How to create a dynamic LINQ join extension method

Question

There was a library of dynamic LINQ extensions methods released as a sample with Visual Studio 2008. I\'d like to extend it with a join method. The code below fail

Answer 1

You can install the nuget package of System.Linq.Dynamic.Core - https://github.com/StefH/System.Linq.Dynamic.Core

This has the join method implemented along with various other helper methods.

Using this library you can do a simple join in the following the way

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

in the result selector outer and inner are key words to access the result of the join.

Using a key with multiple properties and/or selecting a result with multiple properties can be done in the following way

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 2

Here is some sample code showing a join on multiple columns. Using a datatable and datarows you need to always access fields via the indexer.

  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)"
    );

Answer 3

I've fixed it myself now. It was a schoolboy error passing too many parameters to the CreateQuery(... ) call. Paste the following code into the Dynamic.cs file within the DynamicQueryable class for a dynamic Join extension method. You can find the source for the DynamicQuery sample project at http://code.msdn.microsoft.com/csharpsamples.
Enjoy.

    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);
    }