.Contains(term)
// Update accepts new target as first parameter (old target in this case is 
// "s" parameter and new target is "a.ColumnName")
// and list of arguments (in this case it's "term" - we don't need to update that).
// 
var call = containsBody.Update(prop, containsBody.Arguments);
columnFilter = columnFilter.Or(Expression.Lambda<Func<T, bool>>(call, param));

将 Linq to EF 查询构建到可变列数中的可变关键字列表？

Building a Linq to EF query to a variable list of keywords in a variable number of columns?

c#

linq

entity-framework

predicatebuilder