PredicateBuilder返回零记录 [英] PredicateBuilder returning zero records

问题描述

我正在使用PredicateBuilder创建动态的Where子句以从DataTable查询数据.我有一个字典，其中包含我需要搜索的列名和值.我只是简单地遍历字典，如果键与列名匹配，则将该键和值添加到谓词中.在对数据表运行实际查询之前，一切似乎都正常，我得到了零条记录:(但是，如果我用p => p ["Year"] =="2010"之类的东西代替动态谓词，则会得到记录返回.这是代码:

I'm using PredicateBuilder to create a dynamic Where clause to query data from a DataTable. I have a Dictionary that contains my column names and values I need to search for. I'm simply iterating over the dictionary, if the key matches a column name, then add that key and value to the predicate. Everything seems to work fine until the actual query is run against the datatable, I get zero records back:( But if I replace the dynamic predicate with something like p => p["Year"] == "2010", I get records back. Here's the code:

var objectList = table.AsEnumerable();
Func<DataRow, bool> predicate = GetPredicate(parms, table.Columns);

var list1 = objectList.Where(predicate).ToList();

private static Func<DataRow, bool> GetPredicate(Dictionary <string, string> parms, DataColumnCollection dataColumnCollection)
    {
        var predicate = PredicateBuilder.False<DataRow>();
        foreach (var parm in parms)
        {
            if (dataColumnCollection.Contains(parm.Key))
            {
                var copy = parm;
                predicate = predicate.And(p => p[copy.Key] == copy.Value);
            }
        }
        return predicate.Compile();
    }

任何帮助将不胜感激:)

Any help would be greatly appreciated:)

推荐答案

您先从false开始，然后添加and子句.

You're starting with false and then adding and clauses.

false && ...始终返回false，因此您的Where子句将永远不匹配任何内容.

false && ... always returns false, so your Where clause will never match anything.

尝试以

var predicate = PredicateBuilder.True<DataRow>();

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也:

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ALSO:

您正在关闭循环变量，这意味着您的所有谓词都将使用parms中的最后一个parm.

You are closing over a loop variable, which means all your predicates will use the last parm in parms.

您可以通过在循环中创建本地副本来解决此问题:

You can fix this by creating a local copy in your loop:

foreach( var parm in parms )
{
  if (dataColumnCollection.Contains(parm.Key))
  {
    var copy = parm;
    predicate = predicate.And( p => p[ copy.Key ] == copy.Value );
  }

---

更新:

DataRow[ key ]的类型为object，因此在p[ copy.Key ] == copy.Value中，相等运算符是对象引用相等，而不是字符串相等.

DataRow[ key ] is of type object, so in p[ copy.Key ] == copy.Value, the equality operator is object reference equality, not string equality.

您可以通过指定String.Equals来解决此问题:

You can fix this by specifying String.Equals:

predicate = predicate.And( p => String.Equals( p[ copy.Key ], copy.Value ) );

有趣的是，此示例显示您可以具有内容相同的string的多个实例.

Interestingly, this example shows that you can have multiple instances of string with the same contents.

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