Question

有一个动态LINQ扩展程序库已作为样本Visual Studio 2008发布。我想用join方法扩展它。下面的代码在运行时失败并带有参数未匹配异常。问题在哪里？

public static IQueryable Join(this IQueryable outer, IEnumerable inner,
                              string outerSelector, string innerSelector, string resultsSelector,
                              params object[] values)
{
    if (inner == null)
        throw new ArgumentNullException("inner");
    if (outerSelector == null)
        throw new ArgumentNullException("outerSelector");
    if (innerSelector == null)
        throw new ArgumentNullException("innerSelector");
    if (resultsSelector == null)
        throw new ArgumentNullException("resultsSelctor");

    LambdaExpression outerSelectorLambda =
        DynamicExpression.ParseLambda(outer.ElementType, null,
                                      outerSelector, values);
    LambdaExpression innerSelectorLambda =
        DynamicExpression.ParseLambda(inner.AsQueryable().ElementType,
                                      null, innerSelector, values);

    ParameterExpression[] parameters = new ParameterExpression[] {
        Expression.Parameter(outer.ElementType, "outer"),
        Expression.Parameter(inner.AsQueryable().ElementType,
        "inner")
    };
    LambdaExpression resultsSelectorLambda =
        DynamicExpression.ParseLambda(parameters, null,
                                      resultsSelector, values);

    return outer.Provider.CreateQuery(
        Expression.Call(
            typeof(Queryable), "Join", new Type[] {
                outer.ElementType,
                inner.AsQueryable().ElementType,
                outerSelectorLambda.Body.Type,
                innerSelectorLambda.Body.Type,
                resultsSelectorLambda.Body.Type
            },
            outer.Expression, inner.AsQueryable().Expression,
            Expression.Quote(outerSelectorLambda),
            Expression.Quote(innerSelectorLambda),
            Expression.Quote(resultsSelectorLambda))
        );
}

Answer 1

我现在已经修好了。这是一个小学生错误，将过多的参数传递给CreateQuery（...）调用。将以下代码粘贴到DynamicQueryable类中的Dynamic.cs文件中，以获取动态Join扩展方法。您可以在http://code.msdn.microsoft.com/csharpsamples找到DynamicQuery示例项目的源代码。
享受。

    public static IQueryable Join(this IQueryable outer, IEnumerable inner, string outerSelector, string innerSelector, string resultsSelector, params object[] values)
    {
        if (inner == null) throw new ArgumentNullException("inner");
        if (outerSelector == null) throw new ArgumentNullException("outerSelector");
        if (innerSelector == null) throw new ArgumentNullException("innerSelector");
        if (resultsSelector == null) throw new ArgumentNullException("resultsSelctor");

        LambdaExpression outerSelectorLambda = DynamicExpression.ParseLambda(outer.ElementType, null, outerSelector, values);
        LambdaExpression innerSelectorLambda = DynamicExpression.ParseLambda(inner.AsQueryable().ElementType, null, innerSelector, values);

        ParameterExpression[] parameters = new ParameterExpression[] {
            Expression.Parameter(outer.ElementType, "outer"), Expression.Parameter(inner.AsQueryable().ElementType, "inner") };
        LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, resultsSelector, values);

        return outer.Provider.CreateQuery(
            Expression.Call(
                typeof(Queryable), "Join",
                new Type[] {outer.ElementType, inner.AsQueryable().ElementType, outerSelectorLambda.Body.Type, resultsSelectorLambda.Body.Type  },
                outer.Expression, inner.AsQueryable().Expression, Expression.Quote(outerSelectorLambda), Expression.Quote(innerSelectorLambda), Expression.Quote(resultsSelectorLambda)));
    }


    //The generic overload.
    public static IQueryable<T> Join<T>(this IQueryable<T> outer, IEnumerable<T> inner, string outerSelector, string innerSelector, string resultsSelector, params object[] values)
    {
        return (IQueryable<T>)Join((IQueryable)outer, (IEnumerable)inner, outerSelector, innerSelector, resultsSelector, values);
    }

Answer 2

您可以安装System.Linq.Dynamic.Core的nuget包 - https://github.com/StefH/System.Linq.Dynamic.Core

这个连接方法与各种其他辅助方法一起实现。

使用此库，您可以按照以下方式进行简单连接

myContext.TableA.Join(myContext.TableB,'Id','TableAId','outer',null)

结果选择器outer中的

和inner是访问连接结果的关键字。

使用具有多个属性的键和/或选择具有多个属性的结果可以通过以下方式完成

myContext.TableA.Join(myContext.TableB,'new (Id as key1,Code as key2)','new (TableAId as key1,AnotherCol as key2)','new(outer.Id,inner.Desc)',null)

Answer 3

以下是一些显示多列连接的示例代码。使用数据表和数据行，您需要始终通过索引器访问字段。

  DataTable t1 = new DataTable();
  t1.Columns.Add("FundId", typeof(int));
  t1.Columns.Add("Date", typeof(DateTime));
  t1.Columns.Add("CodeA", typeof(string));
  t1.Rows.Add(1, new DateTime(2010, 01, 01), "A1");
  t1.Rows.Add(2, new DateTime(2010, 01, 01), "A2");
  t1.Rows.Add(3, new DateTime(2010, 01, 01), "A3");

  DataTable t2 = new DataTable();
  t2.Columns.Add("FundId", typeof(int));
  t2.Columns.Add("Date", typeof(DateTime));
  t2.Columns.Add("CodeB", typeof(string));
  t2.Rows.Add(1, new DateTime(2010, 01, 01), "B1");
  t2.Rows.Add(2, new DateTime(2010, 01, 01), "B2");
  t2.Rows.Add(3, new DateTime(2010, 01, 01), "B3");

  IQueryable outerTable = t1.AsEnumerable().AsQueryable();
  IEnumerable innerTable = t2.AsEnumerable();

  var query = outerTable.Join
    (
      innerTable, 
      "new(get_Item(0) as FundId, get_Item(1) as Date)",
      "new(get_Item(0) as FundId, get_Item(1) as Date)",
      "new(outer.get_Item(0) as FundId, outer.get_Item(2) as CodeA, inner.get_Item(2) as CodeB)"
    );

如何创建动态LINQ连接扩展方法

3 个答案: